1146 IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 10, OCTOBER 2010

Optically Activated SiC Power Transistorsfor Pulsed-Power Application

Feng Zhao, Member, IEEE, and Mohammad M. Islam

Abstract—To realize the benefits of SiC power electronics andoptical-driving device technology, we present optically activatedSiC power transistors for pulsed-power application. Althoughlimited by the indirect bandgap of SiC, the transistor employs abipolar structure with an internal current gain, which amplifiesthe photogenerated current and therefore has the capability to re-duce the optical-triggering power requirement. The transistors arefabricated on an n-type 4H-SiC substrate with switch-on triggeredby a short UV (337.1 nm) laser pulse with 0.5-mJ optical energy.The transistor successfully switches 1200 V with an FWHM ofabout 180 ns, a rise time of less than 20 ns, and a fall time ofabout 200 ns. This initial work forms the basis for the further de-velopment of high-speed and energy-efficient SiC-based opticallycontrolled power switches for high-temperature and high-powerapplications.

Index Terms—Optically activated, power switch, pulsed power,SiC transistors.

I. INTRODUCTION

DUE TO its superior material properties [1], particularlythe high breakdown voltage and high thermal conduc-

tivity, which is essential to lower both electrical and thermalresistances of the power switch for a given power density, SiChas the potential use as a photoconductive switch material.Combining with the advantages of optical-driving mechanismto provide fast switching and enhanced EMI immunity, a SiC-based optically controlled switch is attractive for pulsed-powerapplications [2], [3]. In the past several years, research in thisdevice technology has been pursued, and significant progresshas been reported, including optical switch-on thyristor [4],bulk photoconductive switches [5], optically activated p-i-ndiodes [6], etc.

One of the critical research issues is that the optical efficiencyof SiC is poor due to its indirect bandgap, which requireshigher optical power to trigger the devices when compared tothe III–V materials such as GaAs and GaN. However, sinceSiC has higher voltage capability and thermal conductivityover GaAs and more mature material growth over GaN, SiCphotoconductive switches still excel GaAs and GaN in certainpower and frequency regime. Hybrid power switch concepthas been discussed [7], and a SiC-based hybrid structure was

Manuscript received June 7, 2010; accepted July 7, 2010. Date of publicationAugust 16, 2010; date of current version September 24, 2010. This work wassupported in part by the Southeast Center for Electrical Engineering Educationunder Grant SCEEE-09-001. The review of this letter was arranged by EditorS.-H. Ryu.

The authors are with the Department of Electrical Engineering, University ofSouth Carolina, Columbia, SC 29208 USA (e-mail: zhaof2@cec.sc.edu).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LED.2010.2058840

reported [8] with a GaAs bipolar transistor electrically coupledto the main SiC power device. Since the GaAs transistor is usedas the triggering device, the hybrid power switch significantlyreduces the optical-triggering power.

In this letter, we present an optically activated SiC powertransistor which employs a bipolar device structure. Unlikeconventional photoconductive switches [4]–[6], collecting thephotogenerated current directly, the bipolar device structureamplifies the photogenerated current by its internal currentgain, which exploits a better optical sensitivity and has the capa-bility to more efficiently switch high electrical power by usingvery low optical-triggering power. Moreover, the transistor isfabricated on an n-type SiC substrate, which lowers the mate-rial cost compared to bulk SiC photoconductors [5] requiringsemi-insulating materials. Furthermore, the bipolar structure isdesirable for photoconductive pulsed-power switches [9], [10],particularly for achieving controllable short pulsewidths sinceits turnoff is controlled by a single base drive. All these advan-tages are testimonies to the potential of optically activated SiCpower transistors for the development of harsh-environmentoperable, long-term reliable, and energy-efficient powerelectronics.

II. DEVICE STRUCTURE AND FABRICATION

The transistors employ a bipolar epitaxial structure(n+-p-n−) grown on a commercial 8◦ off-axis n-type 4H-SiC substrate. The nominal doping and thickness of the epi-taxial layers from the top were 2 × 1019 cm−3/2 μm, 6 ×1017 cm−3/1 μm, and 5 × 1015 cm−3/15 μm, respectively.Both isolation mesa and emitter mesa were patterned by RIE.N-type contact by Ni/Cr (20 nm/40 nm) was annealed by RTAat 1000 ◦C for 1 min in nitrogen ambient. Junction terminationwas realized by partially etching the base layer. A 1.5-μm-thickSiO2 layer was sputtered on the device for both surface pas-sivation and antireflection of optical triggers. A cross-sectionaldrawing of the transistor is shown in Fig. 1 along with an opticalimage of the sample with transistors of different active areas,including 1 × 1 mm2 and 0.5 × 1 mm2 devices for opticalswitching test and some test structures for dc characterizationand design verification.

Due to the large Schottky height, the formation of p-typeohmic contact on SiC is always challenging and usually re-quires high-dose and high-energy Al− implantation to formthe contact region. Since the optically activated transistors aretriggered by optical pulses instead of an electrical bias on thebase, as shown in Fig. 1(a), it is not necessary to make anymetal contact on the p-type region. This simplifies the process

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ZHAO AND ISLAM: OPTICALLY ACTIVATED SiC POWER TRANSISTORS 1147

Fig. 1. (a) Schematic and (b) micrograph of optically activated power transis-tors on an n-type SiC substrate.

Fig. 2. I–V characteristics of a test transistor with its optical image inserted.

and is believed to improve the high-temperature and long-termreliability of the transistor.

III. CHARACTERIZATION AND RESULTS

I–V characterization was performed on the test transistors toqualify the devices and verify the design. The typical forwardand reverse I–V curves are shown in Fig. 2 with a micrographof the test device inserted in the figure. The current gain isabout 11, which is lower than the designed value due to thehigher base doping indicated in the SIMS data. BVCEO is lowerthan BVCBO due to the internal current gain, and both areabove 1600 V, which proves that device design and fabricationprocess, including junction termination, all worked effectively.

The test circuit for switch evaluation is shown in Fig. 3. Thepurpose of the capacitor CS (2 μF), in parallel with the sourcemeter, was to keep the voltage VR constant (VR = VA) whenthe switch was closed. The transistor was activated with a shortUV pulse (wavelength of 337.1 nm, pulse width of 600 ps,and optical energy of 0.5 mJ) by a nitrogen laser. Under UVillumination, electron and hole pairs are generated in the deviceby photon absorption, as shown in Fig. 1(a). Photogeneratedholes (h+), in the base-collector depletion region and within adistance of the diffusion length, flow to and are trapped in thebase region since the transistor is biased in the active regime.This accumulation of holes raises the base potential and allowsa large flow of electrons from the emitter to the collector. Thisis the dominant gain mechanism in optically activated powertransistors.

Fig. 3. Test circuit for optical switching evaluation.

Fig. 4. Collector voltage and current waveforms of the optically activatedtransistor when activated by a single laser pulse. The applied reverse voltagewas 1200 V.

The collector voltage VC on the transistor switch was mea-sured by VC = VR − VL. With a reverse-bias voltage VA =1200 V and RL = 1 kΩ, a transistor of 0.5 mm2 conducted1 A (200 A/cm2) with an FWHM of about 180 ns, a rise timeof < 20 ns, and a fall time of about 200 ns when activated bya single laser pulse. The time evolution of the collector voltageand current waveforms is shown in Fig. 4. Since the opticallyactivated power transistor employs a bipolar structure, thespeed is determined by the emitter and collector charging times[11, pp. 695–696]

τ = τE + τC = β0

[kT

qICEO(CEB + CCB) + RLCCB

](1)

where β0 is the photocurrent gain, and CEB and CCB are theemitter-base and collector-base capacitances, respectively. Asindicated in (1), the response is faster when the value of RL

decreases by reducing the collector charging time RLCCB.This was proven by the collector voltage transient waveformsin Fig. 5(a)–(c) when a transistor was connected with RL ofdifferent values and switched at 1200 V. Reducing RL alsoleads to an increased current conducted by the device and,therefore, the electrical power. Another observation made from(1) is that, when the active area of the transistor increases,

1148 IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 10, OCTOBER 2010

Fig. 5. Time evolution of the collector voltage for transistors connected withdifferent values of RL (1, 2, and 5 kΩ) and with different active areas (0.5 and1 mm2) at 1200 V.

although τE will not be affected by the first-order analysis,the collector charging time RLCCB will increase since CCB

increases, which slows down the transistor. The waveforms inFig. 5(b) and (d) compare the response times of two transistorswith different active areas of 0.5 and 1 mm2 when biased at1200 V with RL = 2 kΩ. Therefore, there is a tradeoff betweenthe response time and the maximum current which can bedelivered by the transistor. Another tradeoff is between theswitch speed and the photocurrent gain since τ is inverselyproportional to β0, as shown in (1). A gain-bandwidth productis another good measure of the performance of the opticallyactivated power transistors and is currently under investigation.

IV. SUMMARY

In this letter, optically activated 4H-SiC power transistorshave been designed, fabricated, and characterized. When ac-tivated by a short (600 ps) UV laser pulse with a wavelengthof 337.1 nm and an optical energy of 0.5 mJ, the transistor

successfully switched 1200 V with an FWHM of 180 ns. Theresponse time is limited by the emitter and collector chargingtimes, which provides the main design parameters to achievesubmicrosecond switches operating at high power levels. Thisinitial work indicates that optically activated SiC power transis-tor, which combines the benefits of both SiC power electronicsand optical-driving mechanism, together with the capability ofreducing the optical-triggering power without compromising onthe switching speed, is a desirable device technology for a widerange of pulsed-power applications.

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