Characterisation
of 4H-SiC Schottky Diodes for IGBT Applications
C. M. Johnson*, M. Rahimo**, N. G. Wright*, D. A. Hinchley**, A. B. Horsfall*, D. J. Morrison*,
A. Knights***
*Department of Electrical and
Electronic Engineering
University of Newcastle
Newcastle-upon-Tyne NE1 7RU
UK.
**Semelab
Coventry Road
Lutterworth
LeicestershireLE17 4JB
UK.
Abstract - Si fast recovery diodes currently limit the
performance of many IGBT powered systems. In this paper SiC
Schottky diodes are proposed as an alternative technology. High
current SiC devices are achieved by parallel connection of a
large number of small elements. The static and dynamic
performance of the SiC Schottky diodes is evaluated and
comparisons made with Si PiN diodes at currents of up to 20A
and de link voltages of up to 600V. The results demonstrate the
effectiveness of the SiC devices in reducing the overall system
losses and the levels of EMI generated by switching transitions.
I.
INTRODucT1ON
Fast recovery diodes play an important role in most power
electronic circuits as freewheeling and/or snubber
components. In high frequency applications careful choice of
diode is required in order to reduce the overall losses of the
circuit and to prevent any failure mechanisms that might
occur during the diode switching transients.
Since the introduction of the Insulated Gate Bipolar
Transistor IGBT, diodes have been required to switch at
higher frequencies (>lOkHz) whilst enduring relatively high
voltage (> 1000V) and current (> 100A) levels. This has
presented new challenges to the diode designer and, as a
result, many state of the art conventional and novel design
techniques have been reported and implemented on an
industrial scale in recent years. Currently, ultra-fast and
hyper-fast Si PiN diodes are available that have been
optimised for minimum static and dynamic losses with high
immunity against diode snappiness and dynamic avalanching
under high stress conditions. In spite of these developments,
diode reverse recovery losses and associated failure modes,
including snappy behaviour and dynamic avalanching,
present a major limitation in the performance of many IGBT
circuits. It is now widely accepted that the optimisation of Si
fast recovery diodes might have reached its limit in terms of
reducing the overall switching losses.
For low voltage applications (<300V), Schottky Si or
GaAs diodes are currently the preferred choice. As majority
carrier devices, Schottky diodes exhibit very low switching
losses and positive temperature coefficient (which makes
them suitable for parallel operation) while maintaining
acceptable levels of leakage current and forward voltage
***School of Electronic Engineering
Information Technology and
Mathematics
University of Surrey
GuildfordGU25XH
UK.
drops. However, at higher voltages, Schottky diodes exhibit
high on-state losses loosing their advantage when compared
to Si PIN diodes.
The potential benefits of SiC as a material for power
semiconductors have been known for some time [1].
However, it is only recently that the quality of SiC has
improved sufficiently for it to be used to fabricate practical
high power devices. High voltage (>600V) Schottky diodes
have been made feasible by utilising the high critical field of
SiC to achieve thin drifi regions. This yields low on-state
losses whilst retaining the advantages of Schottky diodes
with regard to the switching performance. Table I shows a
comparison of static and dynamic characteristics for a state of
the art 1.6kV Si PiN diode and the expected performance of
an equivalent SiC Schottky diode.
Several groups have already reported excellent results for
SiC Schottky diodes [2, 3] and related JBS/MPS structures
[4]. In this paper results are presented comparing high
voltage SiC Schottky diodes with hyper-fast Si PiN diodes
rated at 600V and 1200V. The static performance of the SiC
Schottky diode is evaluated over the temperature range 25C
to 300C and dynamic tests are performed on both the SiC
diode and Si PiN diode at temperatures of 25C and 125C.
Conclusions are drawn concerning the likely benefits to be
derived fi-om the use of SiC Schottky diodes in power
electronic systems<
II. DEVICE FABRICATION
Schottky diodes were fabricated on n-type, 4H-SiC
epitaxial material supplied by Cree Research. The epi-layers
were grown on heavily doped n-type substrates to a thickness
of 10~m with nitrogen doping to 5x10’5cm-3. A Ni ohmic
contact was made to the backside of the samples, while the
topside was patterned with an array of 49 Ti Schottky
contacts, 312~m in diameter. Blanket Ar implantation was
used to form an edge termination, followed by annealing at
600C for 1 minute [5].
0-7803-6404-X/00/$10.00 (C) 2000
SUMMARY
TABLE I
OF EXPECTED PERFORMANCE FOR A 1.6KV SIC SCHOTTKY DIODE AND A 1.6KV SIPIN DIODE
Static Characteristics
On-State Voltage @ 25(;
Temperature Coefflcierut
Leakage Current @ 125C
Max. Junction Tempera{ure
Voltage Rating
SiC Schottky Diode
Ni :1.5 V @ 250A/cm2
Ti: 1,0 V @ 250 A/cm2
Always +ve
10 mA/cm2 @ looov/ 125C
250C
Suitable for (300V - 1600)V
Dynamic Characteristics
Reverse Recove~ Losses
IGBT turn-on Switching Losses
Electromagnetic Interference EMI
Stray Inductance Dependence
Forward Current Dependence
Temperature Dependence
Dynamic Avalanching
Snappy Recovery
SiC Schottky Diode
Small Capacitive Effect
Low
Low
Low
Low
None
None
None
Si PIN Diode
2.0 v @ looA/cm2
-ve or +ve
0.5 mA/cm2 @ 1000V / 125C
150C
Only Suitable for 1600V
Si PIN Diode
High
High
High @ high di/dt
High
High
High
YES @ high di/dt (Limiting)
YES @ high di/dt (Limiting)
terminations [6]. Above this point the characteristic becomes
superlinear with a sharp breakdown occurring at 1530V
(typical breakdown voltages, measured at room temperature
were between 1400V and 1600V for the individual diodes on
each sample). The reverse current at 1000V and room
temperature is below 30~A (equivalent to 1.7mA/cm2),
which compares favorably with existing Si PiN diode
technology. At 125C the leakage current increased to 220pA.
87635-
Fig. 1: Composite SiC Schottky diode mounted carDCB substrate showing
bond wires.
243-
The samples were attached to a Ni plated DCB substrate
using a furnace die attach with Pb-AgSn solder. Individual
diodes on each sample were tested under forward and reverse
bias conditions and selected diodes were bonded using a
1.25mil diameter Al wire. A total of 23 diodes, with a total
active area of 1.68mm2, were selected to form the diode used
in the tests (Fig. 1).
III. STATIC CHARACTERISTICS
21T ---T
Oannn
o
0.5
(Fig.
3),
which
is typical
of Ar
——~
1.5
2
2.5
3
W (v)
Fig. 2: Forward IV characteristics for composite SiC Schottky diode.
Composite diode static I-V characteristics were determined
(Figs. 2 and 3). The upper forward current limit of 8.4A at
V&2.7V is equivalent to a cathode current density of around
500Alcm2 and corresponds to the onset of significant selfheating effects in the SiC die. An equivalent specific series
resistance of 4.6mQcm2 was extracted based upon the total
active area of 1.68mm2. Under reverse bias the current shows
a clear linear dependence on voltage for voltages below
1000V
1
implanted
edge
0-7803-6404-X/00/$10.00 (C) 2000
Diode Voltage (V)
-1200
-1600
-600
———-——
31
-400
0
-—-—t
—-–—
0
2
-50
z
e
-100 ?!
.;
-150 z
~
z
-200 $
E
-250
-5
-6
-300
0.00
0.50
1 .(20
4.50
2.00
Vd (Volts)
Fig. 3: Reverse IV characteristics for composite SiC Schottky diode.
Further static measurements were made on the bare SiC
die, at temperatures between 25C and 400C, using a hot
chuck and probing station. The device dies were found to be
stable under both forward and reverse bias conditions with no
change in characteristics observed after thermal cycling over
the fill temperature range.
Under reverse bias and low forward bias (< 0.5V)
conditions (Fig. 4) the diode current shows a clear
exponential dependence with a coefficient of around 0.02K-’.
At moderate forward bias (Fig. 5) the behaviour is typical of
thermionic emission, whilst for the highest bias levels the
characteristic becomes resistive. The temperature dependence
of the diode equivalent series resistance is exponential with a
coet%cient of 0.006 lK-’.
1.00E+OO r–—–—
1.00E-01
J
1.00E-021
.2
~
8
z
o
k
+
1.00E-03
IV. DYNAMIC CHARACTERISTICS
Dynamic tests were performed using an inductively loaded
chopper circuit (Fig. 6). The IGBT gate drive impedance and
gate drive voltage level were varied to allow different reverse
di/dt to be applied to the diode under test. Tests were
performed on the composite SiC Schottky diode and
commercial ultra-fasthyper-fast Si PiN diodes at current
levels up to 20A and rail voltage levels of up to 600V.
--—
‘E
$
Fig. 5: Effect of temperature on forward characteristic of SiC Schottky
diode. Temperature increases in direction shown by arrows from 75C to
300C in increments of25C.
+
+
+
+
+
+
+
+
+
+
❑
❑
❑
❑
1.00E-04
❑
❑
☞
1Oov
❑
0.2V
❑
5
1.00E-05
❑
❑
❑
1.00E-06
@-R--o
---~-—m
50
100
150
200
250
300
Fig. 6: Inductively loaded chopper test circuit.
Measurement Temperature (C)
Fig. 4: Effect of temperature on SiC Schottky diode leakage current. Open
squares – forward bias of 0.2V, crosses – reverse bias of 100V.
0-7803-6404-X/00/$10.00 (C) 2000
A. Comparison with a 1200E 25A hyper-fast Si PiN diode
o
A 1200V, 25A IGBT was employed as the switching
device and the gate drive adjusted to achieve a dildt of
approximately 500Alps, Figs. 7 and 8 compare, respectively,
the diode current and voltage for the SiC Schottky diode and
a 1200V hyper-fast Si PiN diode during reverse recovery
from a forward current of roughly 20A to a dc level of 600V.
Both diodes exhibit a relatively soft reverse recovery without
significant ringing. It is clear that the SiC diode has a
superior reverse recovery characteristic, exhibiting just 110/0
of the recovered charge and 15°/0of the switching loss of its
Si counterpart. Comparison of the IGBT turn-on losses (Fig.
9) reveals a significant reduction in both the peak switching
power (34Yo) and cumulative loss (31%) in the case of the
SiC Schottky device. A summary of the reverse recovery
parameters is presented in Table II.
-200
--300
~
u
‘ -400
-500
-600
200
100
o
B. Comparison with a 600 V 8A hyper-fast Si PiN diode
400
300
500
Time (ns)
In this case a 1200V, lOOA IGBT was employed as the
switching device and the gate drive adjusted to achieve a
di/dt of approximately 500A/I.ts. Figs. 10 and 11 compare,
respectively, the diode current and voltage for the SiC
Schottky diode and a 600V hyper-fast Si PiN diode during
reverse recovery from a forward current of roughly 14A to a
dc level of 300V. A summary of the reverse recovery
parameters is presented in Table III. The SiC diode has a
superior reverse recovery characteristic, exhibiting just 12°/0
of the recovered charge and 13°/0of the switching loss of its
Si counterpart. In addition, the IGBT turn-on loss is reduced
from 173pJ to 149pJ. This relatively small reduction in IGBT
turn-on loss is a consequence of the large IGBT die used for
the switching tests.
Fig. 8: Diode voltages during reverse recovery at 25C. Open squares – Si
PiN diode, open diamonds – SiC Shottky diode.
‘8~r
’800
16
1600
14
1400
12
600
25
20
4
400
2
200
0
0
0
15
100
200
400
300
500
Tires (ns)
10
Fig. 9: Comparison of IGBT turn-on losses. Open sqaares – instantaneous
power (Si diode), open diamonds – instantaneous power (SiC diode), solid
squares – cumulative loss (Si diode), solid diamonds – cumulative loss (SiC
diode).
5
~.
=
-—.—-——
-5
TABLE 11
SUMMARY
-lo
-15
OF DIODE REVERSE RECOVERY PARAMETERS.
Type
Si PiN
SiC
Ipr (A)
19
4.8
25%
Reverse reeovery time
Trr (ns)
116
35
30%
Recovered charge
Qrr (nC)
1300
140
11%
Diode loss
Eoff Diode (pJ)
600
91
15%
IGBT
Eon IGBT (PJ)
1480
1020
68%
Peak reverse current
-20
-25
0
100
200
300
400
500
Time (ns)
10SS
Fig. 7: Diode currents during reverse rewvery at 25C. Open squares –
1200Vhyper-fsst Si pin diode, open dissnonds - Sic Schottky diode.
0-7803-6404-X/00/$10.00 (C) 2000
SiC vs Si
TABLE 111
SUMMARY OF DIODE REVERSE RECOVERY PARAMETERS AT A MEASUREMENT
TEMPERATURE OF 25C.
SiC sic VsSi
SiRN
15
10
5
Peak reverse current
Ipr (A)
12.5
3
24%
Reverse recovery time
Trr (ns)
37
19
51?’.
Recovered charge
Qrr (nC)
231
28
12%
Diode loss
Eoff Diode @J)
69
9
13’%0
Eon IGBT (~)
173
149
86’ZO
IGBT
-5
-lo
I
I
-15
o
50
100
150
200
250
Time (ns)
Fig. 10: Diode currents during reverse recovery at 25C. Open squares –
600V hyperfast Si pin diode, open diamonds – SiC Schottky diode.
o
50
100
150
200
10SS
Measurements made at a temperature of 125C emphasise
the superior performance of the SiC Schottky diode (Fig. 12).
The extracted parameters for the recove~ process (Table IV)
show that the levels of reverse recovered charge and
switching loss for the Si diode are roughly double those
measured at 25C whilst those for the SiC diode are
unaffected. Fig. 13 compares the IGBT turn-on collector
current waveforms for the Si and SiC diodes at the two
measurement temperatures. The IGBT collector current
overshoot for the Si diode is significant, representing 125°/0
of the switched current at a temperature of 125C. In contrast
the SiC diode exhibits an overshoot of just 2 1°A and is
unaffected by temperature.
250
TABLEIV
AT A MEASUREMENT
SUMMARY
OFDIODE
REVERSE
RSCOVERY
PARAMETERS
0
TEMPERATURE OF 125C.
Si PiN
-50
-1oo
-150
-200
z -250
3
-300
SiC vs Si
sic
Peak reverse current
Ipr (A)
17.5
3
17%
Reverse reeovery time
Trr (ns)
51
19
37%
Recovered charge
Qrr (nC)
446
28
6%
Diode loss
Eoff Diode (IJ)
139
9
IGBT
Eon IGBT (~)
10SS
6%
r
1
1
198
,
149
75%
-350
-400
-450
-500
Tima
(ns)
Fig. 11: Diode voltages during reverse recovery at 25C. Open squares – Si
PiN diode, open diomonds – SiC Shottky diode.
The high level of ringing present on the waveforms,
particularly the diode voltage, is due to resonance between
the test circuit stray inductance and the output capacitance of
the IGBT switch. A significant proportion of the observed
ringing occurs because of magnetic coupling between the
measurement circuit and the power circuit. It is, therefore,
indicative of the level of EMI generated by the switching
transition. Since the level of ringing induced by the SiC diode
is much lower than that observed for the PiN diode it may be
concluded that the level of EMI generated by the IGBT-SiC
diode combination is small compared to the IGBT-PiN diode
combination.
I
-20 J
o
50
100
150
200
250
Time (ns)
Fig. 12: Diode currents during reverse recovery at 125C.Open squares – Si
PiN diode, open diamonds – SiC Shottky diode.
0-7803-6404-X/00/$10.00 (C) 2000
D. Summary
In Si IGBT switched power electronic systems the
enhanced features of the SiC Schottky diode may be used to
good effect in a number of ways:
30-
25-
1.
The peak dildt of 500Aips used in the above tests is
close to the maximum practicable level for Si lPiN
diodes. In the case of the SiC Schottky diodes, a
combination of low peak reverse recovered current,
relatively low EMI levels and insensitivity of switching
characteristics to temperature allow operation at much
higher di/dt. Optimisation of systems employing SiC
Schottky diodes is thus likely to favour increased
switching tlequencies.
2.
Reduced total switching losses allow scope for reduction
of cooling system hardware e.g. smaller heatsink size
and cost.
3.
Reduced IGBT losses allow scope for reduced junction
temperature excursions and hence lower thermal cyclling
stresses. This allows enhanced IGBT reliability without
increasing cooling requirements.
20~
=
15-
10-
5-
0
0
50
100
150
200
250
Time (ne)
Fig, 13: IGBT collector current waveforms. Open squares – Si PiN diode
125C,open triangles – Sil PiN diode 25C, open diamonds – SiC Schottky
diode 25C and 125C (waveforms overlap).
V. CONCLUSIONS
C. Surge current perjbrmance
The positive temperature coefficient of forward voltage
and lack of conductivity modulation limit the ability of
Schottky diodes to endure high surge currents. For many high
speed switching systems this is of little consequence since the
power converter typically operates under current control.
However, where curnents are not controlled they may rise to
the switch de-saturation level, which is typically 2-3 times
the continuous rated {currentof the IGBT switch, under fault
conditions. In tests the SiC Schottky rectifiers employed in
this study successfidly carried short duration (1Oj.M)current
pulses of up to 70A, or nearly 9 times the continuous rated
&rent, wi~hout dama~ge(Fig. 14).
80 ~
i
0
L–
0
5
The key disadvantage is the high cost. Estimates basecl on
present wafer prices show the die cost for the SiC Schottky
diodes to be of the order of 20-30 times that of the equivalent
Si PiN diode technology. However if current trends in SiC
wafer quality and size continue, this cost difference should
fall dramatically over the next few years.
ACKNOWLEDGMENT
15
10
Time
Fig.
the same switching conditions whilst IGBT turn-on losses are
reduced by 25°/0. In addition, the SiC diode dynamic
characteristics are insensitive to variations in junction
temperature and the level of EMI generated by the Si IGIBTSiC diode combination is significantly lower than that
generated using the Si PiN diode.
In Si IGBT switched power electronic systems the
enhanced features of the SiC Schottky diode may be used to
allow higher switching tiequencies, reduced cooling
hardware or improved IGBT reliability
70
10
4H-SiC
Schottky
power
diodes,
capable
of carrying
by parallel
currents
of over 8A, have been fabricated
connection of multiple diode elements on a single die. Static
measurements
show reverse
breakdown
voltages
of cwer
1500V
and
viable
device
operation
up to junction
temperatures
of 250C. Comparative
dynamic measurements,
performed at a temperature
of 125C show levels of reverse
recovered charge and diode switching loss that are just 6’% of
those obtained using a 600V, hyper-fast Si PiN diode under
20
25
(w)
This work was fimded by the UK Engineering and
Physical Sciences Research Council (EPSRC) under research
grant GRJL62320.
14: Single pulse surge current response for SiC Schottky diode.
0-7803-6404-X/00/$10.00 (C) 2000
diodes”, in proceedings of International Conference on Silicon Carbide
and Related Materials (ICSCRW99), abstract 37, 1999.
REFERENCES
[1]
[2]
K. Shenai, R.S. Scott, wdB.J. Bdig4``Optimum semiconductors for
high power electronics’’, L%!L5Trans. Orr Electrorr Device.s,Vol. 36,
1811-1823,1989.
[5]
A.P. Knighk, M.A. Louren~o, K.P. Homewoo4D.J. Momisou N.G.
Wright, S. Ortolland, C.M. Johnson, A.G. ONeill and P.G. Coleman,
“Low temperature annealing of 4H-SiC Schottky diode edge
terminations formed by30keVAr+ implantation’’, Journal of Applied
Physics, in press -to appear April 2000.
[6]
K. J. Schoen, J,M. WoodaB, J. A. Cooper. Jr, M. R. Melloch’’ Design
Considerations
of Experimental
Analysis of High Voltage SiC
Schottky Barrier Rectifiers’’ IEEE Trans. OnElectron Devices Vo145
No7(1998)p1595
R. Hel, M. Fullmann and E. Niemann, “SiC power rectifiers”, in
proceedings of International Conference on Silicon
Related Materials (ICSCRM’99), abstract 7, 1999.
Carbide
and
[3]
Q. WAA, A. Ellison, J. Zhmg, U. Forsberg mdE. Jmzen, ''Designing
physical simulations and fabrication of high-voltage (3.85kV) 4H-SiC
Schottky rectifiers processed on hot-wall andhigh-temperature CVD
films”, in proceedings of International Conference on Silicon Carbide
and Related Materials (ICSCRM99), abstract 39, 1999.
[4]
R. Rupp, M. Treu, A. Mauder, E. GrIebl, W. Werner, W. Bartsch and
D. Stephani, “Performance and reliability issues of SiC-Schottky
0-7803-6404-X/00/$10.00 (C) 2000