U.S. patent application number 15/578595 was filed with the patent office on 2018-06-21 for hetero-junction bipolar transistor and electric device.
This patent application is currently assigned to POWDEK K.K.. The applicant listed for this patent is POWDEC K.K.. Invention is credited to Hiroji KAWAI.
Application Number | 20180175182 15/578595 |
Document ID | / |
Family ID | 59499684 |
Filed Date | 2018-06-21 |
United States Patent
Application |
20180175182 |
Kind Code |
A1 |
KAWAI; Hiroji |
June 21, 2018 |
HETERO-JUNCTION BIPOLAR TRANSISTOR AND ELECTRIC DEVICE
Abstract
This hetero-junction bipolar transistor includes a first n-type
GaN layer, an Al.sub.xGa.sub.1-xN layer (0.1.ltoreq.x.ltoreq.0.5),
an undoped GaN layer having a thickness of not less than 20 nm, a
Mg-doped p-type GaN layer having a thickness of not less than 100
nm, and a second n-type GaN layer which are sequentially stacked.
The first n-type GaN layer and the Al.sub.xGa.sub.1-xN layer form
an emitter, the undoped GaN layer and the p-type GaN layer form a
base, and the second n-type GaN layer forms a collector. During
non-operation, two-dimensional hole gas is formed in a part of the
undoped GaN layer near the hetero interface between the
Al.sub.xGa.sub.1-xN layer and the undoped GaN layer. When the
thickness of the p-type GaN layer is b [nm], the hole concentration
of the p-type GaN layer is p [cm.sup.-3], and the concentration of
the two-dimensional hole gas is P.sub.s [cm.sup.-2],
p.times.b.times.10.sup.-7+P.sub.s.gtoreq.1.times.10.sup.13
[cm.sup.-2] is satisfied.
Inventors: |
KAWAI; Hiroji; (Oyama-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POWDEC K.K. |
Oyama-shi, Tochigi |
|
JP |
|
|
Assignee: |
POWDEK K.K.
|
Family ID: |
59499684 |
Appl. No.: |
15/578595 |
Filed: |
February 3, 2017 |
PCT Filed: |
February 3, 2017 |
PCT NO: |
PCT/JP2017/003888 |
371 Date: |
November 30, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 29/0692 20130101;
H01L 29/66318 20130101; H01L 29/7371 20130101; H01L 29/365
20130101; H01L 29/2003 20130101; H01L 29/205 20130101; H01L 29/7373
20130101; H01L 29/32 20130101; H01L 29/0817 20130101; H01L 29/778
20130101 |
International
Class: |
H01L 29/737 20060101
H01L029/737; H01L 29/205 20060101 H01L029/205; H01L 29/20 20060101
H01L029/20 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 4, 2016 |
JP |
2016-019441 |
Claims
1-14. (canceled)
15: A hetero-junction bipolar transistor, comprising: a first
n-type GaN layer; an Al.sub.xGa.sub.1-xN layer
(0.1.ltoreq.x.ltoreq.0.5) on the first n-type GaN layer; an undoped
GaN layer having a thickness of not less than 20 nm on the
Al.sub.xGa.sub.1-xN layer; a Mg-doped p-type GaN layer having a
thickness of not less than 100 nm on the undoped GaN layer; a
second n-type GaN layer on the p-type GaN layer; an emitter
electrode electrically connected to the first n-type GaN layer; a
base electrode electrically connected to the p-type GaN layer; and
a collector electrode electrically connected to the second n-type
GaN layer, an emitter being formed by the first n-type GaN layer
and the Al.sub.xGa.sub.1-xN layer, a base being formed by the
undoped GaN layer and the p-type GaN layer and a collector being
formed by the second n-type GaN layer, two-dimensional hole gas
being formed in a part of the undoped GaN layer near the hetero
interface between the Al.sub.xGa.sub.1-xN layer and the undoped GaN
layer during non-operation,
p.times.b.times.10.sup.-7+P.sub.s.gtoreq.1.times.10.sup.13
[cm.sup.-2] being satisfied where b denotes the thickness of the
p-type GaN layer, p denotes the hole concentration of the p-type
GaN layer and P.sub.s denotes the concentration of the
two-dimensional hole gas.
16: The hetero-junction bipolar transistor according to claim 15
wherein when the concentration of Mg doped with the p-type GaN
layer is denoted as N.sub.Mg [cm.sup.-3] and the electrical
activation ratio of Mg doped with the p-type GaN layer is denoted
as r,
N.sub.Mg.times.r.times.b.times.10.sup.-7+P.sub.s.gtoreq.1.times.10.sup.13
[cm.sup.-2] is satisfied.
17: The hetero-junction bipolar transistor according to claim 16
wherein the Al composition x and the thickness t [nm] of the
Al.sub.xGa.sub.1-xN layer satisfy the following equation
t.gtoreq..alpha.x.sup..beta. where .alpha. and .beta. are numerals
determined depending on necessary P.sub.s.
18: The hetero-junction bipolar transistor according to claim 17
wherein when P.sub.s.gtoreq.5.times.10.sup.12 [cm.sup.-2], the Al
composition x and the thickness t [nm] of the Al.sub.xGa.sub.1-xN
layer satisfy the following equation:
t.gtoreq.11290x.sup.-1.865.
19: The hetero-junction bipolar transistor according to claim 16
wherein 5.times.10.sup.19
[cm.sup.-3].ltoreq.N.sub.Mg.ltoreq.9.times.10.sup.19 [cm.sup.-3] is
satisfied.
20: The hetero-junction bipolar transistor according to claim 15
wherein the first n-type is formed on a first n.sup.+-type GaN
layer, the second n-type GaN layer is formed on the p-type GaN
layer like a mesa and a second n.sup.+-type GaN layer is formed on
the second n-type GaN layer.
21: The hetero-junction bipolar transistor according to claim 20
wherein the emitter electrode is formed on a surface of the first
n.sup.+-type GaN layer opposite to the first n-type GaN layer, the
base electrode is formed on the part of the p-type GaN layer on
which no second n-type GaN layer is formed and the collector
electrode is formed on the second n.sup.+-type GaN layer.
22: The hetero-junction bipolar transistor according to claim 15
wherein an Al.sub.yGa.sub.1-yN graded layer is formed between the
Al.sub.xGa.sub.1-xN layer and the undoped GaN layer, the Al
composition y of the Al.sub.yGa.sub.1-yN graded layer decreasing
monotonically from x to 0 in the direction from the
Al.sub.xGa.sub.1-xN layer to the undoped GaN layer.
23: The hetero-junction bipolar transistor according to claim 20,
further comprising a p-type GaN layer formed between the second
n-type GaN layer and the second n.sup.+-type GaN layer.
Description
TECHNICAL FIELD
[0001] This invention relates to a hetero-junction bipolar
transistor and an electric device, and particularly relates to a
hetero-junction bipolar transistor using a gallium nitride
(GaN)-based semiconductor and an electric device using the
hetero-junction bipolar transistor.
BACKGROUND ART
[0002] The hetero-junction bipolar transistor (HBT) is known as a
high frequency transistor which can supply a high current. On the
other hand, since GaN-based semiconductor is a widegap
semiconductor, it is possible to apply a high voltage. Therefore,
if a hetero-junction bipolar transistor is made by GaN-based
semiconductor, it is expected that a high frequency power
transistor having high resistance voltage and high power is
realized.
[0003] Until now, the following GaN-based hetero-junction bipolar
transistor is known (see patent literature 1.). The GaN-based
hetero-junction bipolar transistor is made as follows. An AlN
buffer layer, a GaN buffer layer, an emitter layer made of n-type
AlGaN, a base layer made of p-type GaN, a collector layer made of
n-type AlGaN and a subcollector layer made of n-type GaN are grown
in order. Then the collector layer and the subcollector layer are
patterned into a prescribed shape by etching and further the base
layer is patterned into a prescribed shape by etching. Finally, an
emitter electrode is formed on a part of the emitter layer where
the base layer is removed, a base electrode is formed on a part of
the base layer where the collector layer and the subcollector layer
are removed and a collector electrode is formed on the
subcollector.
PRIOR ART LITERATURE
Patent Literature
[0004] [PATENT LITERATURE 1] Laid-open patent gazette
2005-79417
SUMMARY OF INVENTION
Subjects to be Solved by the Invention
[0005] However, according to inventor's consideration, it is
difficult to perform high frequency power amplification or high
frequency power switching of the GaN-based hetero-junction bipolar
transistor proposed by patent literature 1.
[0006] Therefore, the subject to be solved by the invention is to
provide a high performance hetero-junction bipolar transistor which
can easily perform high frequency power amplification or high
frequency power switching.
[0007] Another subject to be solved by the invention is to provide
a high performance electric device using the hetero-junction
bipolar transistor.
Means to Solve the Subjects
[0008] To solve the above subject, according to the invention,
there is provided a hetero-junction bipolar transistor,
comprising:
[0009] a first n-type GaN layer;
[0010] an Al.sub.xGa.sub.1-xN layer (0.1.ltoreq.x.ltoreq.0.5) on
the first n-type GaN layer;
[0011] an undoped GaN layer having a thickness of not less than 20
nm on the Al.sub.xGa.sub.1-xN layer;
[0012] a Mg-doped p-type GaN layer having a thickness of not less
than 100 nm on the undoped GaN layer;
[0013] a second n-type GaN layer on the p-type GaN layer;
[0014] an emitter electrode electrically connected to the first
n-type GaN layer;
[0015] a base electrode electrically connected to the p-type GaN
layer; and
[0016] a collector electrode electrically connected to the second
n-type GaN layer,
[0017] an emitter being formed by the first n-type GaN layer and
the Al.sub.xGa.sub.1-xN layer, a base being formed by the undoped
GaN layer and the p-type GaN layer and a collector being formed by
the second n-type GaN layer,
[0018] two-dimensional hole gas being formed in a part of the
undoped GaN layer near the hetero interface between the
Al.sub.xGa.sub.1-xN layer and the undoped GaN layer during
non-operation,
[0019] p.times.b.times.10.sup.-7+P.sub.s.gtoreq.1.times.10.sup.13
[cm.sup.-2] being satisfied where b denotes the thickness of the
p-type GaN layer, p denotes the hole concentration of the p-type
GaN layer and P.sub.s denotes the concentration of the
two-dimensional hole gas.
[0020] Here, the non-operation can also be said as a thermal
equilibrium state in other words. The two-dimensional hole gas
formed in a part of the undoped GaN layer near the hetero interface
between the Al.sub.xGa.sub.1-xN layer and the undoped GaN layer is
formed by negative fixed charge induced in a part of the
Al.sub.xGa.sub.1-xN layer near the hetero interface between the
Al.sub.xGa.sub.1-xN layer and the undoped GaN layer by
piezopolarization and spontaneous polarization.
[0021] Not only holes supplied from Mg (magnesium), i.e., acceptors
doped with the p-type GaN layer but also the high concentration
two-dimensional hole gas formed in the undoped GaN layer contribute
to holes of the base. Therefore, a sufficient hole concentration of
the base can be obtained. As described in details later, according
to the present inventor's consideration, the hole concentration
(sheet concentration) of the base should be not less than Ix
10.sup.13 cm.sup.-2 in order to realize a hetero-junction bipolar
transistor capable of operating in high frequency. This condition
is p.times.b.times.10.sup.-7+P.sub.s.gtoreq.1.times.10.sup.13
[cm.sup.-2]. Here, 10.sup.-7 on the left side is a multiplier to
convert b in unit of nm into cm. This condition can be satisfied by
selection of the concentration of Mg doped with the p-type GaN
layer and the concentration P.sub.s of the two-dimensional hole gas
formed in the undoped GaN layer. That is, when the concentration of
Mg doped with the p-type GaN layer is denoted as N.sub.Mg
[cm.sup.-3] and the electrical activation ratio of Mg doped with
the p-type GaN layer is denoted as r, this condition can be
expressed as
N.sub.Mg.times.r.times.b.times.10.sup.-7+P.sub.s.gtoreq.1.times.10.sup.13
[cm.sup.-2]. Generally r is about 0.01(1%). If the concentration of
Mg doped with the p-type GaN layer is too high, crystal quality of
the p-type GaN layer deteriorates, which results in deterioration
of characteristics of the hetero-junction bipolar transistor.
Therefore, it is desirable to lower N.sub.Mg by increasing P.sub.s.
For example, when a half of 1.times.10.sup.13 cm.sup.-2, which is
the necessary hole concentration of the base, i.e.,
5.times.10.sup.12 [cm.sup.-2] is supplied by the two-dimensional
hole gas, in other words, when P.sub.s=5.times.10.sup.12
[cm.sup.-2], it is enough that
N.sub.Mg.times.r.times.b.times.10.sup.-7.gtoreq.5.times.10.sup.12
[cm.sup.-2]. Generally, 5.times.10.sup.19
[cm.sup.-3].ltoreq.N.sub.Mg.ltoreq.9.times.10.sup.19 [cm.sup.-3].
On the other hand, in order to prevent the base from depleting
completely and causing punch-through when a reverse bias voltage is
applied between base-collector of the hetero-junction bipolar
transistor during operation, it is necessary that the thickness of
the p-type GaN layer is not less than 100 nm, that is, b.gtoreq.100
nm. For example, in the case where b=100 nm, when r=0.01=10.sup.-2,
N.sub.Mg.gtoreq.5.times.10.sup.19 [cm.sup.-3] is enough to satisfy
N.sub.Mg.times.10.sup.-2.times.100.times.10.sup.-7.gtoreq.5.times.10.sup.-
12 [cm.sup.-2].
[0022] In order to obtain a high concentration two-dimensional hole
gas, the Al composition x and the thickness t [nm] of the
Al.sub.xGa.sub.1-xN layer are selected so as to satisfy the
following equation.
t.gtoreq..alpha.x.sup..beta.
Here, x in the inequality denotes numerals represented in %. For
example, when the Al composition x is 0.25, x in the inequality is
25. .alpha. and .beta. in the inequality are numerals determined
depending on necessary two-dimensional hole gas concentration
P.sub.s and are determined by calculation as described later. For
example, when P.sub.s.gtoreq.5.times.10.sup.12 [cm.sup.-2],
.alpha.=11290 and .beta.=-1.865. In this case, the above inequality
is written as t.gtoreq.11290x.sup.-1.865. The Al composition x is
typically constant for the whole Al.sub.xGa.sub.1-xN layer, but may
be changed, for example, in the thickness direction, if necessary.
The Al.sub.xGa.sub.1-xN layer is typically undoped, but may be the
n-type Al.sub.xGa.sub.1-xN layer doped with donors (n-type
impurities), for example, Si in for example,
1.times.10.sup.16.about.1.times.10.sup.18 cm.sup.-3, if necessary.
Depending on the situation, the Al.sub.xGa.sub.1-xN layer may be,
for example, the p-type Al.sub.xGa.sub.1-xN layer doped with Mg in
extremely low concentration, which is almost the i-type
Al.sub.xGa.sub.1-xN layer.
[0023] Typically, the first n-type GaN layer is formed on a first
n.sup.+-type GaN layer, the second n-type GaN layer is formed on
the p-type GaN layer like a mesa and a second n.sup.+-type GaN
layer is formed on the second n-type GaN layer. Typically, the
emitter electrode is formed on the surface of the first
n.sup.+-type GaN layer opposite to the first n-type GaN layer, the
base electrode is formed on a part of the p-type GaN layer on which
no second n-type GaN layer is formed and the collector electrode is
formed on the second n.sup.+-type GaN layer. The first n.sup.+-type
GaN layer is an ohmic contact layer and its donor concentration is
selected to be sufficiently high so that the emitter electrode can
make ohmic contact. Acceptor concentration of the p-type GaN layer
is selected sufficiently high so that the base electrode can make
ohmic contact. Donor concentration of the second n.sup.+-type GaN
layer is selected to be sufficiently high so that the collector
electrode can make ohmic contact.
[0024] If necessary, formed between the Al.sub.xGa.sub.1-xN layer
and the undoped GaN layer an Al.sub.yGa.sub.1-yN graded layer in
which the Al composition y decreases monotonically from x to 0 in
the direction from the Al.sub.xGa.sub.1-xN layer to the undoped GaN
layer. In this case, the Al.sub.yGa.sub.1-yN graded layer also
forms the emitter. The Al.sub.yGa.sub.1-yN graded layer is
preferably undoped, but may be the n-type Al.sub.xGa.sub.1-xN layer
doped with donors (n-type impurities), for example, Si in
1.times.10.sup.16 1.times.10.sup.18 cm.sup.-3, if necessary. With
the Al.sub.yGa.sub.1-yN graded layer formed between the
Al.sub.xGa.sub.1-xN layer and the undoped GaN layer, discontinuity
of the conduction band and the valence band in the hetero interface
between the Al.sub.xGa.sub.1-xN layer and the undoped GaN layer can
be canceled. Therefore, it is possible to increase the collector
current of the hetero-junction bipolar transistor and it is
possible to cancel offset of the emitter-collector voltage from
which the collector current begins to flow in output
characteristics. The Al composition y of the Al.sub.yGa.sub.1-yN
graded layer may monotonically decrease from x to 0 linearly or
like a curve. If necessary, the hetero-junction bipolar transistor
further comprises a p-type GaN layer formed between the second
n-type GaN layer and the second n.sup.+-type GaN layer. With the
p-type GaN layer formed between the second n-type GaN layer and the
second n.sup.+-type GaN layer, the inclination of the energy band
of the second n-type GaN layer during operation of the
hetero-junction bipolar transistor is decreased, and therefore the
electric field applied to the second n-type GaN layer can be
relaxed. Therefore, it is possible to suppress intervalley
scattering and optical phonon scattering in the second n-type GaN
layer and it is possible to make the hetero-junction bipolar
transistor fast.
[0025] In the hetero-junction bipolar transistor, if necessary,
interlayers having any function may be formed between each of the
layers as far as the characteristics of the hetero-junction bipolar
transistor are not spoiled.
[0026] According to the invention, there is further provided an
electric device, comprising:
[0027] at least a semiconductor element,
[0028] the semiconductor element being a hetero-junction bipolar
transistor, comprising:
[0029] a first n-type GaN layer;
[0030] an Al.sub.xGa.sub.1-xN layer (0.1.ltoreq.x.ltoreq.0.5) on
the first n-type GaN layer;
[0031] an undoped GaN layer having a thickness of not less than 20
nm on the Al.sub.xGa.sub.1-xN layer;
[0032] a Mg-doped p-type GaN layer having a thickness of not less
than 100 nm on the undoped GaN layer;
[0033] a second n-type GaN layer on the p-type GaN layer;
[0034] an emitter electrode electrically connected to the first
n-type GaN layer;
[0035] a base electrode electrically connected to the p-type GaN
layer; and
[0036] a collector electrode electrically connected to the second
n-type GaN layer,
[0037] an emitter being formed by the first n-type GaN layer and
the Al.sub.xGa.sub.1-xN layer, a base being formed by the undoped
GaN layer and the p-type GaN layer and a collector being formed by
the second n-type GaN layer,
[0038] two-dimensional hole gas being formed in a part of the
undoped GaN layer near the hetero interface between the
Al.sub.xGa.sub.1-xN layer and the undoped GaN layer during
non-operation,
[0039] p.times.b.times.10.sup.-7+P.sub.s.gtoreq.1.times.10.sup.13
[cm.sup.-2] being satisfied where b denotes the thickness of the
p-type GaN layer, p denotes the hole concentration of the p-type
GaN layer and P.sub.s denotes the concentration of the
two-dimensional hole gas.
[0040] Here, the electric device includes all devices using
electricity and their uses, functions, sizes, etc. are not limited
and they are, for example, electronic devices, mobile bodies, power
plant, construction machinery, machine tools, etc. The electronic
devices are, for example, communication equipments, robots,
computers, game devices, car devices, home electric products (air
conditioner etc.), industrial products, mobile phones, mobile
devices, IT devices (server etc.), power conditioners used in solar
power generation systems, power supplying systems, etc. The mobile
bodies are railroad cars, motor vehicles (electric car etc.), motor
cycles, aircrafts, rockets, spaceships, etc. Especially, the
electronic devices include high power amplifiers used in a base
station of the next generation mobile communication system, air
traffic control radars, millimeter wave transillumination
examination apparatuses, wireless power transmitters, particle
accelerators, etc.
[0041] In the invention of the electric device, the explanation
concerning the above invention of the hetero-junction bipolar
transistor comes into effect unless it is contrary to its
character.
Effect of the Invention
[0042] According to the invention, the hole concentration of the
base is not less than 1.times.10.sup.13 [cm.sup.-2], that is high
enough, and the thickness of the p-type GaN layer forming the base
is not less than 100 nm. Therefore, when a voltage of not less than
100V is applied between the base and the collector during
operation, it is possible to effectively prevent punch-through
occurring on the base side between the base and the collector and
to lower the base resistance sufficiently, thereby making it
possible to improve the current amplification ratio of the
hetero-junction bipolar transistor. Furthermore, since a part of
the hole concentration of the base can be supplied from the high
concentration two-dimensional hole gas, it is not necessary to
increase the concentration of Mg doped with the p-type GaN layer
too much and therefore the crystal quality of the p-type GaN layer
can be improved, thereby making it possible to improve the
characteristics of the hetero-junction bipolar transistor. Since
the undoped GaN layer having a thickness of not less than 20 nm
formed between the Al.sub.xGa.sub.1-xN layer and the p-type GaN
layer doped with Mg serves as the diffusion barrier layer of Mg, it
is possible to effectively prevent Mg of the p-type GaN layer from
diffusing into the Al.sub.xGa.sub.1-xN layer. With this, it is
possible to prevent the p-n junction interface between the emitter
and the base from locating in the Al.sub.xGa.sub.1-xN layer,
thereby making it possible to decrease the base reactive current.
Since it is possible to easily obtain a collector up structure in
which the collector lies on the upper side, it is possible to
decrease the collector capacitance substantially, thereby making it
possible to make the hetero-junction bipolar transistor fast.
Accordingly, it is possible to realize a high performance
hetero-junction bipolar transistor which can perform high frequency
power amplification or high frequency power switching easily. And a
high performance electric device can be realized by using the
hetero-junction bipolar transistor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] FIG. 1 A cross-sectional view showing the base structure of
the GaN-based HBT according to a first embodiment of the
invention.
[0044] FIG. 2 A cross-sectional view showing a state in which a
base pad electrode and a collector pad electrode are formed in the
GaN-based HBT according to the first embodiment of the
invention.
[0045] FIG. 3 A plan view showing an example of the planar shape of
the base pad electrode and the collector pad electrode shown in
FIG. 2.
[0046] FIG. 4 A cross-sectional view along the A-A' line of FIG.
3.
[0047] FIG. 5 A cross-sectional view along the B-B' line of FIG.
3.
[0048] FIG. 6 A schematic view showing the energy band of the
GaN-based HBT according to the first embodiment of the invention
during non-operation.
[0049] FIG. 7 A schematic view showing the energy band of the
GaN-based HBT according to the first embodiment of the invention
during operation.
[0050] FIG. 8 A schematic view showing a method for making the
GaN-based HBT according to the first embodiment of the
invention.
[0051] FIG. 9 A schematic view showing a method for making the
GaN-based HBT according to the first embodiment of the
invention.
[0052] FIG. 10 A schematic view showing a method for making the
GaN-based HBT according to the first embodiment of the
invention.
[0053] FIG. 11 A schematic view showing a method for making the
GaN-based HBT according to the first embodiment of the
invention.
[0054] FIG. 12 A cross-sectional view showing another method for
making the GaN-based HBT according to the first embodiment of the
invention.
[0055] FIG. 13 A cross-sectional view showing another method for
making the GaN-based HBT according to the first embodiment of the
invention.
[0056] FIG. 14 A cross-sectional view showing the layer structure
of a sample used in an experiment performed to study the GaN-based
HBT according to the first embodiment of the invention. [FIG. 15]A
plan view showing a Hall measurement sample made in the experiment
performed to study the GaN-based HBT according to the first
embodiment of the invention.
[0057] FIG. 16A A schematic view along the A-A' line of the Hall
measurement sample shown in FIG. 15.
[0058] FIG. 16B A schematic view along the B-B' line of the Hall
measurement sample shown in FIG. 15.
[0059] FIG. 16C A schematic view along the C-C' line of the Hall
measurement sample shown in FIG. 15.
[0060] FIG. 17 An energy band diagram of the polarization super
junction obtained by simulation performed based on the
one-dimensional model along the stacking direction of the undoped
GaN layer 52/the Al.sub.xGa.sub.1-xN layer 53/the undoped GaN layer
54 of the Hall measurement sample shown in FIG. 15.
[0061] FIG. 18 A schematic view showing profiles of the 2DHG
concentration and the 2DEG concentration of the polarization super
junction obtained by simulation.
[0062] FIG. 19 A schematic view showing the calculated values and
the measurement values for the remaining thickness of the undoped
GaN layer 54.
[0063] FIG. 20 A schematic view showing the calculation result of
the 2DHG concentration where the remaining thickness of the undoped
GaN layer 54 is 10 nm.
[0064] FIG. 21 A schematic view showing the calculation result of
the 2DHG concentration where the remaining thickness of the undoped
GaN layer 54 is 50 nm.
[0065] FIG. 22 A schematic view showing the calculation result of
the 2DHG concentration where the remaining thickness of the undoped
GaN layer 54 is 100 nm.
[0066] FIG. 23 A schematic view showing the calculation result of
the 2DHG concentration where the remaining thickness of the undoped
GaN layer 54 is 1000 nm.
[0067] FIG. 24 A schematic view showing the relation between the Al
composition x and the thickness t of the Al.sub.xGa.sub.1-xN layer
53 where the remaining thickness of the undoped GaN layer 54 is
changed.
[0068] FIG. 25 A schematic view showing the relation between the Al
composition x and the thickness t of the Al.sub.xGa.sub.1-xN layer
53 which can obtain the 2DHG concentration of 4.times.10.sup.12
[cm.sup.-2] and 5.times.10.sup.12 [cm.sup.-2].
[0069] FIG. 26 A schematic view showing the result of the SIMS
analysis in the depth direction of the sample shown in FIG. 14.
[0070] FIG. 27 A schematic view showing the result of consideration
of the maximum voltage which can be applied between the base and
the collector of the GaN-based HBT according to the first
embodiment of the invention.
[0071] FIG. 28 A cross-sectional view showing the base structure of
a GaN-based HBT according to a second embodiment of the
invention.
[0072] FIG. 29 A schematic view showing the energy band of the
GaN-based HBT according to the second embodiment of the invention
during non-operation.
[0073] FIG. 30 A schematic view showing the base-emitter
voltage-collector current characteristics of the GaN-based HBT
according to the second embodiment of the invention.
[0074] FIG. 31 A schematic view showing the output characteristics
of the GaN-based HBT according to the second embodiment of the
invention.
[0075] FIG. 32 A schematic view showing the base structure of a
GaN-based HBT according to a third embodiment of the invention.
[0076] FIG. 33 A schematic view showing the energy band of the
GaN-based HBT according to the third embodiment of the invention
during operation.
MODES FOR CARRYING OUT THE INVENTION
[0077] Modes for carrying out the invention (hereinafter referred
as "embodiments") will now be explained below.
1. The First Embodiment
[GaN-Based HBT]
[0078] The GaN-based HBT according to the first embodiment is
described. The base structure of the GaN-based HBT is shown in FIG.
1.
[0079] As shown in FIG. 1, in the GaN-based HBT, stacked in order
are an n.sup.+-type GaN layer 11, an n-type GaN layer 12, an
undoped Al.sub.xGa.sub.1-xN layer 13, an undoped GaN layer 14, a
p-type GaN layer 15, an n-type GaN layer 16 and an n.sup.+-type GaN
layer 17. The n.sup.+-type GaN layer 11 may be formed by a grown
layer or may be formed by a GaN substrate used for crystal growth.
The n.sup.+-type GaN layer 11, the n-type GaN layer 12, the undoped
Al.sub.xGa.sub.1-xN layer 13, the undoped GaN layer 14, the p-type
GaN layer 15, the n-type GaN layer 16 and the n.sup.+-type GaN
layer 17 have typically (0001) plane orientation (C-plane
orientation) and the stacking direction is [0001] direction, but
not limited to this. The n-type GaN layer 16 and the n.sup.+-type
GaN layer 17 are selectively formed on the p-type GaN layer 15 and
have a mesa shape. The n-type GaN layer 16 and the n.sup.+-type GaN
layer 17 have, for example, a stripe shape which elongates in one
direction. The elongation direction of the n-type GaN layer 16 and
the n.sup.+-type GaN layer 17 having the stripe shape is typically
[10-10] direction, but not limited to this. Donors (n-type
impurities) doped with the n.sup.+-type GaN layer 11, the n-type
GaN layer 12, the n-type GaN layer 16 and the n.sup.+-type GaN
layer 17 are, for example, Si. Mg is doped with the p-type GaN
layer 15 as acceptors (p-type impurities). The Al composition x of
the undoped Al.sub.xGa.sub.1-xN layer 13 is not less than 0.1 and
not larger than 0.5 (not less than 10% and not larger than 50%).
The undoped GaN layer 14 has a thickness of not less than 20 nm,
preferably not less than 30 nm and serves as the diffusion barrier
layer for preventing Mg doped with the p-type GaN layer 15 from
diffusing into the undoped Al.sub.xGa.sub.1-xN layer 13. The
thickness of the undoped GaN layer 14 is preferably not larger than
1000 nm and is generally selected to be not larger than 100 nm. The
thickness of the p-type GaN layer 15 is selected, taking the
voltage resistance characteristics demanded for the GaN-based HBT
into consideration and at least not less than 100 nm or not less
than 190 nm, and generally not larger than 800 nm. An emitter
electrode 18 is formed on the surface of the n.sup.+-type GaN layer
11 opposite to the n-type GaN layer 12, i.e., the back side of the
n.sup.+-type GaN layer 11 such that it comes in ohmic contact with
the n.sup.+-type GaN layer 11. The emitter electrode 18 may be
formed on a part of the back side of the n.sup.+-type GaN layer 11
or may be formed on the whole back side thereof. In FIG. 1, the
emitter electrode 18 is formed on a part of the back side of the
n.sup.+-type GaN layer 11. A base electrode 19 is formed on a part
of the p-type GaN layer 15 on which the n-type GaN layer 16 and the
n.sup.+-type GaN layer 17 are not formed such that it comes in
ohmic contact with the p-type GaN layer 15. A collector electrode
20 is formed on the n.sup.+-type GaN layer 17 such that it comes in
ohmic contact with the n.sup.+-type GaN layer 17. The collector
electrode 20 may be formed on a part of the surface of the
n.sup.+-type GaN layer 17 or may be formed on the whole surface of
the n.sup.+-type GaN layer 17. In FIG. 1, the collector electrode
20 is formed on a part of the surface of the n.sup.+-type GaN layer
17. The base electrode 19 and the collector electrode 20 typically
have a stripe shape which elongates in the same direction as the
n-type GaN layer 16 and the n.sup.+-type GaN layer 17 having a
stripe shape. In this case, the emitter is formed by the
n.sup.+-type GaN layer 11, the n-type GaN layer 12 and the undoped
Al.sub.xGa.sub.1-xN layer 13, the base is formed by the undoped GaN
layer 14 and the p-type GaN layer 15 and the collector is formed by
the n-type GaN layer 16 and the n.sup.+-type GaN layer 17. The npn
type GaN-based HBT is formed by the emitter, the base and the
collector. The n.sup.+-type GaN layer 11 may be called a subemitter
and the n.sup.+-type GaN layer 17 may be called a subcollector.
[0080] In the GaN-based HBT, during non-operation (thermal
equilibrium state), positive charges 21 are induced in a part of
the undoped Al.sub.xGa.sub.1-xN layer 13 near the hetero interface
between the undoped Al.sub.xGa.sub.1-xN layer 13 and the n-type GaN
layer 11 and negative charges 22 are induced in a part of the
Al.sub.xGa.sub.1-xN layer 13 near the hetero interface between the
undoped Al.sub.xGa.sub.1-xN layer 13 and the undoped GaN layer 14
by piezopolarization and spontaneous polarization. Therefore, in
the GaN-based HBT, during non-operation, two-dimensional electron
gas (2DEG) 23 is formed in a part of the n-type GaN layer 12 near
the hetero interface between the undoped Al.sub.xGa.sub.1-xN layer
13 and the n-type GaN layer 12 and two-dimensional hole gas (2DHG)
24 is formed in a part of the undoped GaN layer 14 near the hetero
interface between the undoped Al.sub.xGa.sub.1-xN layer 13 and the
undoped GaN layer 14.
[0081] In the GaN-based HBT, when the thickness of the p-type GaN
layer 15 is denoted as b [nm], the hole concentration of the p-type
GaN layer 15 is denoted as p [cm.sup.-3] and the concentration of
the two-dimensional hole gas 24 is denoted as P.sub.s [cm.sup.-2],
the condition of
p.times.b.times.10.sup.-7+P.sub.s.gtoreq.1.times.10.sup.13
[cm.sup.-2] is satisfied. When the concentration of Mg doped with
the p-type GaN layer 15 is denoted as N.sub.Mg [cm.sup.-3], the
electrical activation ratio of Mg doped with the p-type GaN layer
15 is denoted as r, the condition can be expressed as
N.sub.Mg.times.r.times.b.times.10.sup.-7+P.sub.s.gtoreq.1.times.10.sup.13
[cm.sup.-2]. For r=10.sup.-2, the condition is expressed as
N.sub.Mg.times.10.sup.-2.times.b.times.10.sup.-7+P.sub.s.gtoreq.1.times.1-
0.sup.13 [cm.sup.-2]. As described later, b is selected as
b.gtoreq.100 [nm] in order to prevent the base from completely
depleting and causing punch-through on the base side when the
reverse bias voltage is applied between the base and the emitter
during operation of the GaN-based HBT.
[0082] In the GaN-based HBT, the Al composition x and the thickness
t [nm] of the undoped Al.sub.xGa.sub.1-xN layer 13 are selected so
as to satisfy the following inequality, where numerals represented
in % are used as x. For example, when x=0.25, 25 is used as x in
the following inequality.
t.gtoreq..alpha.x.sup..beta.
Here, .alpha. and .beta. are determined depending on the
concentration of the 2DHG 24 to be obtained and obtained by
calculation as described later. For example, in order to obtain the
concentration P.sub.s of the 2DHG 24 of 5.times.10.sup.12
cm.sup.-2, .alpha.=11290 and P=-1.865.
[0083] Typically, a plurality of the n-type GaN layers 16 and the
n.sup.+-type GaN layers 17 having a stripe shape are formed
parallel to each other. The collector electrode 20 is formed on
each n.sup.+-type GaN layer 17 and the base electrode 19 is formed
on the p-type GaN layer 15 between a pair of the collector
electrode 20 adjacent to each other. The base electrode 19 and the
collector electrode 20 typically have a stripe shape and are formed
parallel to each other. As shown in FIG. 2, a base pad electrode 25
and a collector pad electrode 26 are formed so as to come in
contact with the base electrode 19 and the collector electrode 20,
respectively. An example of the planar shape of the base pad
electrode 25 and the collector pad electrode 26 is shown in FIG. 3.
FIG. 4 and FIG. 5 show cross-sectional views along the A-A' line
and the B-B' line of FIG. 3, respectively. As shown in FIG. 3, in
this example, the base pad electrode 25 and the collector pad
electrode 26 have a comb like shape and parts of the base pad
electrode 25 and the collector pad electrode 26 corresponding to
teeth of the comb are formed on the base electrode 19 and the
collector electrode 20 so as to come in contact with the base
electrode 19 and the collector electrode 20, respectively. As shown
in FIG. 4 and FIG. 5, a high resistance layer 27, which serves as
electrical insulator substantially, is formed below parts of the
base pad electrode 25 and the collector pad electrode 26 other than
the parts corresponding to the teeth of the comb. The high
resistance layer 27 is made of, for example, insulating film such
as SiO.sub.2 film and Si.sub.3N.sub.4 film and ion-implanted layer
with implanted boron (B) etc.
[0084] The thicknesses of the n.sup.+-type GaN layer 11, the n-type
GaN layer 12, the undoped Al.sub.xGa.sub.1-xN layer 13, the undoped
GaN layer 14, the p-type GaN-layer 15, the n-type GaN layer 16 and
the n.sup.+-type GaN layer 17 are suitably selected. For example,
the thicknesses are 1.about.10 .mu.m for the n.sup.+-type GaN layer
11, 0.2.about.1 .mu.m for the n-type GaN layer 12, 20.about.200 nm
for the undoped Al.sub.xGa.sub.1-xN layer 13, 20.about.100 nm, more
generally 20.about.50 nm for the undoped GaN layer 14 as described
before, 0.1.about.0.9 .mu.m for the p-type GaN layer 15,
0.5.about.5 .mu.m for the n-type GaN layer 16 and 0.05.about.0.5
.mu.m for the n.sup.+-type GaN layer 17. Donor concentrations of
the n.sup.+-type GaN layer 11, the n-type GaN layer 12, the n-type
GaN layer 16 and the n.sup.+-type GaN layer 17 are suitably
selected. For example, donor concentrations are 1.times.10.sup.18
1.times.10.sup.19 cm.sup.-3 for the n.sup.+-type GaN layer 11,
(0.5.about.2).times.10.sup.18 cm.sup.-3 for the n-type GaN layer
12, (0.5.about.5).times.10.sup.16 cm.sup.-3 for the n-type GaN
layer 16 and (1.about.10).times.10.sup.18 cm.sup.-3 for the
n.sup.+-type GaN layer 17. The Mg concentration N.sub.Mg (acceptor
concentration N.sub.A) [cm.sup.-3] of the p-type GaN layer 15 is
generally (1.about.9).times.10.sup.19 cm.sup.-3. The Al composition
x of the undoped Al.sub.xGa.sub.1-xN layer 13 is suitably selected
and, for example, 0.1.about.0.4. At least the lowest layer of the
emitter electrode 18 and the collector electrode 20 are made of
metal which can come in ohmic contact with a n-type GaN, for
example, Ti. For example, the emitter electrode 18 and the
collector electrode 20 are formed by a Ti/Al/Au stacked film. At
least the lowest layer of the base electrode 10 is made of metal
which can come in ohmic contact with a p-type GaN, for example, Ni.
For example, the base electrode 19 is formed by a Ni/Al stacked
film.
[0085] A specific example of the structure of the GaN-based HBT is
described. The n.sup.+-type GaN layer 11 is a thinned n-type GaN
substrate. The thickness and the donor concentration of the n-type
GaN layer 11 are 0.5 .mu.m and 1.times.10.sup.18 cm.sup.-3,
respectively. The thickness and the Al composition x of the undoped
Al.sub.xGa.sub.1-xN layer 13 are 45 nm and 0.25, respectively. The
thickness of the undoped GaN layer 14 is 20 nm. The thickness and
the acceptor concentration of the p-type GaN layer 15 are 0.5 .mu.m
and 5.times.10.sup.19 cm.sup.-3 (hole concentration
5.times.10.sup.17 cm.sup.-3), respectively. The thickness and the
donor concentration of the n-type GaN layer 16 are 1.0 m and
1.times.10.sup.16 cm.sup.-3, respectively. The thickness and the
donor concentration of the n.sup.+-type GaN layer 17 are 0.1 .mu.m
and 5.times.10.sup.18 cm.sup.-3, respectively. The emitter
electrode 18 and the collector electrode 20 are formed by a
Ti/Al/Au stacked film. The base electrode 19 is formed by a Ni/Al
stacked film.
[0086] FIG. 6 shows an energy band structure of the GaN-based HBT
during non-operation. In FIG. 6, E.sub.c denotes an energy of the
bottom of the conduction band, E.sub.v denotes an energy of the top
of the conduction band and F, denotes the Femi energy. As shown in
FIG. 6, a DEG23 is formed in a part of the n-type GaN layer 12 near
the hetero interface between the undoped Al.sub.xGa.sub.1-xN layer
13 and the n-type GaN layer 12 and a 2DHG 24 is formed in a part of
the undoped GaN layer 14 near the hetero interface between the
undoped Al.sub.xGa.sub.1-xN layer 13 and the undoped GaN layer
14.
[Operation of the GaN-Based HBT]
[0087] Voltage applied to the GaN-based HBT during operation is
basically as the same as general npn type bipolar transistors.
[0088] That is, the p-n junction between the emitter and the base
is forward biased and the p-n junction between the base and the
collector is reverse biased. FIG. 7 shows an energy band structure
of the GaN-based HBT when the voltage V.sub.be is applied between
the base and the emitter and the voltage V.sub.ce is applied
between the collector and the emitter during operation. With
application of the voltage V.sub.be the p-n junction between the
base and the emitter is forward biased and electrons are injected
into the undoped GaN layer 14 and the p-type GaN layer 15 which
forms the base from the n-type GaN layer 12 which forms the
emitter. The injected electrons flow in the base and reach the p-n
junction between the base and the collector which is reverse biased
and finally flows to the collector through the p-n junction. In the
p-type GaN layer 15 which forms the base, a part of electrons
injected from the n-type GaN layer 12 which forms the emitter
recombine with holes to disappear.
[Method for Making the GaN-Based HBT]
[0089] First, as shown in FIG. 8, the n-type GaN layer 12, the
undoped Al.sub.xGa.sub.1-xN layer 13, the undoped GaN layer 14, the
p-type GaN layer 15, the n-type GaN layer 16 and the n.sup.+-type
GaN layer 17 are grown in order on, for example, an n-type GaN
substrate 31 having (0001) plane (C-plane) orientation with the
conventionally known MOCVD (metal organic chemical vapor
deposition) method. The n-type GaN substrate 31 has the donor
concentration corresponding to the n.sup.+-type GaN layer 11. For
example, TMG (trimethyl gallium) is used as Ga source, TMA
(trimethyl aluminum) is used as Al source, NH.sub.3 (ammonia) is
used as nitrogen source and N.sub.2 gas and H.sub.2 gas are used as
a carrier gas. The growth temperature is, for example, about
1100.degree. C.
[0090] Then, although not illustrated, an insulating film such as a
SiO.sub.2 film and a SiN film is formed on the whole surface of the
n.sup.+-type GaN layer 17. Thereafter, a resist pattern having a
prescribed shape is formed on the insulating film and the
insulating film is etched and patterned into a prescribed shape by
using the resist pattern as a mask. Then, for example, boron (B) is
ion-implanted into the depth reaching the n-type GaN layer 12 in
the prescribed condition by using the insulating film as a mask.
The resistance of the B-implanted region becomes high and therefore
a high resistance layer 27 is formed in the prescribed part of the
n-type GaN layer 12, the undoped Al.sub.xGa.sub.1-xN layer 13, the
undoped GaN layer 14, the p-type GaN layer 15, the n-type GaN layer
16 and the n.sup.+-type GaN layer 17.
[0091] Then the collector electrode 20 having a stripe shape is
formed on the n.sup.+-type GaN layer 17. More specifically, for
example, a metal film for forming a collector electrode is formed
on the whole surface of the n.sup.+-type GaN layer 17 by vacuum
evaporation method etc., a resist pattern having a stripe shape is
formed on the metal film and the metal film is etched and patterned
by using the resist patter as a mask. In this way, the collector
electrode 20 having a stripe shape is formed. Thereafter, the
resist pattern is removed. Then, the n.sup.+-type GaN layer 17 and
the n-type GaN layer 16 are etched in order by using the collector
electrode 20 as a mask to expose the p-type GaN layer 15. This
etching is performed by, for example, a reactive ion etching (RIE)
using a chlorine-based etching gas. If necessary, an etching stop
layer or an etching monitoring layer is formed between the p-type
GaN layer 15 and the n-type GaN layer 16 in order to make it
possible to automatically stop etching and therefore to stop
etching of the p-type GaN layer 15 when the n.sup.+-type GaN layer
17 and the n-type GaN layer 16 are completely etched. As a result,
the n.sup.+-type GaN layer 17 and the n-type GaN layer 16 are
patterned into a stripe shape as the same as the collector
electrode 20. Plural patterns of the collector electrode 20, the
n.sup.+-type GaN layer 17 and the n-type GaN layer 16 having the
stripe shape are formed in the [10-10] direction parallel to each
other (see FIG. 9). Then, a SiO.sub.2 film is formed on the whole
surface by, for example, vacuum evaporation method. Thereafter, a
resist pattern having a prescribed shape, which has an opening in a
part corresponding to the base electrode 19, is formed on the
SiO.sub.2 film and the SiO.sub.2 film is etched by, for example, an
inductively coupled plasma (ICP) gas etching method by using the
resist pattern as a mask. As a result, only the SiO.sub.2 film 32
which covers the collector electrode 20 is left. Then, side walls
of the n-type GaN layer 16 and the n.sup.+-type GaN layer 17 having
a stripe shape are selectively etched by wet etching in order to
remove etching damage caused to the sidewalls of the n-type GaN
layer 16 and the n.sup.+-type GaN layer 17 and to secure the
flatness of the sidewalls. Here, a mixed solution of KOH and TMAH
(Tetra-methyl-ammonium acid) is used as an etching solution. Then,
a heat treatment is performed to improve the ohmic contact
characteristics of the collector electrode 20 for the n.sup.+-type
GaN layer 17 and to electrically activate Mg doped with the p-type
GaN layer 15. When the collector electrode 20 is formed by a
Ti/Al/Au stacked film, the heat treatment is performed, for
example, in a nitrogen gas (N.sub.2) in a condition of 750.degree.
C. and 5 minutes.
[0092] Then, as shown in FIG. 10, a metal film 33 for forming a
base electrode is formed by performing vacuum evaporation from the
direction perpendicular to the n-type GaN substrate 31, and
thereafter a heat treatment is performed to improve the ohmic
contact characteristics of the metal film 33 formed on the p-type
GaN layer 15 for the p-type GaN layer 15. When the base electrode
19 is formed by a Ni/Au stacked film, the heat treatment is
performed under conditions of, for example, N.sub.2 gas atmosphere,
500.degree. C. and 1 minute. Then, the SiO.sub.2 film 32 is removed
by wet etching and the metal film 33 on the SiO.sub.2 film 32 is
removed. With this, as shown in FIG. 11, the base electrode 19 made
of the metal film 33 is formed on only the p-type GaN layer 15.
[0093] Then, an insulating film 34 such as a SiO.sub.2 film and a
polyimide film is formed on the whole surface by vacuum evaporation
method etc., and thereafter a resist pattern having openings
corresponding to parts for forming a base pad electrode and a
collector pad electrode is formed on the insulating film 34 by
photolithography. Then, a metal film (not illustrated) such as an
Au film etc. serving as an underlayer metal for electroplating is
formed on the whole surface by performing vacuum evaporation from
the direction perpendicular to the n-type GaN substrate 31, and
thereafter the metal film is removed by removing the resist pattern
on which the metal film is formed. Then, a layer such as an Au film
is selectively formed on the metal film serving as the underlayer
metal by electroplating. The thickness of the electroplated layer
is, for example, about 1 .mu.m. With this, formed are the base pad
electrode 25 electrically connected to the base electrode 19 and
the collector pad electrode 26 electrically connected to the
collector electrode 20.
[0094] Then, wrapping, polishing, etc. are performed from the back
side of the n-type GaN substrate 31 and finally a wet etching is
performed to thin the n-type GaN substrate 31 to a prescribed
thickness. The thinned n-type GaN substrate 31 forms the
n.sup.+-type GaN layer 11. Thereafter, a metal film for forming an
emitter electrode is formed on the back side of the n.sup.+-type
GaN layer 11 by vacuum evaporation method etc. and the metal film
is patterned, if necessary. With this, the emitter electrode 18 is
formed. Here, since the back side of the n.sup.+-type GaN layer 11
is an N plane in which N atoms of the GaN crystal are arranged, low
resistance ohmic contact can be realized without performing a heat
treatment (alloying treatment) after the metal film for forming an
emitter electrode is formed.
[0095] As described above, the target GaN-based HBT is made.
[0096] Another method for making the GaN-based HBT is now
described.
[0097] In the method, a conventionally known PENDEO method is used
to grow GaN-based semiconductor layers. That is, as shown in FIG.
12, first, the major plane of a base substrate 41 such as an
insulating substrate such as a sapphire substrate etc. and a Si
substrate is patterned by etching to form convex parts 41a having a
stripe shape parallel to each other. The height and the width of
the convex parts 41a are appropriately selected. Then, an
insulating film 42 such as a SiO.sub.2 film, a SiN film, etc. is
formed on the major plane of the base substrate 41 except the
convex parts 41a. Here, the upper portion of the convex part 41a is
exposed.
[0098] Then, an n.sup.+-type GaN is grown by a MOCVD method. In
this case, the n.sup.+-type GaN first grows on the side of the
upper portion of the convex part 41a and the n.sup.+-type GaN grows
in the lateral direction (direction parallel to the major plane of
the base substrate 41) and the vertical direction, so that the
n.sup.+-type GaN layer 11 grows. In the n.sup.+-type GaN layer 11,
crystal defects such as dislocations etc. (shown by x in FIG. 12)
are generated in the interface (coalescent plane of the grown
layers) in which two grown layers laterally grown from the sides of
the two convex parts 41a are adjacent to each other. Then, grown in
order on the n.sup.+-type GaN layer 11 are the n-type GaN layer 12,
the undoped Al.sub.xGa.sub.1-xN layer 13, the undoped GaN layer 14,
the p-type GaN layer 15, the n-type GaN layer 16 and the
n.sup.+-type GaN layer 17. The crystal defects generated in the
coalescent planes of the n.sup.+-type GaN layer 11 propagate to the
n-type GaN layer 12, the undoped Al.sub.xGa.sub.1-xN layer 13, the
undoped GaN layer 14, the p-type GaN layer 15, the n-type GaN layer
16 and the n.sup.+-type GaN layer 17. Regions of the n.sup.+-type
GaN layer 11, the n-type GaN layer 12, the undoped
Al.sub.xGa.sub.1-xN layer 13, the undoped GaN layer 14, the p-type
GaN layer 15, the n-type GaN layer 16 and the n.sup.+-type GaN
layer 17 which are sandwiched between the two coalescent planes of
the grown layers adjacent to each other are high quality crystal
layers. Then the n-type GaN layer 16 and the n.sup.+-type GaN layer
17 are patterned into a prescribed shape. Then, an insulating film
such as a SiO.sub.2 film, a SiN film, etc. is formed on the whole
surface. Thereafter, a resist pattern having a prescribed shape is
formed on the insulating film and the insulating film is etched by
using the resist pattern as a mask and patterned into a prescribed
shape. Then, for example, ion implantation of boron (B) is
performed in a prescribed condition by using the insulating film as
a mask. With this, high resistance layers 27 made of B-implanted
layers are formed in a part of the n.sup.+-type GaN layer 11, the
n-type GaN layer 12, the undoped Al.sub.xGa.sub.1-xN layer 13, the
undoped GaN layer 14 and the p-type GaN layer 15 in which the
n-type GaN layer 16 and the n.sup.+-type GaN layer 17 are removed
and in a part of the n-type GaN layer 12, the undoped
Al.sub.xGa.sub.1-xN layer 13, the undoped GaN layer 14, the p-type
GaN layer 15, the n-type GaN layer 16 and the n.sup.+-type GaN
layer 17 in which the n-type GaN layer 16 and the n.sup.+-type GaN
layer 17 remain. Then, as the same as the above-mentioned method
for making a GaN-based HBT, the collector electrode 20 is formed on
the n.sup.+-type GaN layer 17, the base electrode 19 is formed on
the p-type GaN layer 15 and the insulating film 43 such as a
SiO.sub.2 film etc. is formed on a part between the collector
electrode 20 and the base electrode 19.
[0099] Then, as shown in FIG. 13, the base substrate 41 is peeled
and further the back side of the n.sup.+-type GaN layer 11 is
polished to obtain a flat surface and finally the emitter electrode
18 is formed on the flat back side.
[0100] Thereafter, although not illustrated, the base pad electrode
25 and the collector pad electrode 26 are formed as the same as the
above-mentioned method for making a GaN-based HBT.
[0101] As described above, the target GaN-based HBT is made.
[0102] Here, described is the ground of setting the hole
concentration of the base not less than 1.times.10.sup.13
[cm.sup.-2], that is,
p.times.b.times.10.sup.-7+P.sub.s.gtoreq.1.times.10.sup.13
[cm.sup.-2]
[0103] In the GaN-based HBT, it is desirable to increase the hole
concentration of the base as high as possible so as to keep the
base potential high. However, if the p-type GaN layer 15 forming
the base is heavily doped with Mg during its growth, Mg diffuses on
the side of the undoped Al.sub.xGa.sub.1-xN layer 13 forming the
emitter by crossing the hetero interface between the p-type GaN
layer 15 and the undoped Al.sub.xGa.sub.1-xN layer 13. As a result,
the p-n junction interface moves into the undoped
Al.sub.xGa.sub.1-xN layer 13 and therefore the gain
.beta.=I.sub.c/I.sub.b decreases due to increase of the reactive
base current. It is therefore indispensable for a high performance
GaN-based HBT to generate holes near the hetero-junction between
the base and the collector without depending on Mg. In the
GaN-based HBT, the subject is solved by preventing diffusion of Mg
into the undoped Al.sub.xGa.sub.1-xN layer 13 with provision of the
undoped GaN layer 14 served as the diffusion barrier layer of Mg
between the p-type GaN layer 15 and the undoped Al.sub.xGa.sub.1-xN
layer 13 and by forming the 2DHG 24 in the undoped GaN layer
14.
[0104] When the GaN-based HBT is in an off state (base-emitter
voltage V.sub.be=0 V), the base/collector p-n junction depletes by
application of the reverse bias voltage. In this case, the high
concentration hole is necessary for the base to keep the base
potential. If the base potential decreases, it becomes impossible
to control the emitter current due to the so-called punch-through
phenomenon.
[0105] Generally, with respect to the p-n junction, the relation
between the depletion layer width W and the applied voltage
V.sub.t, the acceptor concentration N.sub.A and the donor
concentration N.sub.D is expressed as:
W=[(2.epsilon./q){(1/N.sub.A)+(1/N.sub.D)}V.sub.t]
where .epsilon. is the permittivity of GaN, q is the magnitude of
electronic charge and the electrical activation ratios of both
acceptors and donors are supposed to be 100%. The expanse of the
depletion layer is estimated in relation to punch-through. Suppose
now that the acceptor concentration N.sub.A is 1.times.10.sup.18
cm.sup.-3 and the donor concentration N.sub.D is 5.times.10.sup.16
cm.sup.-3. When the applied voltage V.sub.t is 100V, W.about.1500
nm from the above equation. According to the above equation, the
depletion layer extends toward the side of the collector having the
low donor concentration, while the depletion layer extends toward
the side of the base in the ratio of about
5.times.10.sup.16/1.times.10.sup.18=1/20 and therefore the
depletion layer of the base is 1500/20.about.75 nm thick. From
this, in order to prevent punch-through from occurring to the side
of the base, the thickness of the p-type GaN-layer 15 is necessary
to be not less than about 100 nm. Therefore, the thickness of the
p-type GaN layer 15 of the high frequency GaN-based HBT having the
resistance voltage of 100V is roughly estimated to be not less than
about 100 nm when the hole concentration of the p-type GaN layer 15
is p.about.1.times.10.sup.18 cm.sup.-3. Here, the total amount of
holes in the base (sheet concentration) is
1.times.10.sup.18.times.100.times.10=1.times.10.sup.13
cm.sup.-2
[0106] In conclusion, the condition necessary for the high
frequency GaN-based HBT is that the hole concentration of the base
is not less than 1.times.10.sup.13 cm.sup.-2
[0107] In order to set the hole concentration of the p-type GaN
layer 15 to 1.times.10.sup.18 cm.sup.-3 by doping Mg, the Mg
concentration of 1.times.10.sup.20 cm.sup.-3 is necessary because
the electrical activation ratio of Mg is about 1%. However, such a
high concentration Mg doping leads to deterioration of crystal
quality of the p-type GaN layer 15, so that electron-hole
recombination results in the p-type GaN layer 15 and the current
gain lowers. Therefore, it is desirable to lower the Mg
concentration of the p-type GaN layer 15 less than
1.times.10.sup.20 cm.sup.-3. For this purpose, it is indispensable
to make the hole concentration high without relying upon a high
concentration Mg doping. In order to realize this, a
hetero-junction is formed by the undoped Al.sub.xGa.sub.1-xN layer
13 and the undoped GaN layer 14 to form the 2DHG 24 in a part of
the undoped GaN layer 14 near the hetero-interface between the
undoped Al.sub.xGa.sub.1-xN layer 13 and the undoped GaN layer 14,
and holes in the 2DHG 24 are used.
[0108] It is important to increase the hole concentration of the
2DHG 24. Described here the reason why the Al composition x and the
thickness t[nm] of the undoped Al.sub.xGa.sub.1-xN layer 13 are
determined as described above.
Experiment
[0109] For consideration, the relation between the Al composition x
and the thickness t [nm] of the undoped Al.sub.xGa.sub.1-xN layer
13 and the hole concentration of the 2DHG 24 was investigated by
experiment and simulation.
[0110] For this purpose, samples were prepared as described
below.
[0111] First, the layer structure shown in FIG. 14 was prepared. As
shown in FIG. 14, stacked on a (0001) plane, that is, C-plane
sapphire substrate 51 was a low temperature growth (530.degree. C.)
GaN buffer layer (not illustrated) having a thickness of 30 nm by
MOCVD method using TMG as Ga source, TMA as Al source, NH.sub.3 as
nitrogen source, N.sub.2 gas and H.sub.2 gas as a carrier gas. Then
the growth temperature was raised to 1100.degree. C. and an undoped
GaN layer 52 having a thickness of 800 nm, an Al.sub.xGa.sub.1-xN
layer 53 having a thickness of 40 nm and x=0.27, an undoped GaN
layer 54 having a thickness of 80 nm, a Mg-doped p-type GaN layer
55 having a Mg concentration of 5.0.times.10.sup.19 cm.sup.-3 and a
thickness of 50 nm and a Mg-doped p.sup.+-type GaN contact layer 56
having a Mg concentration of 2.0.times.10.sup.20 cm.sup.-3 and a
thickness of 3 nm were grown in order. A 2DEG 57 is formed in a
part of the undoped GaN layer 52 near the hetero interface between
the Al.sub.xGa.sub.1-xN layer 53 and the undoped GaN layer 52 and
the 2DHG 58 is formed in a part of the undoped GaN layer 54 near
the hetero interface between the Al.sub.xGa.sub.1-xN layer 53 and
the undoped GaN layer 54. With respect to the GaN-based HBT shown
in FIG. 1, the undoped GaN layer 52 corresponds to the n-type GaN
layer 12, the Al.sub.xGa.sub.1-xN layer 53 corresponds to the
undoped Al.sub.xGa.sub.1-xN layer 13, the undoped GaN layer 54
corresponds to the undoped GaN layer 14, the 2DEG 57 corresponds to
the 2DEG 23 and the 2DHG 58 corresponds to the 2DHG 24.
[0112] In order to measure the concentration (hereinafter, a
concentration with a unit of cm.sup.-2 denotes a sheet
concentration and a concentration with a unit of cm.sup.-3 denotes
a volume concentration) of the 2DHG by using the layer structure
shown in FIG. 14, a Hall element shown in FIG. 15, FIG. 16A, FIG.
16B and FIG. 16C was made. Here, FIG. 15 is a plan view of the Hall
element and FIG. 16A, FIG. 16B and FIG. 16C are cross-sectional
views along the A-A' line, the B-B' line and the C-C' line of FIG.
15, respectively. Etching was performed to the halfway depth of the
undoped GaN layer 54 and concentrations of the 2DEG 57 and the 2DHG
58 were measured. Four p-electrodes 59 formed on the p.sup.+-type
GaN contact layer 56 on four corners of undoped GaN layer 54 were
used to measure the concentration of the 2DHG 58. Four electrodes
60 formed on four corners of the Al.sub.xGa.sub.1-xN layer 53 were
used to measure the concentration of the 2DEG 57.
[0113] The result of measurement is shown in Table 1. The remaining
thickness of the undoped GaN layer 54 of the sample No. 1 was 60
nm. The remaining thickness of the undoped GaN layer 54 of the
sample No. 2 was 40 nm. The remaining thickness of the undoped GaN
layer 54 of the sample No. 3 was 5 nm. It is understood from Table
1 that in the sample No. 1 and the sample No. 2, the 2DEG 57 and
the 2DHG 58 are induced and accumulated by the polarization super
junction (PSJ) effect. With respect to the sample No. 3, the Hall
voltage for holes was not generated and measurement was
impossible.
TABLE-US-00001 TABLE 1 No. 1 No. 2 No. 3 REMAINING REMAINING
REMAINING THICKNESS THICKNESS THICKNESS OF UNDOPED OF UNDOPED OF
UNDOPED GaN LAYER = GaN LAYER = GaN LAYER = SAMPLE 60 nm 40 nm 5 nm
2DHG CON- 7.08 .times. 10.sup.12 6.60 .times. 10.sup.12 UNMEA-
CENTRATION SURABLE [cm.sup.-2] HOLE 6.05 5.2 UNMEA- MOBILITY
SURABLE [cm.sup.2/Vs] 2DEG CON- 6.78 .times. 10.sup.12 6.43 .times.
10.sup.12 8.15 .times. 10.sup.12 CENTRATION [cm.sup.-2] ELECTRON
863.5 871.0 880.6 MOBILITY [cm.sup.2/Vs]
[0114] Because the 2DHG concentration of the sample No. 2 is less
than the 2DHG concentration of the sample No. 1, it was shown that
the 2DHG concentration depends on the thickness of the undoped GaN
layer 54. This results from the surface pinning effect and the
existence of donor type levels (electron emission type) or hole
trapping levels of the undoped GaN layer 54. The relation between
the amount of 2DHG 58 and the constitution of the
Al.sub.xGa.sub.1-xN layer 53 and the undoped GaN layer 53 was
examined quantitatively.
[Comparison Between Model Calculation and the 2DHG Concentration
Measured]
[0115] In order to derive the relation between the layer structure
of the polarization super junction consisting of the
Al.sub.xGa.sub.1-xN layer 53/the undoped GaN layer 54 and the 2DHG
concentration, the band calculations were carried out. That is, the
calculations were carried out for a one-dimensional model along the
stacking direction in FIG. 16A, FIG. 16B and FIG. 16C. Atlas of
Silvaco, Inc. was used as a simulator software. FIG. 17 shows the
band diagram calculated of the undoped GaN layer 54 (thickness 60
nm)/the Al.sub.xGa.sub.1-xN layer 53 (x=0.24, thickness 40 nm)/the
undoped GaN layer 52 in an equilibrium state. FIG. 18 shows the
concentration profile of the 2DHG and the 2DEG. Band bending occurs
by the positive fixed charge (polarization charge) induced in a
part of the Al.sub.xGa.sub.1-xN layer 53 near the hetero interface
between the undoped GaN layer 52 and the Al.sub.xGa.sub.1-xN layer
53 and the negative fixed charge (polarization charge) induced in a
part of the Al.sub.xGa.sub.1-xN layer 53 near the hetero interface
between the Al.sub.xGa.sub.1-xN layer53 and the undoped GaN layer
54. As a result, the 2DHG is induced in a part of the undoped GaN
layer 54 near the hetero interface between the Al.sub.xGa.sub.1-xN
layer 53 and the undoped GaN layer 54 and the 2DEG is induced in a
part of the undoped GaN layer 52 near the hetero interface between
the Al.sub.xGa.sub.1-xN layer 53 and the undoped GaN layer 52. The
peak concentration of the 2DHG is 1.times.10.sup.20 cm.sup.-3 and
the peak concentration of the 2DEG is 6.times.10.sup.19 cm.sup.-3
and both concentrations decrease exponentially with the distance
from the hetero interface. The 2DEG concentration shows a constant
value, 1.times.10.sup.15 cm.sup.-3 in the deep position of the
undoped GaN layer 52 because the undope level of the undoped GaN
layer 52 was set to 1.times.10.sup.15 cm.sup.-3 for the sake of
calculation. This will not affect discussions below.
[0116] The integral value of the carrier concentration in the depth
direction shows the sheet carrier concentration. FIG. 19 shows the
2DHG concentration as the sheet carrier concentration. In FIG. 19,
the thickness of the undoped GaN layer 54 is shown in the abscissa
and the 2DHG concentration is shown in the ordinate. The 2DHG
concentrations of the sample No. 1 and the sample No. 2 are plotted
in FIG. 19.
[0117] It is understood from FIG. 19 that the result of simulation
(calculated values by the band calculation) reproduces the measured
values well and parameters of physical properties of matter of the
model used in the simulation (its details are not shown) satisfy a
necessary condition for the purpose of exploring the practical
polarization super junction structure.
[0118] As shown in FIG. 19, according to the simulation, the 2DHG
concentration is calculated to be about 1.times.10.sup.12 cm.sup.-2
for the thickness of 7 nm of the undoped GaN layer 54. In this
region, the 2DHG concentration drastically decreases as the
thickness of the undoped GaN layer 54 decreases and is
0.6.times.10.sup.12 cm.sup.-2 for the thickness of 5 nm.
Measurement of the corresponding sample was impossible.
[0119] Its reason is as follows. Suppose that the hole mobility is
about 3 cm.sup.-2/Vs. When the 2DHG concentration of the sample is
0.6.times.10.sup.12 cm.sup.-2 as described above, the sheet
resistance is
1/ne.mu.=1/(0.6.times.10.sup.12.times.1.6.times.10.sup.19.times.3).about.-
3.5M.OMEGA./.quadrature.. It is difficult to carry out Hall
measurement for the sample with such a sheet resistance. Here, n is
the sheet concentration, e is the magnitude of electronic charge
and p is the hole mobility. It is possible that the etching damage
caused during etching for forming a mesa reaches to the hetero
interface between the undoped GaN layer 54 and the
Al.sub.xGa.sub.1-xN layer 53 to decrease further the 2DEG
concentration. This may be another reason for a problem of
impossible measurement. This shows that there is a limit of the
remaining thickness of the undoped GaN layer 54 in an actual device
fabrication and the thickness of 5 nm is too small. In addition,
even if there is no effect of the surface damage, there is still a
limit of the remaining thickness of the undoped GaN layer 54,
taking accuracy of etching during device fabrication into
consideration, and it is considered that the thickness of not less
than 10 nm is actually needed.
[0120] When the 2DHG concentration is 1.times.10.sup.11 cm.sup.-2,
there is no problem in principle. However, in order to obtain the
hole concentration of the base not less than 1.times.10.sup.13
[cm.sup.-2] without increasing the Mg concentration of the p-type
GaN layer 15 too much, the 2DHG concentration is preferably not
less than 1.times.10.sup.12 cm.sup.-2 at least and more preferably
larger. The thickness of the undoped GaN layer 54 is desired to be
large because the 2DHG concentration becomes larger as the
thickness becomes larger. However, when the thickness of the
undoped GaN layer 54 is too large, it becomes impossible to make
the device. Therefore, the thickness of the undoped GaN layer 54 is
desired to be not larger than 1000 nm. This is the reason why the
thickness of the undoped GaN layer 14 is desired to be not larger
than 1000 nm.
[Calculation to Investigate the Relation Between the Al Composition
x and the Thickness t of the Undoped Al.sub.xGa.sub.1-xN Layer 53
and the 2DHG Concentration in the Polarization Super Junction
Structure Consisting of the Undoped GaN Layer 54/the Undoped
Al.sub.xGa.sub.1-xN Layer 53]
[0121] The thickness a of the undoped GaN layer 54 was used as
parameters and set to a=10 nm, 50 nm, 100 nm and 1000 nm,
respectively. The 2DHG concentration was calculated while the Al
composition x and the thickness t of the Al.sub.xGa.sub.1-xN layer
53 were varied. Here, x was varied by 0.05 in a range of
0.05.about.0.5 (5.about.50%) and t was varied by 1 nm in a range of
5.about.10 nm and by 5 nm in a range of 10.about.100 nm. And
calculation was carried out by combining each value of x and each
value of t like a matrix. The result of calculation of the Al
composition x and the thickness t of the Al.sub.xGa.sub.1-xN layer
53 described below can be applied to the Al composition x and the
thickness t of the undoped Al.sub.xGa.sub.1-xN layer 13 of the
GaN-based HBT as it is.
[0122] FIG. 20 shows a table of calculated values of the 2DHG
concentration for the Al composition x and the thickness t (nm) of
the undoped Al.sub.xGa.sub.1-xN layer 53 when the thickness a of
the undoped GaN layer 54 is 10 nm. Needless to say, in FIG. 20, for
example, "1.03E+12" means 1.03.times.10.sup.12 (similar in FIG. 21
to FIG. 23). FIG. 21 shows a similar table of calculated values of
the 2DHG concentration when the thickness a of the undoped GaN
layer 54 is 50 nm. FIG. 22 shows a similar table of calculated
values of the 2DHG concentration when the thickness a of the
undoped GaN layer 54 is 100 nm. FIG. 23 shows a similar table of
calculated values of the 2DHG concentration when the thickness a of
the undoped GaN layer 54 is 1000 nm.
[0123] Inspecting the state of distribution of the 2DHG
concentration shown in FIG. 20 to FIG. 23, it is understood that as
x becomes large and t becomes large, the 2DHG concentration
increases. Values of x and t giving the concentration of
1.00.times.10.sup.12 cm.sup.-2 are extracted. Here, in FIG. 20 to
FIG. 23, cells near the 2DHG concentration of 1.00.times.10.sup.12
cm.sup.-2 are surrounded by a thick line. Since the values of cells
in tables are not exactly 1.00.times.10.sup.12 cm, values of x and
t were obtained from the values of the cells in front of and behind
the cell in proportional distribution.
[0124] FIG. 24 shows the (x, t) coordinate plane in which points of
(x, t) showing values of the 2DHG concentration=1.times.10.sup.12
cm.sup.-2, which were picked up from FIG. 20 to FIG. 23, were
plotted. In FIG. 24, the region on the right side (or the upper
side) of respective points is the area where the 2DHG concentration
2 1.times.10.sup.12 cm.sup.-2. From this, it can be understood that
when the thickness a of the undoped GaN layer 54 is small, the Al
composition x and the thickness t of the Al.sub.xGa.sub.1-xN layer
53 to obtain the 2DHG concentration of not less than
1.times.10.sup.12 cm.sup.-2 are large. It was made clear that as
the thickness a of the undoped GaN layer 54 increases to 100 nm or
more, the change of the 2DHG concentration saturates. This is
understood that the band shape near the hetero interface between
the undoped GaN layer 54 and the Al.sub.xGa.sub.1-xN layer 53 does
not change even if the thickness a of the undoped GaN layer 54
increases.
[0125] Obtained now is an approximate equation expressing values of
coordinate (x, t) in respective series of the thickness a of the
undoped GaN layer 54 shown in FIG. 24. The approximate equation
expresses the approximate curve giving the 2DHG concentration of
1.times.10.sup.12 cm.sup.-2. The approximate equation is expressed
as follows.
t=.alpha.(a)x.sup..beta.(a) (1)
Here, .alpha. and .beta. are functions of the thickness a of the
undoped GaN layer 54.
[0126] The curve shown by the dotted line in FIG. 24 fits to the
approximate equation and values of the parameters .alpha. and
.beta. of the equation (1) are as shown in Table 2.
TABLE-US-00002 TABLE 2 a [nm] .alpha. .beta. 10 30689 -2.1 50 1555
-1.396 100 1011 -1.295 1000 641 -1.196
[0127] Therefore, for an arbitrary thickness a of the undoped GaN
layer 54 ranging from 10 nm to 1000 nm, the thickness t of the
Al.sub.xGa.sub.1-xN layer 53 giving the 2DHG
concentration=1.times.10.sup.12 cm.sup.-2 for the Al composition x
of the Al.sub.xGa.sub.1-xN layer 53 is given by the equation (1).
On the other hand, in the GaN-based HBT, although the p-type GaN
layer 15 is stacked on the undoped GaN layer 14, this structure may
be considered to be equivalent to the undoped GaN layer 14 having
the very large thickness with respect to the 2DHG concentration.
Therefore, with respect to the Al composition x and the thickness t
of the Al.sub.xGa.sub.1-xN layer 53 giving the 2DHG concentration
of not less than 1.times.10.sup.12 cm.sup.-2, values of .alpha. and
.beta. when .alpha.=1000 nm are adopted from .alpha. and .beta. in
Table 2, so that
t [nm].gtoreq.641x.sup.-1.196(x.gtoreq.10%)
is satisfied.
[0128] Obtained now is the relation of the Al composition x and the
thickness t of the Al.sub.xGa.sub.1-xN layer 53 giving the 2DHG
concentration P.sub.s=4.times.10.sup.12 cm.sup.-2 or
5.times.10.sup.12 cm.sup.-2.
[0129] From data of FIG. 20 to FIG. 23, the Al composition x and
the thickness t giving P.sub.s=4.times.10.sup.12 cm.sup.-2 are as
shown in Table 3.
TABLE-US-00003 TABLE 3 x(%) t(nm) 15 57.80 20 31.75 25 21.73 30
16.42 35 13.11 40 10.63 45 9.05 50 7.83
[0130] Data of Table 3 are plotted in FIG. 25. By fitting of the
plotted points with
t=.alpha.x.sup..beta.
.alpha.=4390 and .beta.=-1.631 were obtained. Therefore, the Al
composition x and the thickness t of the Al.sub.xGa.sub.1-xN layer
53 giving the 2DHG concentration P.sub.s of not less than
4.times.10.sup.12 cm.sup.-2 are expressed as
t [nm].gtoreq.4390x.sup.-1.631
[0131] From data of FIG. 20 to FIG. 23, the Al composition x and
the thickness t giving the 2DHG concentration
P.sub.s=5.times.10.sup.12 cm.sup.-2 are as shown in Table 4.
TABLE-US-00004 TABLE 4 x(%) t(nm) 15 85.0 20 39.2 25 24.0 30 18.4
35 14.3 40 11.8 45 9.77 50 8.34
[0132] Data of Table 4 are plotted in FIG. 25. By fitting of the
plotted points with the equation (2), .alpha.=11290 and
.beta.=-1.865 were obtained. Therefore, the Al composition x and
the thickness t of the Al.sub.xGa.sub.1-xN layer 53 giving the 2DHG
concentration P.sub.s of not less than 5.times.10.sup.12 cm.sup.-2
are expressed as
t [nm].gtoreq.11290x.sup.-1.865.
[0133] Described now is the reason why the thickness of the undoped
GaN layer 14 is set to be not less than 20 nm. For the sample shown
in FIG. 14, the depth distribution of Mg was measured by secondary
ion mass spectroscopy (SIMS). The result is shown in FIG. 26. As
shown in FIG. 26, it was confirmed that the Mg concentration of the
undoped GaN layer 54 at a position below the p-type GaN layer 55
having the Mg concentration of 5.times.10.sup.19 cm.sup.-2, in
other words, at a depth of 20 nm from the interface between the
p-type GaN layer 55 and the undoped GaN layer 54 decreases to about
1.0.times.10.sup.16 cm.sup.-3, which is near the detection limit of
SIMS. From this, it was understood that the undoped GaN layer
having a thickness of not less than 20 nm serves as the diffusion
barrier layer of Mg. This is the reason why the thickness of the
undoped GaN layer 14 is set be not less than 20 nm.
[0134] Considered now is the maximum applied voltage between the
base and the collector of the GaN-based HBT. FIG. 27 shows a
distribution of the electric field of the p-n junction between the
p-type GaN layer 15 and the n-type GaN layer 16 of the GaN-based
HBT. In FIG. 27, the vertical axis corresponds to the electric
field E and the horizontal axis corresponds to the distance in the
thickness direction of the p-type GaN layer 15 and the n-type GaN
layer 16. Suppose that the hole concentration of the p-type GaN
layer 15 is p=1.times.10.sup.18 cm.sup.-3 and the electron
concentration of the n-type GaN layer 16 is n=5.times.10.sup.16
cm.sup.-3. For example, in the case where the thickness of the
n-type GaN layer 16 constituting the collector, when a
base-collector voltage V.sub.ce=100V is applied in a state of
base-emitter voltage V.sub.be=0V (off state), the n-type GaN layer
16 completely depletes. In this case, the maximum electric field
E.sub.max applied to the base-collector interface is
E.sub.max=2.times.100/(1.5.times.10.sup.4)=1.3 MV/cm, which is much
less than theoretical breakdown electric field (3.3 MV/cm).
[0135] It was investigated whether the GaN-based HBT can be used as
a power switching device of V.sub.t=400V. When 400-100=300V is
added to V.sub.ce, E.sub.max becomes as E.sub.max=1.3+2=3.3 MV/cm.
However, the distance of the region where the electric field
becomes 3.3 MV/cm is very short, so it is considered that the
GaN-based HBT can be used as the power switching device. On the
other hand, since the depletion layer on the side of the base,
i.e., the p-type GaN layer 15 extends to (3.3/1.3).times.75=190 nm,
the thickness b of the p-type GaN layer 15 needs to be not less
than this. In the GaN-based HBT in which the thickness of the
n-type GaN layer 16 is 0.5 .mu.m, the maximum applied voltage is
56V when E.sub.max is suppressed as E.sub.max=1.3 MV/cm, whereas
the maximum applied voltage is 100+56=156V when the electric field
of E.sub.max=3.3 MV/cm is allowed. As described above, the
thickness b of the p-type GaN layer 15 needs to be not less than
190 nm.
[0136] As described above, according to the first embodiment, the
hole concentration of the base is set to be not less than
1.times.10.sup.13 cm.sup.-2 by selection of the concentration of Mg
doped with the p-type GaN layer 15 and selection of the
concentration of the 2DHG 24 formed in the undoped GaN layer 14 in
the polarization super junction between the undoped
Al.sub.xGa.sub.1-xN layer 13 and the undoped GaN layer 14 and
further the thickness of the p-type GaN layer 15 is set to be not
less than 100 nm. Therefore, when a reverse bias voltage of about
100V is applied to the p-n junction between the base and the
collector in an off state of the GaN-based HBT, it is possible to
effectively prevent punch-through from occurring. Furthermore,
since the hole concentration of the base is not less than
1.times.10.sup.13 cm.sup.-2 and sufficiently high, it is possible
to reduce the base resistance sufficiently and thereby improve the
current gain of the GaN-based HBT. In addition, since the undoped
GaN layer 14 having a thickness of not less than 20 nm is formed
between the undoped Al.sub.xGa.sub.1-xN layer 13 and the p-type GaN
layer 15, it is possible to prevent Mg doped with the p-type GaN
layer 15 from diffusing into the undoped Al.sub.xGa.sub.1-xN layer
13. With this, it is possible to prevent the p-n junction interface
from moving into the undoped Al.sub.xGa.sub.1-xN layer 13 and
therefore prevent reaction base current from flowing. Since the
GaN-based HBT has a collector-up structure, it is possible to
reduce collector capacitance drastically and therefore make
operation speed of the GaN-based HBT high. Accordingly, it is
possible to realize a high performance GaN-based HBT which can
easily accomplish high frequency power amplification and high
frequency power switching and can obtain high voltage resistance
and high output. For example, in the case where the thickness of
the collector is 1 .mu.m, it is possible to realize an extremely
high performance GaN-based HBT having a voltage resistance of not
less than 200V, a transition frequency f.sub.t of not less than 100
GHz and a current gain .beta.=I.sub.c/I.sub.b of not less than 100,
which can accomplish high frequency power amplification. In the
case where the thickness of the collector is 10 .mu.m, it is
possible to realize an extremely high performance GaN-based HBT
having a voltage resistance of not less than 2000V, rise time and
fall time of the collector current of not larger than several ns
and a current gain .beta.=I.sub.c/I.sub.b of not less than 100,
which can accomplish high frequency power switching. And a high
performance electric device can be realized by using the high
performance GaN-based HBT.
2. The Second Embodiment
[GaN-Based HBT]
[0137] The GaN-based HBT according to the second embodiment is
described. The base structure of the GaN-based HBT is shown in FIG.
28.
[0138] As shown in FIG. 28, the GaN-based HBT has the same
structure as the GaN-based HBT according to the first embodiment
except that an Al.sub.yGa.sub.1-yN graded layer 71 of which Al
composition y monotonically increases from x to 0, for example,
linearly or like a curve is formed between the undoped
Al.sub.xGa.sub.1-xN layer 13 and the undoped GaN layer 14. By
forming the Al.sub.yGa.sub.1-yN graded layer 7 between the undoped
Al.sub.xGa.sub.1-xN layer 13 and the undoped GaN layer 14, an
energy band during non-operation is as shown in FIG. 29. As shown
in FIG. 29, at the hetero interface between the Al.sub.yGa.sub.1-yN
graded layer 71 and the undoped GaN layer 14, y of the
Al.sub.yGa.sub.1-yN graded layer 71 is 0 and therefore the
composition of the Al.sub.yGa.sub.1-yN graded layer 71 is the same
as the undoped GaN layer 14, while at the hetero interface between
the Al.sub.yGa.sub.1-yN graded layer 71 and the undoped
Al.sub.xGa.sub.1-xN layer 13, y of the Al.sub.yGa.sub.1-yN graded
layer 71 is x and therefore the composition of the
Al.sub.yGa.sub.1-yN graded layer 71 is the same as the undoped
Al.sub.xGa.sub.1-xN layer 13. Therefore, the conduction band and
the valence band are continuous at the hetero interface between the
Al.sub.yGa.sub.1-yN graded layer 71 and the undoped GaN layer 14
and the hetero interface between the Al.sub.yGa.sub.1-yN graded
layer 71 and the undoped Al.sub.xGa.sub.1-xN layer 13.
[Operation of the GaN-Based HBT]
[0139] Operation method of the GaN-based HBT according to the
second embodiment is basically the same as the GaN-based HBT
according to the first embodiment.
[Method for Making the GaN-Based HBT]
[0140] The method for making the GaN-based HBT according to the
second embodiment is the same as the method for making the
GaN-based HBT according to the first embodiment except that the
Al.sub.yGa.sub.1-yN graded layer 71 is formed between the undoped
Al.sub.xGa.sub.1-xN layer 13 and the undoped GaN layer 14.
[0141] According to the second embodiment, following advantages can
be obtained in addition to the same advantages as the first
embodiment. That is, discontinuity of the conduction band
(.DELTA.E.sub.c) at the hetero interface between the undoped
Al.sub.xGa.sub.1-xN layer 13 and the undoped GaN layer 14 is
eliminated, so that as shown in FIG. 30, it is possible to increase
the collector current compared with the case where .DELTA.E.sub.c
exists in the conduction band and the valence band at the hetero
interface. In FIG. 30, I.sub.c denotes the collector current. As
shown in FIG. 31, in the case where .DELTA.E.sub.c exists, an
offset of collector-emitter voltage appears in output
characteristics, that is, collector-emitter voltage
(V.sub.ce)-collector current (I.sub.c) curve, whereas according to
the second embodiment free from .DELTA.E.sub.c it is possible to
eliminate an offset of the collector-emitter voltage.
3. The Third Embodiment
[GaN-Based HBT]
[0142] The GaN-based HBT according to the third embodiment is
described. The base structure of the GaN-based HBT is shown in FIG.
32.
[0143] As shown in FIG. 32, the GaN-based HBT has the same
structure as the first embodiment except that a p-type GaN layer 81
is formed between the n-type GaN layer 16 and the n.sup.+-type GaN
layer 17. The thickness and the acceptor concentration of the
p-type GaN layer 81 are appropriately selected. For example, the
thickness is 5.about.20 nm and the acceptor concentration is
(1.about.8).times.10.sup.19 cm.sup.-3. The energy band of the
GaN-based HBT during operation is as shown in FIG. 33. As shown in
FIG. 33, by forming the p-type GaN layer 81 between the n-type GaN
layer 16 and the n.sup.+-type GaN layer 17, a slope of the energy
band of the n.sup.+-type GaN layer 17 is drastically decreased
compared with the case where the p-type GaN layer 81 is not formed.
By decreasing the thickness of the p-type GaN layer 81 to
5.about.20 nm, electrons which pass through the p-type GaN layer 13
and reach the n-type GaN layer 16 pass through the p-type GaN layer
81 ballistically and reach the n.sup.+-type GaN layer 17, so that
it is possible to make the operation speed of the GaN-based HBT
high.
[Operation of the GaN-Based HBT]
[0144] The operation method of the GaN-based HBT according to the
third embodiment is basically the same as the GaN-based HBT
according to the first embodiment.
[Method for Making the GaN-Based HBT]
[0145] The method for making the GaN-based HBT according to the
third embodiment is the same as the method for making the GaN-based
HBT according to the first embodiment except that the p-type GaN
layer 81 is formed between the n-type GaN layer 16 and the
n.sup.+-type GaN layer 17.
[0146] According to the third embodiment, following advantages can
be obtained in addition to the same advantages as the first
embodiment. That is, as shown in FIG. 33, a slope of the energy
band of the n.sup.+-type GaN layer 17 is drastically decreased, so
that the electric field applied to the n.sup.+-type GaN layer 17 is
drastically relaxed. Therefore, it is possible to suppress
intervalley scattering and optical phonon scatting in the
n.sup.+-type GaN layer 17 and make the speed of the GaN-based HBT
high.
[0147] Heretofore, embodiments of the present invention have been
explained specifically. However, the present invention is not
limited to these embodiments, but contemplates various changes and
modifications based on the technical idea of the present
invention.
[0148] For example, numerical numbers, structures, shapes,
materials, etc. presented in the aforementioned embodiments are
only examples, and the different numerical numbers, structures,
shapes, materials, etc. may be used as needed.
EXPLANATION OF REFERENCE NUMERALS
[0149] 11 n.sup.+-type GaN layer [0150] 12 n-type GaN layer [0151]
13 Undoped Al.sub.xGa.sub.1-xN layer [0152] 14 Undoped GaN layer
[0153] 15 p-type GaN layer [0154] 16 n-type GaN layer [0155] 17
n.sup.+-type GaN layer [0156] 18 Emitter electrode [0157] 19 Base
electrode [0158] 20 Collector electrode [0159] 23 Two-dimensional
electron gas [0160] 24 Two-dimensional hole gas [0161] 25 Base pad
electrode [0162] 26 Collector pad electrode [0163] 27 High
resistance layer [0164] 31 n-type GaN substrate [0165] 41 Base
substrate [0166] 71 Al.sub.yGa.sub.1-yN graded layer [0167] 81
p-type GaN layer
* * * * *