U.S. patent application number 14/870311 was filed with the patent office on 2016-08-25 for transistor body control circuit and an integrated circuit.
The applicant listed for this patent is FREESCALE SEMICONDUCTOR, INC.. Invention is credited to EDOUARD DENIS DE FRESART, HUBERT MICHEL GRANDRY, EVGUENIY STAFANOV.
Application Number | 20160247799 14/870311 |
Document ID | / |
Family ID | 55272209 |
Filed Date | 2016-08-25 |
United States Patent
Application |
20160247799 |
Kind Code |
A1 |
STAFANOV; EVGUENIY ; et
al. |
August 25, 2016 |
TRANSISTOR BODY CONTROL CIRCUIT AND AN INTEGRATED CIRCUIT
Abstract
An integrated circuit comprises a transistor body control
circuit for controlling a body of a bidirectional power transistor.
The transistor body control circuit comprises switches connected
between a body terminal and a first current terminal, with a
control terminal for controlling the current flowing through the
switch. The control terminal of the switch is connected to
alternating current, AC capacitive voltage divider. The AC
capacitive voltage dividers are connected to the control terminals
and arranged to control the switches to switch the voltage of the
body terminal as a function of the voltage between the first
current terminal and the second current terminal. The integrated
circuit further comprises a bi-directional power transistor
connected to the transistor body control circuit.
Inventors: |
STAFANOV; EVGUENIY; (VIEILLE
TOULOUSE, FR) ; DE FRESART; EDOUARD DENIS; (TEMPE,
AZ) ; GRANDRY; HUBERT MICHEL; (ESPANES, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FREESCALE SEMICONDUCTOR, INC. |
Austin |
TX |
US |
|
|
Family ID: |
55272209 |
Appl. No.: |
14/870311 |
Filed: |
September 30, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 29/7827 20130101;
H01L 29/7813 20130101; H03K 17/687 20130101; H03K 2217/0018
20130101; H01L 27/0727 20130101; H01L 29/086 20130101; H01L 29/407
20130101; H03K 17/102 20130101; H03K 2217/0009 20130101 |
International
Class: |
H01L 27/07 20060101
H01L027/07; H01L 29/78 20060101 H01L029/78; H03K 17/687 20060101
H03K017/687 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 23, 2015 |
IB |
PCT/IB2015/001389 |
Claims
1. A transistor body control circuit for controlling a body of a
bidirectional power transistor, comprising: a first switch
connectable between a body terminal and a first current terminal of
the bidirectional power transistor, comprising a control terminal
for controlling the current flowing through the first switch; a
second switch connectable between the body terminal and a second
current terminal of the bidirectional power transistor, comprising
a control terminal for controlling the current flowing through the
second switch; the control terminal of the first switch being
connected to a first alternating current, AC capacitive voltage
divider and the control terminal of the second switch being
connected to a second AC capacitive voltage divider; said AC
capacitive voltage dividers being connectable to the first control
terminal and the second current terminal and arranged to control
the first switch and the second switch to switch the voltage of the
body terminal to the first current terminal or the second current
terminal as a function of the voltage between the first current
terminal and the second current terminal.
2. A circuit as claimed in claim 1, where the AC capacitive voltage
dividers both comprise a series connection of a resistor
connectable to a respective current terminal of the transistor and
at least two capacitive elements in series, a node between said
capacitive elements being connected to the control terminal.
3. A circuit as claimed in claim 2, wherein the at least one of the
first or second AC capacitive voltage divider comprises a diode, a
cathode of said diode being connected to said resistor and an anode
of said diode being connected to said node.
4. A circuit as claimed in claim 1, wherein for at least one of the
first or second AC capacitive voltage divider a capacitive part of
the switch to which the divider is connected forms a capacitive
element of said AC capacitive voltage divider connecting the
control terminal and the respective current terminal.
5. A circuit as claimed in claim 1, further comprising clamp
circuitry for clamping the control terminals relative to said body
terminal to below the breakdown voltage of the control
terminal.
6. A circuit as claimed in claim 5, wherein further comprising, for
at least one of the first or second switch, a resistor between the
control terminal and the body terminal.
7. An integrated circuit, comprising a bi-directional power
transistor, the integrated circuit comprising: a transistor body
control circuit for controlling a body of a bidirectional power
transistor, the transistor body control circuit comprising: a first
switch connectable between a body terminal and a first current
terminal of the bidirectional power transistor, comprising a
control terminal for controlling the current flowing through the
first switch; a second switch connectable between the body terminal
and a second current terminal of the bidirectional power
transistor, comprising a control terminal for controlling the
current flowing through the second switch; the control terminal of
the first switch being connected to a first alternating current, AC
capacitive voltage divider and the control terminal of the second
switch being connected to a second AC capacitive voltage divider;
said AC capacitive voltage dividers being connectable to the first
control terminal and the second current terminal and arranged to
control the first switch and the second switch to switch the
voltage of the body terminal to the first current terminal or the
second current terminal as a function of the voltage between the
first current terminal and the second current terminal; the
integrated circuit further comprising a bi-directional power
transistor connected with a body to said body terminal, with said
first current terminal to a drain terminal and with a second
current terminal to a source terminal.
8. An integrated circuit as claimed in claim 7, wherein the
bi-directional power transistor comprises: a substrate with a
substrate top surface; a layer stack extending over the substrate
top surface, in which stack a first vertical trench and a second
vertical trench are present, each of said vertical trenches
extending in a vertical direction from a top layer of the stack
towards the substrate; an electrical path which can be selectively
enabled or disabled to allow current to flow in a first direction
or a second direction, opposite to the first direction, between the
drain terminal and the source terminal, the electrical path
comprising: wherein the drain terminal is situated, in said
vertical direction, below the source terminal and the source
terminal being situated on or above the top layer; and the body
extends laterally between the first and second vertical trenches
and vertically between said drain terminal and said source
terminal; the electrical path comprising: the body, a first drift
region extending, in said vertical direction, between the body and
the drain terminal, and a second drift region extending, in said
vertical direction, between the body and the source terminal.
9. The integrated circuit as claimed in claim 8, comprising a
control die on which the transistor body control circuit is
provided and a power die on which the bi-directional power
transistor is provided.
10. The integrated circuit as claimed in claim 7, where the AC
capacitive voltage dividers both comprise a series connection of a
resistor connectable to a respective current terminal of the
transistor and at least two capacitive elements in series, a node
between said capacitive elements being connected to the control
terminal.
11. The integrated circuit as claimed in claim 10, wherein at least
one of the first or second AC capacitive voltage divider comprises
a diode, a cathode of said diode being connected to said resistor
and an anode of said diode being connected to said node.
12. The integrated circuit as claimed in claim 7, wherein for at
least one of the first or second AC capacitive voltage divider a
capacitive part of the switch to which the divider is connected
forms a capacitive element of said AC capacitive voltage divider
connecting the control terminal and the respective current
terminal.
13. The integrated circuit as claimed in claim 7, further
comprising clamp circuitry for clamping the control terminals
relative to said body terminal to below the breakdown voltage of
the control terminal.
14. The integrated circuit as claimed in claim 13, wherein further
comprising, for at least one of the first or second switch, a
resistor between the control terminal and the body terminal.
15. The integrated circuit as claimed in claim 7, wherein the
bi-directional power transistor has a breakdown voltage of at least
25 V and the first or second switch has a breakdown voltage below
10 V.
16. The integrated circuit as claimed in claim 7, wherein the
bi-directional power transistor has a breakdown voltage from the
drain terminal to the source terminal of at least 40 V and in
reverse direction from the source terminal to the drain terminal of
at least 25 V.
17. The integrated circuit as claimed in claim 16, wherein the
breakdown voltage from the drain terminal to the source terminal is
higher than in the reverse direction.
18. The integrated circuit as claimed in claim 7, wherein the AC
capacitive divider has a division ratio which varies depending on a
polarity of the voltage across the AC capacitive divider.
19. A circuit as claimed in claim 2, wherein for at least one of
the first or second AC capacitive voltage divider a capacitive part
of the switch to which the divider is connected forms a capacitive
element of said AC capacitive voltage divider connecting the
control terminal and the respective current terminal.
20. A circuit as claimed in claim 3, wherein for at least one of
the first or second AC capacitive voltage divider a capacitive part
of the switch to which the divider is connected forms a capacitive
element of said AC capacitive voltage divider connecting the
control terminal and the respective current terminal.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a transistor body control circuit
and an integrated circuit.
BACKGROUND OF THE INVENTION
[0002] Bi-directional switches switch high currents through their
conduction electrodes while blocking high voltages applied to the
conduction electrodes. Bi-directional switches are used in a
variety of electrical systems. A typical bi-directional switch is
specified to supply high currents, which can range from several
Amperes of maximum current to several hundreds of Amperes depending
on the specific switch and application, while blocking relatively
high voltages, e.g. of at least 25 V without breaking down.
[0003] Bi-directional switches are typically implemented using
electromechanical switches or a configuration of semiconductor
devices, e.g. power transistors. However, standard power
transistors have no technically meaningful blocking voltage in one
direction, making them unidirectional devices. Consequently,
current bi-directional switches typically are implemented using two
separate serially coupled power MOSFETs. The separate MOSFETs are
formed on separate semiconductor dice, and often housed in separate
packages, which results in a high manufacturing cost and a large
area occupied on a circuit board. When the separate MOSFET dice are
housed in a single package and interconnected with wire bonds, the
area occupied on a circuit board is reduced but the manufacturing
cost is still too high for many applications.
[0004] U.S. Pat. Nos. 7,282,406, 7,297,603, 7,537,970, 7,910,409,
8,101,969 and 8,530,284 all disclose an integrated circuit with
several different transistors integrated on the same circuit,
including a p-channel bi-directional trench power transistor for
battery charging protection. The transistor comprises two vertical
trenches between which a body is present. The body is separated
from current carrying electrodes above and below the body by
high-voltage regions with a lesser doping concentration than the
electrodes. However, this bi-directional trench power transistor
has an inherent parasitic bipolar transistor formed by the body and
the high voltage regions. Furthermore, it is not suitable for
operation with high voltages, such as of at least 20 or more, e.g.
up to 40 V or more, and/or high currents, e.g. above 1 mA, up to 1
A or more.
[0005] U.S. Pat. No. 8,101,969 discloses a body bias switch
including two MOSFETs connected in parallel to the native diodes in
the bi-directional trench power transistor. The MOSFETs are
integrated on the same die as the bi-directional trench power
transistor. When the drain voltage is the highest voltage (i.e. out
of the drain voltage and the source voltage), the body is
referenced to the source voltage, and vice versa when the source
voltage is the highest voltage, the body is referenced to the drain
voltage. However, this bias switch comes with a risk of
over-voltages damaging the MOSFETs since the gates of the MOSFETs
are connected directly to the drain and source respectively.
Accordingly, in case the maximum drain-source voltages of the
bidirectional transistor exceed the breakdown voltages of the
MOSFETs the latter risk irreversible damage.
SUMMARY OF THE INVENTION
[0006] The present invention provides a transistor body control
circuit and an integrated circuit as described in the accompanying
claims.
[0007] Specific embodiments of the invention are set forth in the
dependent claims.
[0008] These and other aspects of the invention will be apparent
from and elucidated with reference to the embodiments described
hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Further details, aspects and embodiments of the invention
will be described, by way of example only, with reference to the
drawings. In the drawings, like reference numbers are used to
identify like or functionally similar elements. Elements in the
figures are illustrated for simplicity and clarity and have not
necessarily been drawn to scale.
[0010] FIG. 1 schematically shows a cross-sectional view of an
example of an embodiment of a bidirectional power transistor.
[0011] FIG. 2 shows a circuit-diagram of a first example of a
transistor body control circuit suitable for the example of FIG.
1
[0012] FIG. 3 shows a circuit-diagram of a second example of a
transistor body control circuit suitable for the example of FIG.
1
[0013] FIG. 4 shows a circuit-diagram of a third example of a
transistor body control circuit suitable for the example of FIG.
1
[0014] FIG. 5 shows a diagram of an example of an integrated
circuit comprising a power transistor and a transistor body control
circuit.
[0015] FIG. 6 shows a graph illustrating simulated current-voltage
characteristics of a bidirectional power transistor without a
transistor body control circuit.
[0016] FIG. 7 shows a graph illustrating simulated current-voltage
characteristics of a bidirectional power transistor with a
transistor body control circuit.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] Because the illustrated embodiments of the present invention
can for the most part, be implemented using electronic components
and circuits known to those skilled in the art, details will not be
explained in any greater extent than that considered necessary as
illustrated for the understanding and appreciation of the
underlying concepts of the present invention and in order not to
obfuscate or distract from the teachings of the present
invention.
[0018] FIG. 1 shows an example of a bidirectional power transistor
100. The power transistor 100 shown in FIG. 1 is a bi-directional
transistor, as is explained below in more detail, which can support
high energies, i.e. high currents and/or voltages both from the
source towards the drain and vice-versa and. The power transistor
can for example have a current maximum of more than 1 A, such as 10
A or more, such as 100 A or more, such as at least 200 A and/or a
positive drain-source break down voltage of at least 25 V, for
example 50 V or more, and a negative drain-source break down
voltage of at least 25 V, for example 30 V or more, such as 50 V or
more, for example 100 V or more, e.g. 300 V or more.
[0019] FIGS. 2-4 show circuits suitable to switch the body 103. As
illustrated with the I-V characteristics in FIGS. 6 and 7, in which
the breakdown voltage in drain to source direction is denoted with
BVdss and in the reverse direction with BVr, the bi-directional
transistor will exhibit higher breakdown voltages in both
directions when the body 103 is actively switched to the source or
drain voltage, compared to a body of which the potential is left
floating. This is observed both at ambient temperature (25 degrees
Celsius) and becomes even more significant at elevated temperatures
(175 degrees Celsius). Accordingly, the transistor body control
circuit allows to improve the breakdown voltages.
[0020] The transistor body control circuit shown in FIGS. 2-4
comprises first and second switches M1, M2, e.g. field effect
transistors operated in switched mode, connectable between the body
terminal B and a respective one of the first and second current
terminals D,S of the bidirectional power transistor. The current
flowing through the switches M1, M2 can be controlled through a
respective control terminal, e.g. a gate, of the switch M1, M2. The
control terminal of the first switch is connected to a first
alternating current or transient, hereinafter AC, capacitive
voltage divider 210 and the control terminal of the second switch
is connected to a second AC capacitive voltage divider 220. The AC
capacitive voltage divides 210,220 are connectable to the current
terminals D,S of the transistor, as shown the AC capacitive voltage
dividers 210,220 are connected between respective contacts 201,203
of the circuit 200 connectable to the current terminals D,S. The AC
capacitive voltage dividers 210,220, when in operation, control the
switches M1, M2 to switch the voltage of the body terminal to the
first current terminal or the second current terminal as a function
of the voltage between the first current terminal and the second
current terminal. The AC capacitive voltage dividers reduce the
risk that the switches M1, M2 are damaged by a too high voltage
applied to the current terminals D,S because at the control
terminal of the switches M1, M2 a voltage will be applied that is
only a fraction of the voltage between the current terminals
D,S.
[0021] Referring back to FIG. 1, the example of a bidirectional
power transistor shown therein comprises a first current terminal
105, e.g. a drain, and a second current terminal 101, e.g. a
source. An electrical path is present between the first current
terminal 105 and the second current terminal 101, through a first
drift region 104, a body 103 and a second drift region 102. The
electrical path can be selectively enabled or disabled to allow
current to flow in a first direction, e.g. from the first current
terminal to the second terminal or a second direction, opposite to
the first direction, by applying suitable signals and power to the
electrodes B, D, G, Sh and S, which are connected to parts
101,103,105,107,108 of the power transistor.
[0022] The power transistor 100 comprises a first current
electrode, e.g. in this example the drain electrode D, which is
connected to a first current terminal 105, e.g. the drain, of the
power transistor 100. A second current electrode S, e.g. the source
electrode, is connected to a second current terminal 101, e.g. the
source, of the power transistor shown. A gate or control electrode
G is connected to the control terminal, e.g. gate 108, of the power
transistor 100. As shown in the example of FIG. 1, the
semiconductor product can further comprise a body electrode B
connectable to an external power supply and connected to the body
103 of the power transistor 100. A separate shield electrode Sh is
provided via which the voltage of the shield plate 107 can be
controlled separately from the voltage and/or current of the other
electrodes. However, the second current electrode S can
alternatively be connected to the shield plate 107 of each of the
power transistors 100, as indicated in FIG. 1 with the dashed line
and hence the voltage of the shield plate be coupled to the second
current terminal.
[0023] In FIG. 1 a cross-sectional view is shown to explain the
bidirectional operation of the bidirectional power transistor, but
it will be apparent that the transistor has an elongated finger
like shape and that the different electrodes may connect to the
respective element at a location of the finger suitable for the
specific implementation and not necessarily at the section shown in
FIG. 1, e.g. the body 103 may be connected at opposite ends of the
finger to the body electrode B, the source 101 to the source
electrode S at the opposite ends of the finger and a position in
the middle of the finger, etc.
[0024] It should be apparent that in FIG. 1 only a single
transistor or "cell" is shown, and that an actual semiconductor
product can comprise an arrangement of a plurality of these cells.
Depending on the specific implementation, the product can comprise
several tens, hundreds, thousands or more cells in a suitable
arrangement (e.g. 2-dimensional matrix) and connected in parallel
to form a single power transistor device. The product can for
example be implemented as described in applicant's co-pending
International Patent Application PCT/IB2013/002209, the entire
contents of which are incorporated herein by reference. In case the
product comprises a plurality of cells, the terminals of each of
the different cells can be connected to the electrodes, to allow
the different cells to be controlled simultaneously to conduct
current through a layer stack from a first current terminal 101 to
a second current terminal 105 or vice versa. Each of the electrodes
or feeds B,D,G,Sh and S present in the semiconductor product is
connectable to external circuitry, such as a power supply or
control logic circuitry, not shown. The connection between the
electrodes and feed and the external circuitry can be provided in
any conventional manner, and is not described in further
detail.
[0025] The power transistor 100 can be used to control the flow of
current. The shown example of power transistor can for example be
used in a method for operating a power transistor as described
below, although it will be apparent that other types of
bi-directional power transistors can be used as well to perform
such a method and that the power transistor can be used in other
methods. The power transistor can be operated intermittently in a
first direction or a second direction, i.e. bi-directional. The
bi-directional power transistor can be symmetric with positive and
negative break down voltages that have the same absolute value, or
be asymmetric, with different values, depending on the specific
implementation. For instance, depending on the specific
implementation the thickness of the first and/or second drift
region can be adapted to obtain a breakdown voltage for the
specific implementation. For an asymmetric transistor, a suitable
positive breakdown voltage has found to be between 1.5 and 2 times
that of the negative breakdown voltage, such as 45 V for a 25 V
negative breakdown voltage.
[0026] The bi-directional nature of the power transistor 100 will
now be described in operation, using the example of an n-type power
transistor. In a first direction and in respect of switching the
power transistor 100 on, a positive voltage (relative to the
source) can be applied to the drain electrode D. The body electrode
B can be connected to the source electrode S, so as to electrically
couple the body 103 to the source 101 of the transistor 100, as
explained below in more detail with reference to the operation of
the circuits shown in FIGS. 2-4. To the shield plate a voltage
lower than the voltage of the drain electrode (e.g. 0 V or the
source voltage if the drain electrode is at a positive voltage) can
then be provided to shield the gate from the voltage applied to the
drain electrode D. By applying a positive gate-source bias voltage,
Vgs>0 V, to the gate electrode G by an external gate driver
circuit (not shown) a depletion field effect can be caused through
the gate dielectric at an interface between the body 103 and the
first and second trenches 106 in which the gate 108 is provided.
When the gate bias voltage exceeds a threshold voltage Vth, an
inversion conducting n-layer is formed along the interface of the
trench 106 and the body 103, which conducts the majority of
carriers injected from the source 101 to be collected by the drain
105.
[0027] In an off-state, a positive voltage can be applied to the
drain 105. The body 103 can still be electrically tied to the
source and so be subjected to a source potential. The gate bias
voltage can be set to a lowest potential, e.g. Vgs=0 V. A first
depletion layer can be formed around a bottom p-n junction formed
by the interface of the body 103 and the first drift region 104. By
increasing the drain-source bias voltage, Vds, a first space charge
region of the depletion layer can increase to the low-doped bottom
part of the first drift region 104. The electrical field in the
region thereby increases and when a critical field is reached, an
avalanche phenomena by carrier impact ionization can be observed
causing breakdown of the reverse biased junction mentioned
above.
[0028] In the second direction and in relation to an on-state, the
body electrode can be set such that the drain potential is coupled
to the body 103, as explained below in more detail with reference
to the operation of the circuits shown in FIGS. 2-4. A positive
voltage can be applied to the source 101. In the second direction,
a positive bias voltage, e.g. the source voltage, can be provided
to the shield plate and the gate biased relative to the drain 105.
This allows to reduce the electrical field in at least a part of
the first drift region 104, and accordingly the breakdown voltage
can be increased.
[0029] A positive gate bias voltage, Vgd>0 V, can be applied to
the gate by the external gate driver circuit, thereby causing a
depletion field effect through the gate dielectric into the body
along the inner sidewalls of the trenches 106. When the gate bias
voltage exceeds the threshold voltage Vth an inversion conducting
layer can be formed along the interface of the trench dielectric
and the body, which can conduct the majority of the carriers
injected from the substrate 102 and collected by the source
101.
[0030] In an off state, a positive voltage can be applied to the
source 101. The body 103 can still be electrically tied to the
potential of the drain. The gate-drain bias voltage, Vgd, can be
set to the lowest potential, namely, Vgd=0V. A second depletion
layer can be formed around a top p-n junction formed by the
interface of the body and the second drift region 102. By
increasing the source-drain bias voltage, Vsd, a second space
charge region of the depletion layer can increase to the low-doped
top part of the second drift region 102. The electrical field in
the region can thereby increase and when a critical field is
reached, an avalanche phenomena by carrier impact ionization can be
observed causing breakdown of the reverse biased junction mentioned
above, thereby implementing the blocking voltage.
[0031] In the example of FIG. 1, the first current terminal 105 is
formed in a wafer substrate. On the bottom of the substrate, also
referred to as the back-side, a metal layer 110 is provided which
constitutes the electrode for the first current terminal 105 and
allows to connect the first current terminal 105 to an external
voltage or current supply. A suitably patterned and structured
layer stack has been provided on top-side of the substrate and the
other components of the power transistor 100 are formed in the
layer stack e.g. by processing, such as successive patterning,
doping, deposition, etching, etc of the stack. The top surface of
the layer stack is covered by a passivation layer 109 of a suitable
dielectric material. The passivation layer 109 shields the rest of
the power transistor 100 from ambient influences, such as
oxidations or otherwise.
[0032] The layer stack can be implemented in any manner suitable
for the specific implementation. In the shown example, the layer
stack 102 comprises a bulk layer of a base material of the first
conductivity type with a concentration of majority charge carriers
equal to a concentration in the first drift region or in the second
drift region. The bulk layer is provided with one or more doped
layers in which the doping differs from the base material, e.g. in
conductivity type and/or concentration of majority charge carriers.
Thus, in the example the layers of the layer stack 102 are formed
from the same base material. The doped layers in the bulk layer can
for example comprise one or more of the group consisting of: a
buried layer of the second conductivity type, in which the body is
present; a source layer of the first conductivity type with a
concentration of majority charge carriers higher than the base
material, in which the second current terminal is present, the
source layer is separated from the buried layer by a drift layer of
the base material which the second drift region is present; a drain
layer of the first conductivity type with a concentration of
majority charge carriers higher than the base material, in which
the first current terminal is present, the drain layer is separated
from the buried layer by a drift layer of the base material in
which the first drift region is present. However, alternatively,
the layer stack can comprise a plurality of different layers of
different base materials, for example individually grown on top of
each other during consecutive phases of manufacturing of the power
transistor.
[0033] In the example of FIG. 1, in the stack 102 vertical trenches
106 are present in which the gate is buried and the bidirectional
power transistor is a bidirectional vertical trench field effect
power transistor. Each of the vertical trenches 106 extends in the
vertical direction from a top layer of the stack towards the first
current terminal 105. The power transistor is defined in lateral
direction by the vertical trenches. Hereinafter, the vertical
sidewalls of the trenches 106 closest to, and facing towards, the
body 103 are referred to as the inner sidewalls and the vertical
sidewalls facing away from the body are referred to as the outer
sidewalls. The inner sidewalls of the vertical trenches 106 confine
the current laterally and thus form the lateral boundaries of the
current path.
[0034] In the shown example, each of the first and second vertical
trenches 106 comprises a gate electrode 108 in a first part of the
vertical trench 106. The gate electrode 108 is electrically
isolated from the body 103 by a gate dielectric, in this example
formed by a gate dielectric layer lining the inner sidewall in a
first part of the trench. The gate electrode 108 is coupled to the
body 103 and, when a suitable voltage is applied to the gate, e.g.
through the gate electrode, a vertical conductive channel is formed
in the body 103. Through the vertical channel a current can flow
from the first drift region 104 to the second drift region 102,
when the first current terminal 105 is at a positive voltage with
respect to the second current terminal 101, or vice versa when the
second current terminal 101 is at a positive voltage with respect
to the first current terminal 105.
[0035] The first and second vertical trenches 106 extend, in the
vertical direction, from the top layer beyond an upper boundary of
the first drift region 104, and in a lateral direction parallel to
the substrate top-surface electrically isolate, and define, the
first drift region 104. Thereby, the risk on an unexpected
breakdown of a transistor 100 can be reduced. Without being bound
to theory, it is believed that unexpected breakdown can be caused
by voltage and/or current events in other power transistors (e.g.
adjacent cells) or devices. Furthermore, it is believed that by
isolating the first drift region 104 in the lateral direction,
switching speed can be increased since the gate-drain capacitance
is reduced drastically and less charge carriers need to be
recombined when switching off, i.e. only those in the region
between the first and second vertical trenches 106, rather than in
the entire drift region.
[0036] The vertical trenches 106 further comprise a shield plate
107. The shield plate 107 is capable of generating a vertical
accumulation layer in the first drift region 104, e.g. along the
inner sidewall of the trench, at the interface between the first
drift region 104 and the vertical trench 106 when the voltage
shield plate 107 is biased with respect to the voltage of the first
current terminal 105 in a first polarity. For example, in case the
first current terminal is an n-doped semiconductor material, the
accumulation layer can be generated when the shield plate 107 is
sufficiently positively biased. In case the first current terminal
is a p-doped semiconductor material, the accumulation layer can be
generated when the shield plate 107 is sufficiently negatively
biased. In the shown examples the accumulation layer will extend in
vertical direction through the whole first drift region, from the
bottom limit of the body region 103 up to the first current
terminal 105. Thus, a conductive path between the body and the
first current terminal 105 can be established in a relatively fast
manner. However, depending on the specific implementation, the
accumulation layer can extend in vertical direction through a part
of the first drift region 104 only, and e.g. be spaced from the
body or the first current terminal. The shield plate 107 can
further locally reduce the electrical field density in parts of the
first drift region when the shield plate is biased with respect to
the first current terminal in a second polarity. For example, in
case the first current terminal is an n-doped semiconductor
material, the reduction is obtained when the shield plate 107 is
sufficiently negatively biased. For example, in case the first
current terminal is an n-doped semiconductor material, the
reduction is obtained when the shield plate 107 is sufficiently
negatively biased. Thus, unexpected breakdown can be reduced
because overly high electric fields in the first drift region can
be avoided while the speed of switching can be improved since the
current path through the drift region can be enabled more rapidly
by creating the accumulation layer.
[0037] In the shown example, the shield plate 107 is situated in a
lower part of the trench 100. This lower part is closer to the
first current terminal 105 than the first part. The shield plate
107 is in this example additional to the lateral isolation of the
first drift region 104 by the vertical trench 106. However, it
should be apparent that the shield plate 107 can be used without
the lateral isolation of the first drift region 104, and that the
lateral isolation of the first drift region can be used without a
shield plate.
[0038] The first drift region 104 and the second drift region 102
can be implemented in any manner suitable for the specific
implementation. The first and second drift region can be of a first
conductivity type having a first type of majority charge carriers,
while the body is of a second conductivity type having a second
type of majority charge carriers opposite to the first type. For
example the drift regions can be n-type semiconductors and the body
be a p-type or vice versa.
[0039] In the example of FIG. 1, the first drift region 104 extends
in lateral direction between the vertical trenches and is defined
by the inner sidewalls of the vertical trenches 106. The first
drift region 104 extends in vertical direction from the bottom of
the body 103 until the top-surface of the substrate 120. Suitable
lower limits for the thickness have been found to 2 micron or more,
such as 5 micron or more, for example 10 micron or more, and
suitable upper limits 10 micron or less, such as 5 micron or less,
such as 2 micron or less. The drift region can for example be
mono-crystalline, and grown on the substrate through for instance
an epitaxial process. The drift region can be of the same material,
e.g. Si, as the first current terminal 105 but with a lower doping
concentration. A suitable dopant has found to be P or As with a
resistivity of 0.05 Ohm*cm or more, e.g. 0.1 Ohm*cm or more, such
as 0.2 ohm*cm or more. A suitable upper limit has been found a
resistivity of 1 Ohm*cm or less. A particularly effective
resistivity has been found to be 0.2 Ohm*cm on average but it will
be apparent that other values may be used depending on the desired
breakdown voltage of the transistor and that the doping
concentration does not need to be uniform over the entire drift
region.
[0040] The second drift region 102 can, as in the examples, have
essentially the same characteristics as the first drift region 104.
In the example, the thickness of the second drift region is much
less than of the first drift region but the thickness may be more
or less the same, depending on the desired breakdown voltage of the
bidirectional power transistor. A suitable thickness has found to
be 1 micron or more, for example 1.5 micron.
[0041] The first current terminal 105 and second current terminal
101 can be implemented in any manner suitable for the specific
implementation. The first current terminal 105 is in the example
situated, in a vertical direction from a top of the stack towards
the first current terminal 105, below the second current terminal
101. In the shown examples, the first and second current terminal
101, 105 are of the same, first, conductivity type as the drift
regions 102,104 and opposite to the conductivity type of the body
103. The concentration of majority charge carriers in the first
current terminal 105 is higher than in the first drift region 104.
The concentration of majority charge carriers in the second current
terminal 101 is higher than in the second drift region 102. The
current terminals can for example be doped or otherwise be provided
with a resistivity which is at least one order of magnitude smaller
than the resistivity of the drift regions.
[0042] In this example, the first current terminal 105 is of a
semiconductor material provided with a dopant of the same type as
the first drift region 211 (e.g. an n-type doping or a p-type
doping) but a higher concentration. This makes the first current
terminal 105 highly conductive compared to the first drift region
104. For instance, the doping concentration can be at least 2.5
orders of magnitude higher than in the drift region, 3 orders or
more have been found to be particularly effective. The first
current terminal 105 can be any suitable type of substrate such as
a mono-crystaline Si substrate with a <100>orientation, and
doped with a suitable dopant, such as in case of an N-doped current
terminal Arsenic (As), to obtain a resistivity of less than 1 milli
Ohm/com, such as less than 0.005 ohm/cm, for example 0.03 Ohm*cm or
less.
[0043] The second current terminal 101 can be implemented in any
manner suitable for the specific implementation, and be of similar
constitution as the first current terminal 105, but in terms of
conductivity and doping concentration different, for example with a
doping concentration which is an order of magnitude higher. In this
example, the second current terminal 101 is formed as the area of
the top layer of the layer stack between, in lateral direction, the
inner sidewalls of the trenches 106 and in vertical direction
between the top of the second drift layer 102 and the top-surface
of the layer stack (in this example covered by passivation layer
109). However, depending on the specific implementation the second
current terminal 101 can be implemented above the top layer, for
example by local formation or deposition of a suitable material on
the layer stack, in the area in lateral direction between the inner
sidewalls of the vertical trenches 106.
[0044] The body 103 can be implemented in any manner suitable for
the specific implementation. In the shown example, the body is
defined in lateral direction by the inner sidewalls of the vertical
trenches 106 and in vertical direction between by the bottom of the
second drift region, and the top of the first drift region. The
body 103 extends laterally between vertical trenches 106. The body
103 extends vertically between the first current terminal 105 and
the second current terminal 101. The first drift region 104 thus
extends, in the vertical direction, between the body 103 and the
first current terminal 105, while the second drift region 102
extends, in the vertical direction, between the body 103 and the
second current terminal 101. The body can for example be formed by
a doping a semiconductor material, e.g. Si, with a suitable dopant
(e.g. p-type if the current terminals 101,105 are of an n-type). A
suitable dopant has been found Boron, such as B.sub.11. A suitable
concentration has been found to be 2 orders of magnitude smaller
than that of the first current terminal 105.
[0045] As mentioned above, the breakdown voltage is increased if
the voltage of the body 103 is actively controlled and not left
floating. Referring to the examples of body control circuits shown
in FIGS. 2-4, the switches M1, M2 are controlled by the AC
capacitive voltage dividers such that when the one is open, the
other is closed and accordingly the body 103 (when the circuit is
connected to the bidirectional power transistor) is either set to
the voltage of the first current terminal or of the second current
terminal, more specifically to the lowest of the voltages of the
first or second current terminal.
[0046] Referring to the example of FIG. 2, the body 103 can for
example be switched using an AC capacitive voltage divider
comprising a chain 210,220 of at least two capacitors C1,C2;C3,C4
respectively. One end of the chain may be connected to a respective
one of the contacts 201,203 to respectively receive the voltage of
the first current terminal 105 and the second terminal 101. The
other end of the chain may be connected the control terminal of a
respective switch M1, M2 to set to voltage of that end and hence
control the state of the switch to be conductive (closed) or
non-conductive (open). The voltage of the node is a fraction of the
voltage between the contacts 201,203 and by selecting a suitable
ratio of the capacitances (i.e for this example a suitable value
for C1/C2 resp. C3/C4) the switch M1 may then be switched to e.g.
be closed if the voltage at the contact 201 is positive relative to
the voltage of contact 203 and open if the voltage at the contact
201 is negative relative to the voltage of contact 203. Similarly
the switch M2 may then be switched to e.g. be open if the voltage
at the contact 201 is positive relative to the voltage of contact
203 and closed if the voltage at the contact 201 is negative
relative to the voltage of contact 203.
[0047] Referring to the example of FIG. 3, the AC capacitive
voltage dividers 210,220 may both comprise a series connection of a
resistor R1, R2 connectable to a respective current terminal of the
transistor and at least two capacitive elements in series. A node
between the capacitive elements can be connected to the control
terminal and form the divided voltage node.
[0048] A capacitive part of the switch M1, M2 to which the AC
capacitive voltage divider 210,220 is connected can be used as a
capacitive element of the AC capacitive voltage. In this example,
the switches M1, M2 are field effect transistors ((FETs), in this
example n-type operated in depletion mode, and the inherent
capacitance between the gate and body of the FET is used as a
capacitive element of the voltage divider, between (in this
example) source contact 203 and the divided voltage node for the
first switch M1 and between drain contact 201 and the divided
voltage node for the second switch M1. Thereby the need for an
additional capacitive element in the circuit is avoided, which
especially allows to reduce the size of the circuit when it is
implemented as an integrated circuit since capacitors occupy a
relative large amount of die surface.
[0049] The AC capacitive voltage divider in the example of FIG. 4
further has a diode D1 resp. D2 which is used as another capacitive
element. The diode is connected with its cathode to the resistor
R1,R2 and with an anode to the divided voltage node. Thus, when
diode D1 connected to the second switch M2 is in reverse, i.e. the
voltage of the source contact 203 is high, the gate of the second
switch will receive a divided voltage V.sub.g2 proportional to the
ratio of the gate capacitance and the diode capacitance, and more
specifically:
V g 2 .varies. C iss 2 C iss 2 + C D 1 V out ##EQU00001##
where V.sub.out is the voltage between drain and source of the
power transistor (i.e. the supply voltage0, and C.sub.iss.sup.2 is
the transistor input capacitance or C.sub.gd.sup.2+C.sub.gs.sup.2
of the second transistor M2. When the diode D1 is in forward mode
(.e. the voltage of the source contact 203 is low) the capacitance
of the diode is high and the gate will receive a voltage below the
threshold voltage. In this respect, it should be noted that the
diode capacitance in reverse is mainly the junction capacitance
whereas in the forward mode the diode capacitance is high because
the junction capacitance increases due to the narrowing of the
depletion layer, and the, far higher, diffusion capacitance adds to
that. Accordingly, the AC capacitive voltage divider has a division
ratio which varies depending on the polarity of the supply
voltage.
[0050] The first switch M1 is operated in a similar manner relative
to the drain contact 201, depending on the mode of the diode D2
(forward or reverse). Thus, when diode D2 connected to the first
switch M1 is in reverse, i.e. the voltage of the drain contact 201
is high, the gate of the first switch M1 will receive proportional
to the ratio of the gate capacitance C.sub.iss.sup.1 and the diode
capacitance C.sub.D2, and more specifically:
V g 1 .varies. C iss 1 C iss 1 + C D 2 V out ##EQU00002##
where the superscript 1 denotes the first switch M1. When the diode
D2 is in forward mode (.e. the voltage of the drain contact 201 is
low) the capacitance of the diode is high and the gate will receive
a voltage below the threshold voltage.
[0051] It will be apparent that a suitable value for the
capacitance of the diodes D1,D2 may be set to choosing a suitable
size of the diodes. In a practical example, with breakdown voltages
roughly similar to FIG. 7, a diode D2 with a breakdown voltage BV
of about 35 V when the power transistor has a drain-source
breakdown voltage BVdss of about 45 V while for a reverse breakdown
voltage BVr of about 30 V diode D1 with a BV of 17 V have shown to
effectively protect the switches M1, M2 while avoiding the diodes
to breakdown.
[0052] Referring to FIG. 4, the transistor body control circuit 200
can further comprise clamp circuitry 230 for clamping the control
terminals of the switches M1, M2 relative to the body terminal B to
below the breakdown voltage of the control terminal. In the example
of FIG. 4, the clamp circuitry 230 comprises Zener diodes Z1,Z2
connected to each other with their anodes, while connected with the
cathode to a respective one of the switches M1,M2. The Zener diodes
Z1,Z2 clamp the control terminal of the switches M1, M2 relative to
their anodes, while the anodes of the Zener diodes Z1,Z2 in turn
are clamped relative to the body contact 202 by a diode D3
connected with its cathode to the body contact and with the anode
to the anodes of the Zener diodes. The breakdown voltage of the
Zener diode plus the forward voltage of the diode D3 are chosen to
be below the breakdown voltage of the switches M1, M2, i.e. in case
the switches are FETs to be below the gate oxide breakdown voltage
BVox. For example, the clamping voltage may be 5 V when the gate
oxide breakdown voltage BVox is 8 V.
[0053] Furthermore, the control terminals of the switches M1, M2
are connected to the body contact 202 through pull-down resistors
R3,R4 which prevents the voltage of the control terminals from
floating and thus ensures that the switches are always in a defined
state.
[0054] Referring now to FIG. 5, the example of an integrated
circuit 240 comprises a bi-directional power transistor and a
transistor body control circuit. In this example, the transistor
body control circuit is implemented as shown in FIG. 4 however it
will be apparent that the integrated circuit may use another type
of transistor body control circuit, such as those of FIG. 2 or 3.
For sake of simplicity, only a single bi-directional power
transistor is shown, but it will be apparent that this may consist
of an arrangement of a plurality of power transistor cells
connected to operate as a single power transistor. As shown in FIG.
5, the bi-directional power transistor has a transistor T which can
be opened or closed via the gate G and inherent diodes in common
anode connecting the current terminals and having their anodes
connected to the body of the transistor. The body of the transistor
T is connected with a body terminal B to the body contact 202 of
the body control circuit, with the first current or drain terminal
D to a drain contact 201 and to an IC contact pin 243 allowing the
first current terminal to be connected to an external power or
signal source. A second current or source terminal S of the
transistor is connected to a source contact 203 of the control
circuit and to an IC contact pin 244 allowing the second current
terminal to be connected to e.g. a load to be driven by the power
transistor 100.
[0055] The integrated circuit 240 shown therein comprises two dice
241,242 on which respective electronic circuitry is provided, and
more specifically comprises a power die 242 on which the
bi-directional power transistor is provided, and a control die 241
on which control circuitry is provided which controls the
bi-directional power transistor. In the example, only a body
control circuit is shown on the control die 241 but other control
circuitry may be present as well, such as control circuitry that
becomes active in specific conditions such as overload, over
temperature, short circuits, etc. and controls the bi-directional
power transistor to operate in a state that avoids permanent damage
to the transistor. The AC capacitive voltage divider allows to use
a control die with a lower maximum gate-source voltage than the
maximum voltage across the source-drain in both directions of the
power transistor while avoiding the risk of damage to the switches
M1,M2 and accordingly enables a wide variety of choice for the
characteristics of the control die 241.
[0056] In the foregoing specification, the invention has been
described with reference to specific examples of embodiments of the
invention. It will, however, be evident that various modifications
and changes can be made therein without departing from the scope of
the invention as set forth in the appended claims and that the
claims are not intended to be limited to the specific examples.
[0057] For example, the vertical trench 106 can be implemented in
any manner suitable for the specific implementation and have any
suitable shape, size and configuration. For instance, trench can be
without a shield plate or the trench can be provided with more
shield plates, for example with a shield plate above the gate.
Also, in thee example of FIG. 1 the first and second vertical
trenches 106 are very deep trenches which extend from the top of
the layer stack into the first current terminal 105, i.e. the
substrate in that example. However the vertical trenches can be
less deep, and for example extend until the top surface of the
first current terminal 105, e.g the bottom of the trench touching
the substrate on which the layer stack is provided. Likewise, the
vertical trenches can terminate slightly above the first current
terminal 105, for example at a vertical position closer to the
substrate top surface than to the middle of the vertical drift
layer 106, or expressed mathematically dtrench>0.75 ddrift,
where dtrench represents the depth of the trench in the drift
region, and ddrift the vertical thickness of the drift region.
Also, the vertical trenches can be filled, e.g. with the electrodes
and dielectrics in any suitable manner.
[0058] Furthermore, the semiconductor substrate described herein
can be any semiconductor material or combinations of materials,
such as gallium arsenide, silicon germanium, silicon,
monocrystalline silicon, the like, and combinations of the
above.
[0059] Moreover, the terms "front," "back," "top," "bottom,"
"over," "under" and the like in the description and in the claims,
if any, are used for descriptive purposes and not necessarily for
describing permanent relative positions. It is understood that the
terms so used are interchangeable under appropriate circumstances
such that the embodiments of the invention described herein are,
for example, capable of operation in other orientations than those
illustrated or otherwise described herein. For example, the
transistor shown in FIG. 1 may be used in an up-side down or
rotated position relative to that shown in the drawing without
affecting its operation.
[0060] Also for example, in one embodiment, the illustrated
examples can be implemented as circuitry located on a single
integrated circuit or within a same device. For instance, the power
die 242 and the control die 241 of the example of FIG. 4 can be
implemented as separate dice in a single integrated circuit
package, connected using e.g. bondwires or other connecting
techniques. Alternatively, the examples can be implemented as any
number of separate integrated circuits or separate devices
interconnected with each other in a suitable manner. For example,
the control circuitry on the control die 241 can be provided
outside an integrated circuit package in which the power die 242 is
present or the control circuitry composed of at least two separate
components, e.g. mounted in a printed circuit board.
[0061] However, other modifications, variations and alternatives
are also possible. The specifications and drawings are,
accordingly, to be regarded in an illustrative rather than in a
restrictive sense.
[0062] In the claims, any reference signs placed between
parentheses shall not be construed as limiting the claim. The word
`comprising` does not exclude the presence of other elements or
steps then those listed in a claim. Furthermore, the terms "a" or
"an," as used herein, are defined as one or more than one. Also,
the use of introductory phrases such as "at least one" and "one or
more" in the claims should not be construed to imply that the
introduction of another claim element by the indefinite articles
"a" or "an" limits any particular claim containing such introduced
claim element to inventions containing only one such element, even
when the same claim includes the introductory phrases "one or more"
or "at least one" and indefinite articles such as "a" or "an." The
same holds true for the use of definite articles. Unless stated
otherwise, terms such as "first" and "second" are used to
arbitrarily distinguish between the elements such terms describe.
Thus, these terms are not necessarily intended to indicate temporal
or other prioritization of such elements The mere fact that certain
measures are recited in mutually different claims does not indicate
that a combination of these measures cannot be used to
advantage.
* * * * *