U.S. patent application number 10/237596 was filed with the patent office on 2003-01-23 for gaas mesfet having ldd and non-uniform p-well doping profiles.
This patent application is currently assigned to Anadigics, Inc.. Invention is credited to Li, Weiqi.
Application Number | 20030017660 10/237596 |
Document ID | / |
Family ID | 25358024 |
Filed Date | 2003-01-23 |
United States Patent
Application |
20030017660 |
Kind Code |
A1 |
Li, Weiqi |
January 23, 2003 |
GaAs MESFET having LDD and non-uniform P-well doping profiles
Abstract
A MESFET has a conduction channel provided with a first doping
profile in a first portion which extends between the source and the
gate, and a second doping profile in a second portion which extends
between the gate and the drain. A background p-type region is
provided beneath the first portion, but not necessarily behind the
second portion.
Inventors: |
Li, Weiqi; (Easton,
PA) |
Correspondence
Address: |
PENNIE & EDMONDS LLP
1667 K STREET NW
SUITE 1000
WASHINGTON
DC
20006
|
Assignee: |
Anadigics, Inc.
|
Family ID: |
25358024 |
Appl. No.: |
10/237596 |
Filed: |
September 10, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10237596 |
Sep 10, 2002 |
|
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09871740 |
Jun 4, 2001 |
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6458640 |
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Current U.S.
Class: |
438/174 ;
257/E21.453; 257/E29.05; 257/E29.321; 438/167; 438/527 |
Current CPC
Class: |
H01L 29/8128 20130101;
H01L 29/66871 20130101; H01L 29/1029 20130101 |
Class at
Publication: |
438/174 ;
438/167; 438/527 |
International
Class: |
H01L 021/338; H01L
021/425 |
Claims
What is claimed is:
1. A metal-semiconductor field effect transistor (MESFET)
comprising: a substrate; a source region formed in the substrate
and having a source electrode; a drain region formed in the
substrate and having a drain electrode; a conduction channel formed
in the substrate between the source region and the drain region;
and a gate electrode positioned between the source region and the
drain region, the gate electrode also being above the conduction
channel; wherein the conduction channel has a first doping profile
in a first portion thereof between the source region and the gate
electrode, and a second doping profile in a second portion thereof
between the gate electrode and the drain region.
2. The MESFET according to claim 1, wherein: a p-type background
region is implanted in the substrate beneath the first portion, but
not beneath the second portion.
3. The MESFET according to claim 2, wherein the first portion is
doped with n-type ions.
4. The MESFET according to claim 3, wherein the first portion is
more heavily doped than the second portion.
5. The MESFET according to claim 1, wherein: the p-type background
region merges with the first portion.
6. The MESFET according to claim 1, wherein the first portion is
more heavily doped than the second portion.
7. The MESFET according to claim 1, wherein the substrate is formed
from GaAs.
Description
RELATED APPLICATIONS
[0001] This is a Divisional of U.S. patent application Ser. No.
09/871,740, filed Jun. 4, 2001, now U.S. Pat. No. ______.
TECHNICAL FIELD
[0002] The present invention is directed to the general field of
forming gallium arsenide (GaAs) semiconductor devices. More
particularly, it is directed to forming GaAs Metal-Semiconductor
Field Effect Transistors (MESFETs).
BACKGROUND OF THE INVENTION
[0003] FIG. 1 illustrates a simplified structure of a conventional
GaAs MESFET 100. The MESFET 100 has a GaAs substrate 102, a source
region 104, a drain region 106, an n-type channel 108, and a p-type
background region 110 and. A source electrode 112 is formed above
the source region 104, a drain electrode 114 is formed above the
drain region 106 and a gate electrode 116 is formed between the
source and drain electrodes on a surface of the GaAs substrate, and
above the n-type channel 108. As seen in FIG. 1, the gate electrode
116 is formed in a depressed area 118 formed in the upper surface
of the device. When a voltage is applied to the gate electrode 116,
the width of the n-type channel changes, thereby affecting the flow
of current between the source electrode 112 and the drain electrode
114.
[0004] In conventional ion implanted, or epitaxially grown, GaAs
MESFET devices, such as that depicted in FIG. 1, the channel 108 is
doped uniformly between the source 104 and drain 106 regions. The
result is that the p-type background forms a p-n junction with the
n-type channel doping underneath the channel. When the MESFET 100
is used as an amplifier, it normally operates with high electrical
field intensity in the gate-drain region. In high RF power
amplifiers, the electrical field in the gate-drain region may be
high enough to initiate impact ionization, in which both excessive
electrons and holes are generated. In such case, the holes become
trapped in the p-n junction, thereby forming a virtual back-gating,
which results in a pinch-off the n-channel 108. This phenomenon is
termed a power transient in RF amplifiers, which is detrimental to
normal operation.
SUMMARY OF THE INVENTION
[0005] The present invention uses selective ion implantation
techniques to create a GaAs MESFET device with non-uniform doping
profiles in the conduction channel. In the Source-Gate region of
the MESFET, a conventional p-type implantation is used as the
background, and one or more n-type implantations form the
conduction channel. In the Gate-Drain region of the device, there
is either no, or a reduced, background p-type implantation, and the
n-type implantation dose is also reduced, resulting in lower doping
concentration between the gate and the drain.
[0006] The present invention is also directed to a method for
forming a GaAs MESFET having non-uniform doping profiles in the
conduction channel. This is accomplished by forming a lightly-doped
first conduction channel of a first type, forming a moderately
doped second conduction channel of the first type along a first
portion of the first conduction channel, forming a background
region of a second type beneath the second conduction channel,
forming source and drain regions at opposite ends of the first
conduction channel, forming source and drain contacts over
corresponding source and drain regions, and forming a gate contact
between the source and drain contacts, the gate contact being
positioned approximately over an end of the second conduction
channel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The present invention is next described with reference to
the following figures, in which:
[0008] FIG. 1 shows a prior art GaAs MESFET with uniform channel
doping; and
[0009] FIGS. 2a-2d show various stages in forming a GaAs MESFET in
accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0010] The process for forming a GaAs MESFET having a non-uniformly
doped channel is now described.
[0011] As seen in FIG. 2a, a substrate 202 is first provided. The
substrate is preferably formed from GaAs, although it may instead
be formed of such materials as Al.sub.xGa.sub.(1-x)As,
In.sub.xGa.sub.(1-x)As, x.about.[0.0-1.0], and InP.
[0012] A first photoresist layer 204 is placed over selected
regions of the upper surface 206 of the substrate. The photoresist
204 is deposited using a mask (not shown) and is configured to
expose a first, preferably continuous upper surface area of the
substrate above what will eventually become the channel.
[0013] Next a lightly doped n-channel 208 having a first length
defined between first end 208a and second end 208b is formed in the
substrate. To do this, n-type ions 206 are implanted into the
substrate, as depicted by the arrows. The n-type ions, preferably
in form of silicon ions, are implanted at an energy of between
approximately 25 KeV and 200 Kev, and so penetrate the substrate to
a depth of between approximately 0.5 nm and 1.2 .mu.m. The n-type
ions preferably are implanted at a relatively low dosage of between
approximately 1E12/cm.sup.2 and 5E12/cm.sup.2, thereby forming the
lightly doped n-channel 208.
[0014] As seen in FIG. 2b, a second photoresist layer 212 is then
placed over the resulting structure. The second photoresist layer
212 is configured to expose a first portion 214 of the lightly
doped channel 208 while a second portion 216 of the lightly doped
channel 208 is covered. Next, a p-type background region 218 having
a second length shorter than the first length and extending from
proximate to the first end 208a of the lightly-doped n-channel 208
is formed in first portion 214. The p-type background region 218 is
formed at or near the boundary between the first portion 214 of the
lightly doped n-channel 208 and the substrate 202 below. To do
this, p-type ions 220 are implanted into the substrate, as depicted
by the arrows. The p-type ions, preferably in the form of beryllium
or magnesium ions, are implanted at an energy of between
approximately 30 KeV and 200 KeV, and so penetrate to a depth of
between approximately 0.1 nm and 1.5 .mu.m The p-type ions
preferably are implanted at a dosage of between approximately
1E11/cm.sup.2 and 1E12/cm.sup.2, thereby forming the p-type
background region 218, a "p-well", in only the first region 214 of
the n-channel 208. As seen in the figures, the p-type background
region 218 extends along the first portion 214 in a direction
parallel to the upper surface, at one end of the n-channel 208.
[0015] Next, using the same photoresist mask, a moderately doped
n-type channel region 222 is formed in the first region 214 of the
lightly doped n-channel 208, above the p-type background region
218. The moderately doped n-type channel region 222 has a third
length which is substantially similar to the second length and
extends from proximate to the first end 208a of the lightly-doped
n-channel 208. To form the channel region 222, n-type ions 224 are
implanted into the first portion 214 of the lightly doped n-channel
208, as depicted by the arrows. The n-type ions, preferably in the
form of silicon ions, are implanted at the substantially same
energy as that used to create the lightly doped n-channel 208 and
so penetrate to about the same depth, just above the p-type
background region 218. The n-type ions preferably are implanted at
a dosage of between approximately 1E12/cm.sup.2 and 5E12/cm.sup.2,
thereby converting the original lightly doped n-channel 208 into a
moderately doped n-channel region 222 in only the first region 214
of the n-channel 208. It should be noted here that one can reverse
the order in which the p-type background region 218 and the
moderately doped n-type channel regions 222 are formed, without
substantially impacting the performance of the ultimate device.
While FIG. 2b shows the regions 218 and 222 to be distinct and
non-overlapping, it should be kept in mind that due to distribution
of ion energies, the regions do not always have a crisp boundary,
but rather somewhat merge together.
[0016] As seen in FIG. 2c, a third photoresist layer 230 is then
placed over the resulting structure. The third photoresist layer
substantially covers the first 214 and second 216 regions of the
original lightly doped n-channel 208, and leaves exposed a pair of
lateral areas 232a, 232b of the substrate on either side of the
original n-channel 208. The lateral areas are situated over what
will eventually become the source region 234 and the drain region
236. To convert the substrate below lateral areas 232a, 232b into
the source 234 and drain 236 regions, n-type ions 238 are implanted
into the regions of the substrate below the lateral areas 232a,
232b, as depicted by the arrows. This results in the formation of a
source region 234 adjacent to one end of the moderately doped
n-channel 222 and the p-type background region, and also results in
the formation of a drain region 236 adjacent to an end of the
lightly doped n-channel 208. The n-type ions, preferably in the
form of silicon ions, are implanted at an energy of between
approximately 50 KeV and 100 KeV, and so penetrate to a depth of
between approximately 0.5 .mu.m and 1.0 .mu.m. Furthermore, the
n-type ions preferably are implanted at a dosage of between
approximately 5E12/cm.sup.2 and 1E13/cm.sup.2, thereby converting
the substrate into highly doped n-type regions 234, 236. It should
be noted here that while the source 234 and drain 236 regions
preferably are formed in a single step, it may also be possible to
form them in separate step, especially in the event that the two
regions are to be differently doped, or have different depths.
[0017] As seen in FIG. 2d, source 242 and drain contacts 244,
preferably made of germanium gold (GeAu), are formed over
respective source 234 and 236 drain regions. In addition, a gate
contact 246 is formed between the source and drain contacts. As is
known to those skilled in the art, the gate contacts are typically
formed from Ti/Pt/Au, or other refractory metal, such as Mo, W,
TiW, and the like. Preferably, the gate contact 246 is positioned
near the second end of the moderately doped n-channel 222 extending
between the source and the gate; the gate contact may even straddle
the boundary 248 between the channel 222 and the lightly doped
n-channel 208 extending between the gate and the drain, or be
positioned entirely above the lightly-doped n-channel adjacent to
the boundary 248. Also, as seen in FIG. 2d, the gate is formed in a
depression 250 created in the upper surface of the device, the
depression having the effect of physically limiting the width of
the channel below. While the source 242 and drain 244 contacts are
preferably formed at the same time using a single photoresist mask,
they may be made in separate steps. Furthermore, the gate contact
248 preferably is formed after the source and drain contacts are
formed.
[0018] The final device has a conduction channel between the source
and the drain which has a first doping profile between the source
and the gate, and a second doping profile between the drain and the
gate. More particularly, the MESFET of the present invention has
p-type background region between the source and the gate, forming a
p-well profile. The n-type channel implant dosage is reduced in the
gate-drain region to form a lightly doped drain (LDD), as compared
to the n-type channel implant dosage in the source-gate region.
[0019] The design of the present invention helps mitigate the p-n
junction in the gate-drain region, while the LDD profile helps
minimize the peak electric field intensity in the drain region. The
LDD profile may also assist in increasing the gate-drain breakdown
voltage, and alleviate the initiation of impact ionization, thereby
mitigating the power transients caused by excessive hole trapping
in the drain region.
[0020] In general, the P-well LDD GaAs MESFET design of the present
invention does not severely degrade the device DC and RF
performance, as compared to conventionally implanted GaAs MESFETs.
This is because the channel current and the transconductance of a
GaAs MESFET are mainly determined by the doping profiles in the
source-gate region, where it is the same for both the P-well LDD
GaAs MESFET of the present invention and the conventional MESFET.
Furthermore, in normal amplifier operation, the electrons travel at
saturation velocity in the gate-drain region and so the LDD doping
profile generally does not negatively affect the channel electron
transport process.
[0021] Also, although the final MESFET is an n-channel
semiconductor device, this is not intended as a limitation of the
present invention and as those skilled in the art will appreciate,
a P-channel semiconductor device may be achieved by converting
P-type regions to N-type regions, and vice versa.
[0022] Finally, while the above invention has been described with
reference to certain preferred embodiments, it should be kept in
mind that the scope of the present invention is not limited to
these. One skilled in the art may find variations of these
preferred embodiments which, nevertheless, fall within the spirit
of the present invention, whose scope is defined by the claims set
forth below.
* * * * *