Solid-State Electronics 53 (2009) 1169–1172

Contents lists available at ScienceDirect

Solid-State Electronics

journal homepage: www.elsevier .com/locate /sse

71 W (19.7 W/mm) SiC BJTs for long-pulse UHF radar applications

Feng Zhao *

Department of Electrical Engineering, University of South Carolina, Columbia, SC 29208, United States

a r t i c l e i n f o

Article history:Received 23 April 2009Received in revised form 24 July 2009Accepted 28 July 2009Available online 21 August 2009

The review of this paper was arranged byProf. A. Zaslavsky

Keywords:4H–SiCBipolar junction transistors (BJTs)UHFLong pulseRadar

0038-1101/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.sse.2009.07.008

* Tel.: +1 803 777 6303; fax: +1 803 777 8045.E-mail address: zhaof2@cec.sc.edu

a b s t r a c t

In this paper, we present 71 W SiC bipolar junction transistors (BJTs) using state of the art technology. Thedevices were fabricated on a commercial n-type 4H–SiC substrate using a double-mesa etch and interdig-itated emitter–base finger design. When operating under common-emitter configuration and long pulseRF conditions of 15 ms pulse width and 25% duty cycle, the packaged devices without internal matchingexhibited 8.5 dB power gain and 71 W output power with a 50.7% power added efficiency (PAE) at500 MHz. The power density is 19.7 W/mm normalized to total emitter finger length. The normally-offcharacteristic and superior long pulse RF performance makes these SiC transistors promising for use incompact power amplifiers in long-pulse UHF radar systems.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Silicon carbide (SiC) is a wide bandgap semiconductor attract-ing researchers by its superior physical and electrical propertiesincluding higher breakdown field, thermal conductivity and satu-ration electron velocity as compared to both Si and GaAs counter-parts [1–5]. Together these material properties provide the basisfor the potential of SiC-based power RF devices for applicationssuch as radar, avionics and communication.

SiC RF BJTs have advantages over other SiC RF devices, such asits normally-off characteristics for simplified driving when com-pared to SITs [6], vertical structure for higher current capabilitywhen compared to MESFETs [7], absence of high quality insulatorfor high radiation tolerance, and temperature and field reliabilitywhen compared to MOSFETs [8], and conductivity modulation fora lower on-state resistance. Good progress has been made in thelast few years on SiC UHF BJTs, as exemplified by device demon-strations with an output power of 215 W and 7.5 dB gain undershort 500 ns pulse width and 0.1% duty cycle at 450 MHz [9], and2100 W and 6.3 dB under 2 ls pulse width and 1% duty cycle at425 MHz [10]. However, the power densities in these devices areonly 4.13 W/mm and 7 W/mm, respectively, when normalized tothe total emitter length, and the RF performance was limited tovery short pulse and duty cycle. This is far from the theoretical lim-

ll rights reserved.

it for 4H–SiC bipolar transistors. In radar systems, longer pulsewidth could either increase detection range or the transmitterpower could be reduced to keep the radar detection range to thesame level. However, long pulse width would reduce the range res-olution of radar system. Pulse compression techniques are usuallyused to solve this resolution issue, but the system becomes morecomplex with such pulse compression. Therefore, modern technol-ogy especially the utilization of SiC based compact devices is crit-ical to further improve the radar detection range and at the sametime simplify the whole radar system.

In 2006, the author presented BJTs [11] delivering 41 W outputpower at 500 MHz with a power density of 22.8 W/mm and 8.3 dBgain when operated under 150 ls and 5% duty. With recent designand fabrication process development, the author reports in this pa-per SiC BJTs with 71 W output power and 8.5 dB power gain whenoperated in class AB mode with a pulse width of 15 ms and a dutycycle of 25%. The PAE is 50.7% and the power density is 19.7 W/mm. This long pulse RF performance is, to the author’s knowledge,the highest ever published to date for SiC bipolar transistors.

2. Device structure, fabrication, and packaging

The epitaxial structures (n–p–n) were grown on a commerciallyavailable 2-in. 8� off-axis n-type 4H–SiC conductive substrate. Thenominal doping and thickness of emitter, base, and collector are400 nm/1 � 1019 cm�3, 150 nm/4 � 1018 cm�3, and 4 lm/1 � 1016

cm�3. The minority electron mobility for such a base doping is

Fig. 2. A RF SiC BJT die in a flanged package.

1170 F. Zhao / Solid-State Electronics 53 (2009) 1169–1172

215 cm2/V s based on the author’s prior study [12]. Transistorswere fabricated using a double-mesa etch and an interdigitatedemitter–base finger structure. Both isolation mesa and emittermesa were patterned by reactive ion etch (RIE) in a SF6 gas andthe etch depth was accurately controlled by an innovative tech-nique [13]. Both emitter and base finger width is 2 lm, and thebase to emitter alignment is 1 lm. The emitter finger length is75 lm, and there are 24 emitter fingers in each single transistor.The device surface was passivated with a thin layer of dry thermaloxide, followed by a layer of deposited oxide by plasma enhancedchemical vapor deposition (PECVD). Ni/Cr was used for the emitterand collector (back of the wafer) contacts, and Ti/Al for the basecontacts. Both n- and p-type ohmic contacts were prepared by ra-pid thermal annealing (RTA) at 950 �C in the high-purity Ar ambi-ent. Ti/Au was deposited for the wiring and pads. The entire devicesurface was covered by another layer of PECVD oxide, followed bya forming gas anneal process. The windows for bonding pads wereopened by RIE etching. More detailed fabrication procedures can befound elsewhere [14]. A cross-sectional drawing of a single fingerstructure and an optical image of a 2-in. SiC wafer after fabricationis shown in Fig. 1.

After fabrication, the wafer was diced and each SiC BJT die wasattached to a package, which incorporates a 40-mil thick BeO layerover a 65-mil thick CuW flange. Fig. 2 shows a RF BJT die in aflanged package. There are six identical transistors on each die.For large-signal RF measurements, two transistors were bondedin the package through emitter and base pads by Au bond wireswhich are straight and directly go to leads of the package. Therewas no internal matching used in the package during this study.The impedance matching was performed with a pre-matching cir-cuit on the test fixture and two triple-stub tuners at both input andoutput.

Fig. 1. (a) Schematic cross-sectional structure of a 4H–SiC BJT and (b) a 2-in. SiCwafer after device fabrication.

3. Results and discussion

3.1. DC characterization

An on-wafer DC current–voltage (I–V) characterization for thecommon-emitter configuration was performed on a single transis-tor under low and high voltage conditions with an Agilent 4155esemiconductor parameter analyzer to qualify the RF transistorsas well as to identify the proper DC bias points for RF measure-ments. A typical I–V is illustrated in Fig. 3. The maximum DC gainis 20 and the theoretical breakdown voltage BVCEO based on collec-tor thickness and doping is over 400 V. However, devices were onlytested up to 200 V due to the voltage limitation of the test equip-ment (4155e).

3.2. Small-signal RF characterization

Small-signal RF measurements were performed at room tem-perature using an Agilent E5071B network analyzer. The networkanalyzer was calibrated using a Thru-Reflect-Line (TRL) technique.The s-parameters were measured from 50 MHz to 4 GHz at a biasof VCE = 50 V and JC = 3.7 kA/cm2. The package itself is ‘‘purely”capacitive and was de-embedded from the devices using y-param-eters [15,16] converted from the measured s-parameters. Theemitter inductance due to the bond wires was also de-embeddedby subtracting it from the resulting z-parameters. The transit fre-quency fT was extrapolated from the 20 dB/decade fitted line ofcurrent gain |h21|, while the maximum oscillation frequency fMAX

was obtained from the unilateral power gain U and maximal avail-able gain GMAX at the frequency where the gain has decreased to 0-dB. The results are shown in Fig. 4 with fT = 2.1 GHz andfMAX = 3.9 GHz. The maximal available power gain GMAX is 12.7 dBat 500 MHz.

3.3. Large-signal RF characterization

The RF performance of the transistors was also characterized bylarge-signal measurements. Two identical transistors on a RF BJTdie were bonded in common-emitter configuration. DC voltagewas fed to the collector lead of the transistor; base was biased witha positive voltage and synchronized with a RF pulsing. The pulsewidth of the base pulsing was 0.2 ms wider than the RF pulse tocompensate for the RF delay. This bias configuration makes surethat the RF signal is not chopped off. The pre-matching circuit wasdesigned on the test fixture with a three-section transformation of

Fig. 3. I–V characteristic of a 4H–SiC RF BJT on the wafer.

0

10

20

30

40

0.01 0.1 1 10Frequency (GHz)

|h21

| (dB

)

(a) |h21| and fT

0

10

20

30

40

0.01 0.1 1 10

Frequency (GHz)

U a

nd G

MA

X (

dB)

(b) U, GMAX and fMAX

VCE = 50 V JC = 3.7

fT = 2.1 GHz

20dB/dec

12.7dB@500MHz

U

VCE = 50 V JC = 3.7 kA/cm2

GMAX

fMAX = 3.9 GHz

kA/cm2

Fig. 4. Small-signal results with fT and fMAX extrapolated from s-parameters of apackaged 4H–SiC BJT.

Fig. 5. Output power, power gain and PAE of two 4H–SiC BJTs bonded in thepackage under pulsed operation at 500 MHz. The pulse width is 15 ms and the dutycycle is 25%.

F. Zhao / Solid-State Electronics 53 (2009) 1169–1172 1171

8 X – 25 X – 50 X. Two triple-stub tuners at the input and outputmatch both input and output impedances to 50 X to obtain maxi-mum gain at the required input power. The input and output peakpower was measured by Agilent E4417A series power meter. A30 dB coupler was employed at the input side to monitor inputpower. A water-cooling heat sink was used to cool down the test

fixture and the device. The thermal-couple clapped on the packageflange showed that the flange temperature was maintained to be atroom temperature during the RF test.

Fig. 5 shows the RF performance at 500 MHz under pulsed ClassAB operation mode with 15 ms pulse width and 25% duty cycle.The transistors were biased at VCE = 100 V and JC = 5.5 kA/cm2. Ata power gain of 10 dB, the devices can deliver 52 W output powerand a PAE of 43%. The pulse droop is less than 0.5 dB as shown inFig. 6. When the output power is increased to 71 W correspondingto a power density of 19.7 W/mm, the transistors can still deliver8.5 dB power gain and the PAE increases to 50.7%. The author be-lieves that the improved RF performance compared to the resultsfrom [11] is due to the optimized design, both in epi (doping andthickness) and lateral layout, as well as fabrication process devel-opment. The narrower emitter finger width (2 lm) and tighteralignment (1 lm) between base contact to emitter mesa lead to asmaller total isolation-mesa area which reduces the base–collectorjunction capacitance and base resistance. Base ohmic contacts withlower contact resistivity (an average of 8 � 10�4 X cm2 across thewafer) were obtained which are lower than those in [11]. Previ-ously in [11], the author also reported 4H–SiC BJTs characterizedwith IR scan measurements on a test stage of 70 �C. The devicescan deliver 50 W output power with more than 7.5 dB gain whenoperated with a 150 ls pulse and 5% duty cycle, even though the

Pin=5WPout=50W

10dB

/div

Gp=10 dB

Droop ≤ 0.5 dB

Fig. 6. Characterization of pulse droop at 500 MHz and an output power of 50 W.

1172 F. Zhao / Solid-State Electronics 53 (2009) 1169–1172

temperature of the devices increases to more than 250 �C. Thismuch better thermal performance as opposed to Si devices, whencombined with the RF performance reported in this paper, showsthat 4H–SiC BJT is a promising candidate for long-pulse UHF radarapplications. Compared to the state-of-the-art 1 kW Si RF BJTs [17],the output power from 4H–SiC BJTs is still much lower. Furtherdevelopment work especially in power scaling is required toachieve a comparable output power.

4. Conclusion

4H–SiC RF BJTs on a n-type substrate demonstrated an outputpower of 71 W, a power gain of 8.5 dB and an efficiency of 50.7%under long pulse RF conditions of 15 ms pulse width and 25% dutycycle at 500 MHz. The power density is 19.7 W/mm and the pulsedroop is less than 0.5 dB. These results are testimonies to the po-tential of SiC BJTs for compact RF power amplifiers and to replaceSi BJTs in long-pulse UHF radar applications.

Acknowledgment

The author would like to thank his former colleagues at Micro-semi Corporation for their assistance in the work.

References

[1] Sze SM. Physics of semiconductor devices. John Wiley & Sons; 1981.[2] Shenai K, Scott RS, Baliga BJ. Optimum semiconductors for high-power

electronics. IEEE Trans Electron Dev 1989;36:1811–22.[3] Neudeck PG. Progress towards high temperature, high power SiC devices. Inst

Phys Conf Ser 141: Compd Semicond 1994:1–6.[4] Harris GL. Properties of SiC. London: Institute of Electrical Engineers; 1995.[5] Weitzel CE. Silicon carbide high frequency devices. Mater Sci Forum

1998;264–268:907–12.[6] Shi TF, Mallinger M, Leverich L, Chang J, Leader C, Caballero M. 800 W UHF SiC

sit transistor for radar applications. In: Dig. 2008 IEEE MTT-S internationalmicrowave symposium; 2008. p. 69–72.

[7] Müller SG, Brady MF, Brixius WH, Glass RC, Hobgood HM, Jenny JR, et al.Sublimation-grown semi-insulating SiC for high Frequency devices. Mater SciForum 2003;433–436:39.

[8] Gudjónsson G, Allerstam F, Ólafsson HÖ, Nilsson PÅ, Hjelmgren H, AnderssonK, et al. High-power-density 4H–SiC RF MOSFETs. IEEE Electron Dev Lett2006;27(6):469–71.

[9] Huang CF, Perez-Wurfl I, Zhao F, Torvik J, Irwin R, Torvik K, et al. 215 W pulsedclass a UHF power amplification based on SiC bipolar technology. In: Dig. IEEEDRC late news papers, Notre Dame, IN, June, 2004.

[10] Agarwal AK, Haley J, Bartlow H, McCalpin B, Krishnaswami S, Capell C, et al.First demonstration of 2100 W output power at 425 MHz using 4H–SiC RFpower BJTs. In: Dig. int. conf. on silicon carbide and related materials,Pittsburgh, Pennsylvania, September; 2005. p. 72.

[11] Zhao F, Perez-Wurfl I, Torvik K, Chiu J, Mallinger M, Torvik J, et al. Rugged UHF4H–SiC BJTs with record 22.8 W/mm power density and 8.3 dB Gain, In: Dig.device research conf., University Park, Pennsylvania, June, 2006. p. 153–4.

[12] Zhao F, Perez-Wurfl I, Huang CF, Torvik J, Van Zeghbroeck B. Analysis of transittimes and minority carrier mobility in npn 4H–SiC bipolar junction transistors.IEEE Trans on Electron Dev 2005;52(12):2541–5.

[13] Goulakov AB, Zhao F, Perez-Wurfl I, Torvik J, Van Zeghbroeck B. Optimizedreactive ion etch process for high performance SiC bipolar junction transistors.J Vac Sci Technol A 2007;25(July/August):961–6.

[14] Zhao F. Design, analysis and experimental study of RF 4H–SiC npn bipolarjunction transistors. PhD dissertation, Dept. Elect. Eng., Univ. Colorado,Boulder, CO, December 2004.

[15] Liu W. Handbook of III–V heterojunction bipolar transistors. John Wiley &Sons, Inc.; 1998.

[16] van Wijnen PJ, Claessen HR, Wolsheimer EA. A new straightforward calibrationand correction procedure for on wafer high frequency s-parametermeasurements (45 MHz–18 GHz). In: proc. IEEE bipolar circuits andtechnology meeting; 1987. p. 70–3.

[17] Shi TF, Chang J, Leverich L, Mallinger M, Leader C. 1 KW push–pull highefficiency RF BJT transistor for radar applications. In: IEEE/MTT-S internationalmicrowave symposium, San Francisco, CA, June, 2007.