# Large Area Silicon Carbide Vertical JFETs for 1200 V Cascode Switch Operation

Authors: Victor Veliadis; Ty McNutt; Megan Snook; Harold Hearne; Paul Potyraj; Jeremy Junghans; Charles Scozzie
Journal: International Journal of Power Management Electronics
 (2008)

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## Abstract

SiC VJFETs are excellent candidates for reliable high-power/temperature switching as they only usepn junctions in the active device area where the high-electric fields occur. VJFETs do not suffer from forward voltage degradation, exhibit excellent short-circuit performance, and operate at 300∘C. 0.19 cm2 1200 V normally-on and 0.15 cm2 low-voltage normally-off VJFETs were fabricated. The 1200-V VJFET outputs 53 A with a forward drain voltage drop of 2 V and a specific onstate resistance of 5.4 mΩ cm2. The low-voltage VJFET outputs 28 A with a forward drain voltage drop of 3.3 V and a specific onstate resistance of 15 mΩ cm2. The 1200-V SiC VJFET was connected in the cascode configuration with two Si MOSFETs and with a low-voltage SiC VJFET to form normally-off power switches.  At a forward drain voltage drop of 2.2 V, the SiC/MOSFETs cascode switch outputs 33 A. The all-SiC cascode switch outputs 24 A at a voltage drop of 4.7 V.


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## Body

## 1. Introduction

Wideband gap semiconductors like silicon
carbide (SiC) and the III-IV nitrides are currently being developed for high-power/temperature
applications. Silicon carbide (SiC) is ideally suited for power-conditioning
applications due to its high saturated drift velocity, its mechanical strength,
its excellent thermal conductivity, and its high critical field strength.  For power devices, the tenfold increase in
critical field strength of SiC relative to Si allows high-voltage blocking
layers to be fabricated significantly thinner than those of comparable Si
devices.  This reduces device onstate resistance,
and the associated conduction and switching losses, while maintaining the same
high-voltage blocking capability.  Figure1 shows the theoretical specific onstate resistance of blocking regions
designed for certain breakdown voltages in Si and 4H-SiC, under optimum punch-through
conditions [1].  The specific onstate resistance
of 4H-SiC is approximately 400 times lower than that of Si at a given breakdown
voltage.  This allows for high current operation
at relatively low-forward voltage drop.  In
addition, the wide band gap of SiC allows operation at high temperatures where
conventional Si devices fail. Forward voltage drop versus current density of
Northrop Grumman’s all-SiC vertical junction field effect transistor-(VJFET-) based cascode switch, and those of commercial Si MOSFET, Si IGBT, and Si
CoolMOS switches are
shown in Figure 2. The SiC switch has a lower voltage drop at a given current
density, even at the elevated temperature of 150°C. The low-loss and the high-temperature
operational capabilities of SiC devices can potentially eliminate the costly
cooling systems present in today’s Si based power electronics.Figure 1
Theoretical specific onstate resistance of blocking regions designed for certain breakdown voltages in Si and 4H-SiC, under optimum punch-through conditions.Figure 2
Forward voltage drops versus current densities of Northrop Grumman’s all-SiC VJFET-based cascode switch and of commercial Si MOSFET, Si IGBT, and Si CoolMOS switches. The SiC switch has a lower measured voltage drop at a given current density, even at the elevated temperature of 150°C.Presently, several SiC devices are being developed for 600 V
(1200 V rating) power switching applications. SiC MOS-based devices show
promise as normally-off power switches but suffer from low-MOS mobility and
native oxide issues that limit reliable operation to below 175°C [2].  Furthermore, several temperature-dependant
factors result in a decrease of the SiC MOSFET threshold voltage with
temperature.  This may lead to unwanted
MOSFET turnon at temperatures over 200°C.The SiC bipolar junction transistor is another normally-off
power switching candidate.  However, as
with all SiC bipolar devices, its long term performance deteriorates due to
forward bias voltage degradation [3].
Also, the BJT is a current controlled device that can require
substantial base drive current [4].The SiC VJFET is a very promising candidate for high-power/temperature
switching as it only usespn junctions in the active device area, where the high-electric fields occur, and
can therefore fully exploit the high-temperature properties of SiC in a gate
voltage controlled switching device.
VJFETs for high voltage applications are typically normally-on devices,
and an all-SiC normally-off power switch can be implemented by combining a high-voltage
normally-on VJFET with a low-voltage normally-off VJFET in the cascode
configuration.In this paper, we review the reliability and high
temperature characteristics of1.25×10−3 cm2 area
unipolar ion-implanted SiC VJFETs. Subsequently, we present the forward current
and blocking voltage characteristics of 0.19 cm2 area 1200 V normally-on and 0.15 cm2 area low-voltage normally-off SiC VJFETs.  The 0.19 cm2 1200-V VJFETs have
been connected in the cascode configuration with Si MOSFETs and 0.15 cm2 low-voltage SiC VJFETs to form normally-off power switches.


## 2. SiC VJFET Structure

A cross-section schematic of a high-voltagep+ ion-implanted 4H-SiC VJFET is shown
in Figure 3.Figure 3
Simplified cross-section schematic of a normally-on ion-implanted SiC VJFET. The layer dimensions are not to scale.The channel layer is
doped to low 1016 cm−3, and the drift layer is doped to
mid 1015 cm−3. To
ensure >1200 V blocking, a 12 μm drift layer thickness is
used.  The substrates and epitaxy are
grown by commercial vendors. In the on-state, majority carriers (electrons)
flow vertically from source to drain.  To
control the current through the device, the gates are subjected to a voltage,
which adjusts the width of the depletion regions between the p-type gates in the n-type channel. In normally-off VJFETs, the p+ implant depletion regions must overlap at 0 V gate bias.
Reducing the gate-to-gate spacing leads to higher depletion region overlap and
the VJFET blocks increasingly higher drain voltages. For larger gate-to-gate
separations, the 0 V gate bias depletion regions do not overlap and the VJFET
is normally-on.As the normally-off VJFET need only block low voltages in
cascode switching operation, its drift layer is approximately 2.5μm to minimize onstate resistance
and losses.To ensure high-voltage operation  with minimum associated onstate
resistance, a robust, self-aligned, multiple floating guard-ring edge
termination was designed and fabricated.
The high-voltage VJFETs  (12μm drift layer doped at mid 1015 cm−3)
exhibited breakdown voltages of up to 2022 V, which corresponds to a record 93%
of the calculated 4H-SiC material limit [5].  The
measured specific onstate resistance was 2.1 mΩ cm2, a value
close to the theoretical limit of the 4H-SiC material, Figure 1 [6].Initially, limited by the low 4H-SiC material quality, and
the high micropipe defect density in particular, “small”1.25×10−3 cm2 area VJFETs were fabricated and paralleled to increase current
output.  The small area VJFET
manufacturing was optimized and high yield with excellent wafer parameter
uniformity were achieved [7, 8].


## 3. SiC VJFET Cascode Reliability and High-Temperature Operation

To assess the reliability of the 1.25× 10−3 cm2 area VJFETs, the forward voltage drops across the VJFET’s
gate-to-drain and gate-to-source pn junctions were measured at constant junction current densities of 100 A/cm2.
After 500 hours of continuous room
temperature operation under this DC bias condition, no measurable forward
voltage drift was detected [9].Additionally, 1200 V
SiC VJFETs were subjected to short-circuit testing to determine the
survivability time prior to the onset of catastrophic device failure.  The SiC VJFETs exhibited hold-off times in
excess of 1 millisecond, a sixfold improvement over Si
MOSFETs of similar voltage rating [9].To implement all-SiC
normally-off power switches, high voltage normally-on and
low-voltage normally-off VJFETs were connected in the cascode configuration.  A schematic of the cascode switch and its
constituent VJFETs are shown in Figure4.
The switches are voltage-driven, and have exhibited excellent power
switching characteristics including low onstate resistance, high speed, and low
switching losses [10].  A typical breakdown
voltage curve of a normally-off (1250 V at Vgs= 0 V) all-SiC cascode
switch is shown in Figure 5.Figure 4
Schematic of Northrop Grumman’s all-SiC power switch consisting of high-voltage normally-on and low-voltage normally-off  VJFETs connected in the cascode configuration.Figure 5
An all-SiC cascode switch blocking 1250 V when normally-off (gate-to-source biasVgs = 0 V).The cascode switch’s internal PiN diode has exhibited a very
fast 100 nanoseconds reverse recovery time, Figure6, which can potentially eliminate
the need for external diodes in power switching circuits [11]. A half-bridge
inverter was demonstrated using SiC cascode switches with no external antiparallel
diodes.  The inverter consisted of
high-side and low-side cascode switches that were pulse-width modulated from a
500 V bus to produce a 60 Hz sinuso at the output [11].Figure 6
The very fast 100 nanoseconds reverse recovery time of the cascode switch’s internal diode.The high-temperature operational capability of SiC VJFETs is
crucial in eliminating costly cooling in power systems.  To investigate the effect of temperature on
blocking voltage, the blocking characteristics of a normally-off VJFET were
measured at 25°C and 300°C junction temperatures.  At a given gate-to-source bias Vgs,
the blocking voltage decreases with temperature as shown in Figure 7.  This is in agreement with theory as the
reverse-bias drain-to-source leakage current increases with temperature, due to
the higher number of thermally generated carriers. The measurements were
performed using a Tektronix 371 A curve tracer. As the 300°C measurement setup
required modification of the curve tracer’s looping compensation, the 25°C and
300°C leakage current levels cannot be directly compared.Figure 7
Blocking voltage characteristics of the cascode switch’s Noff VJFET at 25°C (blue line), and 300°C (red line).  The gate-to-source bias is Vgs = 0 V.The
effect of temperature on the onstate drain current of the cascode switch is
illustrated in Figure8 at junction temperatures of 25°C and 300°C, for
gate-to-source biases of 0 to 3 V in steps of 0.5 V.Figure 8
Onstate drain current curves of a 1200 V SiC cascode switch at 25°C (blue) and 300°C (red) junction temperatures.  The gate-to-source bias ranges from 0 to 3 V in steps of 0.5 V.As VJFETs do not have gate oxides, they reliably operate at junction
temperatures of 300°C.  The measured drop
in current with increasing temperature in Figure 8 is in good agreement with
the theoretical reduction in SiC electron mobility.  As the channel and drift regions are designed
independently in VJFETs, the onstate resistance can be tuned for maximum
current output [12].


## 4. Large Area VJFETs

To meet the current handling requirements of modern power conditioning
systems, 1200 V normally-on VJFETs of 0.19 cm2 area (4.4 mm × 4.33 mm) were manufactured. Excluding the bonding pads and edge termination
region, the active area as defined by the pn-junctions is 0.143 cm2.
A photograph of large area VJFETs fabricated on a 3-inch 4H-SiC wafer is shown
in Figure 9.Figure 9
0.19 cm2 VJFETs fabricated on a 3-inch 4H-SiC wafer.Large area VJFETs were soldered into packages
and wire bonded using thick aluminum wires, Figure10.Figure 10
A 0.19 cm2 area VJFET soldered into a package and wire bonded using 10 mil thickness aluminium wires.To attain the desirable 1200 V blocking voltage capability, a 12μm drift layer with a doping concentration in the mid 1015 cm−3 was used.  The blocking voltage characteristics
of the 0.19 cm2 VJFET were measured with a Tektronix 371 A curve
tracer and are shown as a function of gate voltage in Figure 11.  At a gate-to-source bias of −24 V, the VJFET blocks 1680 V at  a drain current density of 1 mA/cm2.Figure 11
Blocking voltage characteristics of a 0.143 cm2 active area VJFET at gate biases of −4 V to −24 V, in steps of −2 V.  At a gate-to-source bias of −24 V and a drain current density of 1 mA/cm2, the VJFET blocks 1680 V.Room-temperature pulsed onstate drain current measurements
were performed on packaged 1200-V VJFETs (single chip), at a gate bias range of
0 to 3 V in steps of 0.5 V, Figure12.  At a gate bias of 2.5 V, the VJFET’s drain
current is 40 A with a forward drain voltage drop of 1.5 V and a specific onstate
resistance of 5.4 mΩ cm2.
The current density is 280 A/cm2, and the power density is
420 W/cm2. The gate current
at Vgs= 2.5 V is 12 mA, which results in a transistor current gain
of 3333.  At the same gate bias of 2.5 V,
the VJFET outputs a drain current of 53 A at a forward drain voltage drop of 2 V.  The current density is 371 A/cm2,
and the power density is 741 W/cm2, which is within the heat
load capability of advanced water-cooled packages [13].  The specific onstate resistance is
5.4 mΩ cm2, and the transistor current gain is 4417.  A drain current of 100 A at a forward drain
voltage drop of 4.8 V is measured at a gate bias of 2.5 V (gate current of 12 mA).Figure 12
Onstate drain current characteristics versus drain voltage characteristics of a high-voltage 0.143 cm2 active area packaged VJFET, at a gate-to-source bias range of 0 to 3 V in steps of 0.5 V.  At a gate-to-source bias of 2.5 V (with a gate current of 12 mA), the VJFET outputs 53 A and 100 A at forward drain voltage drops of 2 V and 4.8 V, respectively.Finally, a record high onstate current of
161 A is measured at a drain voltage drop of 16 V, for a gate bias of 3 V.
Although biasing the gatepn junction
above its ~2.7 V built-in potential increases drain current, it can
seriously degrade current gain. Therefore, in practical power switching
circuits, the gate driver biases the pn junction below its built-in potential.As pointed out earlier and evident from the
blocking voltage characteristics presented in Figure11, the 0.143 cm2 1200-V VJFET is designed normally-on to minimize onstate resistance and maximize
current gain.  Presently, inherently safe
operation gate-drive circuits are being developed to utilize normally-on SiC
VJFETs as power switches [14, 15]. However, most circuit designers require a
normally-off SiC-based switch as a direct replacement to silicon MOSFETs or IGBTs.  Connecting a high-voltage, low onstate resistance
SiC VJFET in the cascode configuration with a low-voltage silicon power MOSFET
creates a normally-off power switch with a control characteristic similar to a
silicon MOSFET or IGBT [16].  The cascode
circuit diagram is similar to the one that appears in Figure 4, with the
low-voltage normally-off part being a silicon MOSFET.In the cascode configuration and with the
MOSFET being biased in the on state, the 1200-V SiC VJFET and the Si MOSFET operate
in series, with the gate of the 1200-V SiC VJFET automatically biased at a voltage value equal to the negative of the drain-to-source voltage drop across
the low-voltage Si MOSFET.  In the
offstate, the MOSFET’s drain-to-source blocking voltage provides the necessary
negative gate bias to pinch off the 1200-V SiC VJFET.  After the VJFET is pinched off, further
increase in reverse voltage at the drain of the cascode is supported by the 1200-V
SiC VJFET.The 1200-V SiC VJFET, whose onstate characteristics
appear in Figure12, was connected in the cascode configuration with two
paralleled commercial 75-V/97-A rated silicon MOSFETs.The onstate drain current characteristics versus drain voltage of the resulting cascode switchwere measured at MOSFET
gate biases of 0 V, 5 V, 10 V, and 15 V, Figure 13.  At a MOSFET gate-to-source bias of 15 V, the cascode
switch outputs 33 A at a forward drain voltage drop of 2.2 V.  Under these biasing conditions, the gate of
the 1200-V SiC VJFET experiences a bias equal to the negative of the measured 0.2 V
drain-to-source voltage drop across the MOSFETs. The forward voltage drop
across the 1200-V SiC-VJFET component of the cascode switch is 2 V. Its current
and power densities are 231 A/cm2 and 462 W/cm2,
respectively.Figure 13
Onstate drain current characteristics of a switch consisting of a single 0.143 cm2 active area 1200-V SiC VJFET connected in the cascode configuration with two paralleled 75-V/97-A commercial silicon MOSFETs. At a MOSFET gate-to-source bias of 15 V, the cascode switch outputs 33 A at a forward drain voltage drop of 2.2 V.The SiC-VJFET/Si-MOSFET normally-off power switch exploits
the high-blocking voltage with low onstate resistance capability of the 1200-V SiC
VJFET.  However, the Si MOSFET sets an
upper limit on temperature operation and introduces gate oxide capacitance.  To overcome these limitations and exploit the
high-temperature capability of SiC, a 0.15 cm2 SiC VJFET was fabricated
to be used as the low-voltage normally-off component of the cascode switch (Figure 4).  Excluding the bonding pads and edge
termination regions, the low-voltage normally-off VJFET’s pn-junction active
area is 0.13 cm2.  Its blocking
voltage characteristics at different gate biases are demonstrated in Figure 14.  The device has a thin 2.5 μm drift layer to minimize onstate
resistance and losses, and blocks 44 V at zero gate-to-source bias with a
drain current density of 1 mA/cm2.Figure 14
Blocking voltage versus gate bias characteristics of a 0.13 cm2 active area low-voltage normally-off SiC VJFET.  At a gate-to-source bias of 0 V and a drain current density of 1 mA/cm2, the VJFET blocks 44 V (normally off to 44 V).Room
temperature onstate drain current measurements were performed on the low-voltage
normally-off 0.15 cm2 VJFETs at a gate bias range of 0 to 3.5 V, Figure 15.Figure 15
Onstate drain current characteristics versus drain voltage characteristics of a 0.13 cm2 active area low-voltage normally-off packaged VJFET. Gate biases of 0 to 3.5 V were applied, in steps of 0.5 V. At a gate bias of 2.5 V, the VJFET outputs 28 A at a forward drain voltage drop of 3.3 V.At a gate bias of 2.5 V, the VJFET’s drain
current is 28 A with a forward drain voltage drop of 3.3 V and a specific onstate
resistance of 15 mΩ cm2. The current
and power densities are 215 A/cm2 and 711 W/cm2,
respectively. A drain current of 50 A at a forward drain voltage drop of 4 V is
measured at a gate bias of 3.5 V.  As the
low-voltage VJFET is designed for normally-off operation, it is more resistive
than the normally-on 1200-V VJFET of Figure 12, and consequently outputs less
current under similar drain biasing conditions.To implement a 1200 V all-SiC normally-off
power switch similar to the one schematically shown in Figure4, a single
0.143-cm2 1200-V SiC VJFET was connected in the cascode
configuration with a single 0.13 cm2 low-voltage SiC VJFET. Room
temperature onstate drain current measurements were performed at cascode gate
biases of 0 to 3.5 V, Figure 16.Figure 16
Onstate drain current characteristics versus drain voltage characteristics of an all-SiC cascode switch consisting of a low-voltage 0.13 cm2 active area VJFET and a 1200-V 0.143 cm2 active area VJFET.  The measurements were taken at cascode gate biases of 0 to 3.5 V, in steps of 0.5 V. At a cascode gate bias of 2.5 V, the switch outputs 24 A with a forward drain voltage drop of 4.7 V.At a cascode gate bias of 2.5 V, the all-SiC
cascode switch outputs 24 A at a forward drain voltage drop of 4.7 V. At
this biasing condition, drain voltages of 2.8 V and 1.9 V are dropped across
the 1200-V and low-voltage VJFETs, respectively. The current and power
densities are 168 A/cm2 and 470 W/cm2 for the 1200-V
VJFET, and 185 A/cm2 and 351 W/cm2 for the low-voltage
VJFET. In forward cascode operation, the gate of the 1200-V VJFET is biased at
a voltage value equal to the negative of the drain-to-source voltage drop
across the low-voltage VJFET (Figure 4). Thus, the gate of the 1200-V VJFET is
biased at −1.9 V when 4.7 V are dropped across the cascode switch.In the SiC-VJFET/Si-MOSFETs cascode, a current of 33 A
passes through the switch at a forward drain bias of 2.2 V, Figure13.  This is higher than the 24 A current of the
all-SiC cascode under similar 1200-V VJFET power density biasing conditions.
The mature Si wafer technology allows the fabrication of MOSFETs of a larger
size, which minimizes their resistance and voltage drop. Consequently, at a
power density of about 470 W/cm2 on the 1200-V VJFET of the cascode,
the gate of the 1200-V SiC VJFET is biased at the −0.2 V MOSFET voltage drop in
the SiC-VJFET/Si-MOSFETs case, and at the −1.9 V low-voltage VJFET voltage drop
in the all-SiC cascode case. This difference in 1200-V VJFET gate bias is
responsible for the disparity in cascode current outputs under similar power
density biasing conditions.  Paralleling
multiple low-voltage SiC VJFETs will minimize the voltage drop on the
low-voltage portion of the all-SiC cascode switch and lead to higher cascode
current output.Operating the 1200-V SiC VJFET as a switch in an
inherently safe gate-drive circuit eliminates the need for a low-voltage normally-off
SiC VJFET cascode component.  Moreover,
in a 1200-V normally-on VJFET
switch a gate bias of 2.5 V (12 mA gate current) can be applied, which allows
for high-current/high-gain operation with low onstate resistance, Figure12.In a cascode switch, the gate of the
high-voltage VJFET is biased at a voltage value equal to the negative of the
drain-to-source voltage drop across the low-voltage component. Hence, in forward cascode operation,
the high-voltage VJFET’s gate is always at a negative bias, which lowers current
output and increases onstate resistance.  To visualize the impact of
negative gate bias on the 1200-V VJFET of the cascode, the onstate drain
current characteristics of the 0.143-cm2 1200-V VJFET are plotted at
a gate bias range of 0 to −4.5 V in steps of 0.5 V, Figure 17.Figure 17
Onstate drain current versus drain voltage characteristics of a  1200-V 0.143-cm2 active area packaged VJFET at a gate bias range of 0 to −4.5 V, in steps of 0.5 V.   At a gate-to-source bias of −4.5 V, the VJFET is pinched off and negligible current flows through its drain.It is evident from Figure17 that 1200-V
VJFET current output decreases nonlinearly with negative bias on its gate.  Thus, the voltage drop across the low-voltage
cascode component limits the current output of the entire cascode by reverse
biasing the gate of the 1200-V VJFET.
At −4.5 V gate bias, the VJFET turns off and negligible current flows
through its drain.


## 5. Conclusion

The SiC VJFET is a very promising candidate for reliable high-power/temperature
switching as it only usespn junctions in the active device area where the high-electric fields occur.  VJFETs do not suffer from forward voltage
degradation, and exhibit holdoff times higher than those of their Si
counterparts in short circuit testing.  The
VJFET based all-SiC normally-off cascode switch’s internal diode has exhibited
a very fast 100 nanoseconds reverse recovery time, eliminating the need for
antiparallel diodes in power switching circuits. VJFETs were successfully operated
at 300°C junction temperature. The measured reduction in onstate current is in
good agreement with the theoretical reduction in SiC electron mobility.To meet the current handling requirements of modern power
conditioning systems, 1200 V normally-on VJFETs of 0.19 cm2 and low-voltage
normally-off VJFETs of 0.15 cm2 areas were fabricated.  At a gate bias of 2.5 V, the 1200-V VJFET outputs
53 A with a forward drain voltage drop of 2 V and a specific onstate resistance
of 5.4 mΩ cm2.  The low-voltage VJFET’s drain current is 28 A,
at a gate bias of 2.5 V, with a forward drain voltage drop of 3.3 V and a
specific onstate resistance of 15 mΩ cm2.A 1200-V SiC VJFET was connected in the cascode
configuration with two commercial Si MOSFETs to form a normally-off power switch.
At a MOSFET gate-to-source bias of 15 V,
the cascode switch outputs 33 A at a forward drain voltage drop of 2.2 V.  To fully exploit the high-temperature
capability of SiC in a normally-off power switch, a 0.15 cm2 low-voltage normally-off SiC VJFET was connected with a 0.19 cm2 1200-V normally-on VJFET in the cascode configuration.  At a forward drain voltage drop of 4.7 V, the
all-SiC cascode switch outputs 24 A at 2.5 V cascode gate bias.  Operating
the 1200 V normally-on SiC VJFET as a switch in an inherently safe gate-drive
circuit eliminates the need for a low-voltage normally-off SiC VJFET cascode
component, and enables high-current/high-gain operation with low voltage drop and
low onstate resistance.



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*Source: 523721-2008-06-09.xml*