Janis M. Niedra

QSS Group, Inc., Cleveland, Ohio

Fast Turn-Off Times Observed in

Experimental 4H SiC Thyristors

NASA/CR—2006-214259

November 2006
NASA STI Program . . . in Profile

Since its founding, NASA has been dedicated to the

advancement of aeronautics and space science. The

NASA Scientific and Technical Information (STI)

program plays a key part in helping NASA maintain

this important role.

The NASA STI Program operates under the auspices

of the Agency Chief Information Officer. It collects,

organizes, provides for archiving, and disseminates

NASA’s STI. The NASA STI program provides access

to the NASA Aeronautics and Space Database and its

public interface, the NASA Technical Reports Server,

thus providing one of the largest collections of

aeronautical and space science STI in the world.

Results are published in both non-NASA channels and

by NASA in the NASA STI Report Series, which

includes the following report types:

• TECHNICAL PUBLICATION. Reports of

completed research or a major significant phase

of research that present the results of NASA

programs and include extensive data or theoretical

analysis. Includes compilations of significant

scientific and technical data and information

deemed to be of continuing reference value.

NASA counterpart of peer-reviewed formal

professional papers but has less stringent

limitations on manuscript length and extent of

graphic presentations.

• TECHNICAL MEMORANDUM. Scientific

and technical findings that are preliminary or

of specialized interest, e.g., quick release

reports, working papers, and bibliographies that

contain minimal annotation. Does not contain

extensive analysis.

• CONTRACTOR REPORT. Scientific and

technical findings by NASA-sponsored

contractors and grantees.

• CONFERENCE PUBLICATION. Collected

papers from scientific and technical

conferences, symposia, seminars, or other

meetings sponsored or cosponsored by NASA.

• SPECIAL PUBLICATION. Scientific,

technical, or historical information from

NASA programs, projects, and missions, often

concerned with subjects having substantial

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific and

technical material pertinent to NASA’s mission.

Specialized services also include creating custom

thesauri, building customized databases, organizing

and publishing research results.

For more information about the NASA STI

program, see the following:

• Access the NASA STI program home page at

http://www.sti.nasa.gov

• E-mail your question via the Internet to

help@sti.nasa.gov

• Fax your question to the NASA STI Help Desk

at 301–621–0134

• Telephone the NASA STI Help Desk at

301–621–0390

• Write to:

 NASA STI Help Desk

 NASA Center for AeroSpace Information

 7121 Standard Drive

 Hanover, MD 21076–1320
NASA/CR—2006-214259

November 2006

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared under Contracts NAS3–98008 and NAS3–00145

Janis M. Niedra

QSS Group, Inc., Cleveland, Ohio

Fast Turn-Off Times Observed in

Experimental 4H SiC Thyristors
Acknowledgments

This work was sponsored by the NASA Glenn Research Center under NAS3–98008 and NAS3–00145

with G.E. Schwarze as the Project Manager.

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Available electronically at http://gltrs.grc.nasa.gov

Level of Review: This material has been technically reviewed by expert reviewer(s).
NASA/CR—2006-214259 1

Fast Turn-Off Times Observed in Experimental 4H SiC Thyristors 

Janis M. Niedra 

QSS Group, Inc. 

Cleveland, Ohio 44135 

Abstract 

Room temperature measurements of the turn-off time (tq) are reported for several packaged, npnp 

developmental power thyristors based on 4H-type SiC and rated 400 V, 2 A. Turn-off is effected by a 

50 V pulse of applied reverse voltage, from a state of a steady 1 A forward current. Plots of tq against the 

ramp rate (dVAK/dt) of reapplied forward voltage are presented for preset values of limiting anode-tocathode voltage (VAK,max). The lowest tq measured was about 180 ns. A rapid rise of these tq curves was 

observed for values of VAK,max that are only about a fifth of the rated voltage, whereas comparative tq plots 

for a commercial, fast turn-off, Si-based thyristor at a proportionately reduced VAK,max showed no such 

behavior. Hence these SiC thyristors may have problems arising from material defects or surface 

passivation. The influence the R-C-D gate bypass circuit that was used is briefly discussed. 

Introduction 

The superior thermal conductivity, breakdown electric field, saturation drift velocity and wide 

bandgap of 4H-type silicon carbide (table I) continue to motivate intense efforts to exploit this material 

for high temperature capable, fast-switching power devices. Years have passed since the first 

demonstrations of vertical npnp thyristors in 4H-SiC (refs. 1 and 2), still the development of large-area 

power devices continues to be hindered by crystalline defect problems, in addition to the fact that SiC has 

no naturally passivating oxide. Although there exist studies of the turn-on process in such thyristors (refs. 

3 and 4), data reports on turn-off from a steady forward conduction state are scarce. This paper reports 

turn-off time (tq) measurements at room temperature on experimental npnp thyristors built on n-doped 

4H-type silicon carbide substrates. These devices, packaged on gold plated kovar TO-220 headers with 

welded lids, are nominally rated 2 A, 400 V forward blocking and are the products of Cree Research, Inc. 

(ref. 5). Some tq data is reported in (ref. 5), but for zero applied reverse voltage. No information is 

available on the crystalline defect (e.g., micropipe) statistics, the doping profiles, or on the surface 

passivation for these devices. 

Method of Measurement 

Except for details of the gate drive, the tq measurement circuit used is fully described in the 

NASA/CR—2006-214260. Briefly, this circuit uses a fast MOSFET switch to divert the current from 

an inductively driven current source away from the thyristor under test (TUT), while simultaneously 

applying a constant reverse bias voltage of adjustable amplitude and duration to the TUT. Following the 

reverse bias period, the current from the source is switched to an adjustable array of capacitors in parallel 

with the TUT, generating a voltage ramp of controlled slope across the TUT. The height of this ramp is 

limited by an adjustable array of energy absorbing Zener diodes. 

As the duration of the reverse bias is decreased, with the other parameters kept constant, a point is 

reached where the reapplied voltage causes the TUT to turn on again. The tq is then the time from the zero 

crossing of the current to the zero crossing of the reapplied voltage ramp. An illustration of this 

measurement process is given in figure 1. The circuit allows a cyclic repetition of this measurement, such 

that a reliable value for tq can be obtained. 
NASA/CR—2006-214259 2

It is well known that impedance conditions in the gate firing circuitry have a great effect on the tq. 

This is due to the influence of any gate bias on minority carrier injection at the p-n junction included in 

the gate firing circuit loop. The reapplied voltage ramp induces a capacitive displacement current through 

the TUT and hence a minority carrier injection at any p-n junction oriented in the forward direction, such 

as the junction involved in the gate firing circuit. Indeed, both this injection effect, and hence the tq, can 

be reduced by bypassing this junction or by the application of a low level reverse bias to this junction at 

the start of the reapplied forward voltage. Both bypassing and such a bias were effected here by use of a 

small (5 nF) gate capacitor; however, see later discussion. A bypassing diode was finally added to protect 

this junction from the transient multiampere reverse breakdown currents induced by the sudden 

application of reverse voltage to the TUT. 

Since in the thyristors tested here had their gates connected to the n-layer next to the p-layer of the 

anode (the so called ‘anode gate’) and the cathode was grounded, the gate firing circuit was at the high 

potential of the anode and had to be isolated from ground. This was done by a small transformer, based on 

a high frequency ferrite core. Capacitive effects to ground through this transformer were not evaluated. A 

diagram of the gate firing circuit is shown in figure 2. 

Experimental Observations 

Initially, 5 SiC thyristors of the same structure and nominal ratings were selected for measurement, 

but 2 of these devices failed early during measurement, leaving only the 3, whose data is shown in 

figure 3. The histories of these devices were not identical, however. Thyristor ST/SQR-1 had been aged 

for 1000 hr at 350 °C by the manufacturer (Cree Research, Inc.), and thyristor TC/SQR-17 had been 

subjected 5 times to a thermal shock cycle (Mil-Spec 202F, method 107F) from –65 to 350 °C, whereas 

thyristor VG/SQR-3 had no prior history of such aging or testing. 

A tq below 400 ns could be obtained in all the tested devices and below 200 ns for the VG/SQR-3, 

provided the rate (dVAK/dt) of forward reapplied voltage was sufficiently low. For low rates below about 

200 V/μs, the retriggering would normally be on the rising slope of the reapplied forward voltage (VAK), 

provided its limiting value VAK,max was set sufficiently high. Under these conditions, the VAK,max has no 

effect, explaining the confluence of the tq curves for a particular device at low dVAK/dt. As dVAK/dt and tq

increase, retriggering is normally at the preset limiting voltage VAK,max and the curves separate according 

to this limiting voltage. 

The second notable observation is that the rate of increase of tq with dVAK/dt becomes very large once 

some value of VAK,max is exceeded. Qualitatively, this says that above some value of VAK,max even below 

the maximum dc rating, the thyristor can not be kept off, except possibly at uselessly low rates of 

reapplied forward voltage. Such behavior may well be expected for VAK,max close to the absolute dc rating 

of the device. Here, however, 72 V is only but a fifth of the absolute rating of these SiC thyristors. To get 

more insight into this problem, a comparative plot of the tq of a fast turn-off Si thyristor is included in 

figure 3. The MCR2150A is an 800 V, 15 A commercial thyristor (now discontinued). And at VAK,max of 

153 V, this device is also being operated at about a fifth of its absolute rating. But there is no tendency yet 

for its tq plot to curve upward. For the MCR2150A, the nearly uniform shift up of the 153 V curve as 

compared to the 63 V curve is anomalous (unknown cause), since a 72 V curve (not plotted) had no such 

consistent shift relative to the 63 V curve shown. 

Summary and Discussion 

Turn-off times (tq) were measured at room temperature for several developmental thyristors based on 

4H-type SiC and depending on the rate (dVAK/dt) of reapplied forward voltage, values below 200 ns were 

observed. Plots are presented of tq versus dVAK/dt up to 3000 V/μs, for preset values of limiting anode-to 

cathode voltage (VAK,max). Turn-off was always effected from a steady state of 1 A forward conduction by 

a reverse applied voltage pulse of 50 V and of a duration according to the tq. 
NASA/CR—2006-214259 3

A rapid upward curvature of these tq plots is evident in figure 3 for values of VAK,max that are only 

about a fifth of the absolute rating (~400 V) of these SiC thyristors. This strongly suggests a problem with 

either surface passivation or else a basic SiC defect problem, such as micropipes (severe screw 

dislocations). When a commercial fast turn-off, Si-based thyristor was tested at a proportionately low 

VAK,max, no such upward curvature of tq was observed. This further points to the existence of a materials or 

device fabrication problem in these SiC devices. Nevertheless, the potential for an enormous reduction in 

turn-off time by going to SiC, as compared to the fastest Si thyristors, is evident in figure 3. 

As cautioned above, impedance conditions in the gate firing circuitry have a great effect on the 

measured tq. Initially, gate bypass capacitors of 5 nF and higher were tried, without the diode (see fig. 2). 

This capacitor bypasses displacement currents, induced by the reapplied forward voltage ramp, around the 

anode-to gate p-n junction, thus hindering minority carrier injection there and hence reducing tq. Indeed, 

this capacitor can get charged by the reverse current transient (caused by the sudden application of a 

reverse bias), up to the breakdown voltage of this junction. Besides making the gate more difficult to fire, 

such an overly large (>5 nF) ‘cheating’ capacitor was found to cause under certain conditions, a double 

valued tq! The cause is simply that while this capacitor retains sufficient charge to significantly reverse 

bias the said junction, it is difficult for the rising forward voltage to retrigger the thyristor. However, once 

this charge is gone, retriggering can be easy. Also, if the duration of the reverse bias pulse is very short, 

the capacitor has less influence and retriggering can again occur. A value of 5 nF gave acceptable 

performance, especially after the diode was added. The purpose of the diode is to protect the anode-gate 

junction from the transient multiampere reverse breakdown currents induced by the sudden application of 

reverse voltage to the thyristor. After observing that thyristors failed during measurements usually had 

deteriorated anode-gate junctions, the diode seemed desirable. With the diode in place, the capacitor still 

bypasses to some extent the ramp-induced currents, but the problem of two values for tq was gone. 

References 

1. J.W. Palmour, J.A. Edmund, H.S. Kong and C.H. Carter, Jr., “Vertical Power Devices in Silicon 

Carbide,” in Silicon Carbide and Rel. Mater., Inst. of Phys. Conf. Series 137, 1994, p. 499. 

2. J.W. Palmour, S.T. Allen and D.G. Waltz, “4H-SiC Power Switching Devices,” in Silicon Carbide 

and Rel. Mater., Inst. of Phys. Conf. Series 142, 1995, p. 813. 

3. M.E. Levinshtein, J.W. Palmour, S.L. Rumyanetsev and R. Singh, “Turn-on Process in 4H-SiC 

Thyristors,” IEEE Trans. On Electron Devices 44, no. 7, July 1997, p. 1177. 

1. 4 M.E. Levinshtein, J.W. Palmour, S.L. Rumyanetsev and R. Singh, “The Critical Charge Density of 

4H-SiC Thyristors,” IEEE Trans. On Electron Devices 45, no. 1, January 1998, p. 307. 

4. J.W. Palmour and C.H. Carter Jr., “A 350 C Silicon Carbide Power Thyristor,” Cree Research, Inc., 

Durham, NC, 6 May 1998. NASA SBIR Phase II contract NAS3–26927. 

TABLE I.—APPROXIMATE MATERIAL PROPERTIES OF 4H-TYPE SiC AND Si*

 4H-SiC Si 

Band gap (eV) 3.26 1.12 

Breakdown electric field (V/cm) 2.2×106

 2.5×105

Thermal conductivity W/(cm K) 

(at room temperature) 

4.9 1.5 

Saturated electron drift speed (cm/s) 

(for E > 2×105

 V/cm) 

2×107

 1×107

*

Data provided by Cree Research, Inc. 
NASA/CR—2006-214259 4

Figure 1.—Typical current (upper) and voltage (lower) 

waveforms observed during tq measurement. Here the thyristor 

turns on spontaneously on the plateau of maximum reapplied 

voltage. Turn-on may also occur on the rising slope of the 

reapplied voltage. 

Figure 2.—Gate firing circuit used to measure tq of the SiC thyristors. 

The diode was added to protect the gate junction from breakdown 

during the reverse current transient. The pulse generator has an output 

impedance of 50 Ω. 

TUT 

1N5811

To pulse generator

5 nF 22Ω
NASA/CR—2006-214259 5

Figure 3.—Turn-off time (tq) as a function of the rate of rise (dVAK/dt) of reapplied forward voltage for 

three 4H-type SiC thyristors, using the gate trigger circuit shown in figure 2. In all cases, the initial 

forward current was a steady 1 A and turn-off was effected by a reverse applied voltage (VAK,rev) of 

−50 V. For each thyristor, results are shown for 3 limiting anode-to-cathode forward voltages (VAK,max). 
NASA/CR—2006-214259 6

Figure 4.—Comparative plots of tq for a fast turn-off, commercial Si thyristor and the ST/SQR-1 

developmental 4H-type SiC thyristor. The cause for the persistence of a vertical separation of the two 

MCR2150A curves for low values of dVAK/dt may be experimental error. 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

REPORT DOCUMENTATION PAGE

2. REPORT DATE

19. SECURITY CLASSIFICATION

 OF ABSTRACT

18. SECURITY CLASSIFICATION

 OF THIS PAGE

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18

298-102

Form Approved

OMB No. 0704-0188

12b. DISTRIBUTION CODE

8. PERFORMING ORGANIZATION

 REPORT NUMBER

5. FUNDING NUMBERS

3. REPORT TYPE AND DATES COVERED

4. TITLE AND SUBTITLE

6. AUTHOR(S)

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT

13. ABSTRACT (Maximum 200 words)

14. SUBJECT TERMS

17. SECURITY CLASSIFICATION

 OF REPORT

16. PRICE CODE

15. NUMBER OF PAGES

20. LIMITATION OF ABSTRACT

Unclassified Unclassified

Final Contractor Report

Unclassified

1. AGENCY USE ONLY (Leave blank)

10. SPONSORING/MONITORING

 AGENCY REPORT NUMBER

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546–0001

Available electronically at http://gltrs.grc.nasa.gov

November 2006

NASA CR—2006-214259

E–15542

WBS 423799.03.01

NAS3–98008

NAS3–00145

12

Fast Turn-Off Times Observed in Experimental 4H SiC Thyristors

Janis M. Niedra

4H-SiC thyristor; Turn-off time tq; Reapplied voltage rate; Device defects; Gate firing

circuits

Unclassified -Unlimited

Subject Category: 33

QSS Group, Inc.

21000 Brookpark Road

Cleveland, Ohio 44135

Project Manager, Gene E. Schwarze, Power and Propulsion Division, NASA Glenn Research Center, organization

code RPE, 216–433–6117.

Room temperature measurements of the turn-off time (tq) are reported for several packaged, npnp developmental power

thyristors based on 4H-type SiC and rated 400 V, 2 A. Turn-off is effected by a 50 V pulse of applied reverse voltage,

from a state of a steady 1 A forward current. Plots of tq against the ramp rate (dVAK/dt) of reapplied forward voltage are

presented for preset values of limiting anode-to-cathode voltage (VAK,max). The lowest tq measured was about 180 ns. A

rapid rise of these tq curves was observed for values of VAK,max that are only about a fifth of the rated voltage, whereas

comparative tq plots for a commercial, fast turn-off, Si-based thyristor at a proportionately reduced VAK,max showed no

such behavior. Hence these SiC thyristors may have problems arising from material defects or surface passivation. The

influence the R-C-D gate bypass circuit that was used is briefly discussed.