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First Demonstration of 4H-SiC RF Bipolar Junction Transistors on a Semi-insulating Substrate with fT/fMAX of 7/5.2 GHz Feng Zhao1, 2, Ivan Perez-Wurfl1, Chih-Fang Huang1, John Torvik1, and Bart Van Zeghbroeck1, 2 1 Boulder Advanced Technology Center*, Advanced Power Technology, Boulder, CO, 80301, USA Department of Electrical and Computer Engineering, University of Colorado, Boulder, CO 80309, USA 2 Abstract — 4H-SiC RF BJTs on a semi-insulating (>105 Ω-cm) substrate were designed and fabricated for the first time using an n-p-n triple mesa-etch and interdigitated emitter-base finger design. On-wafer small signal s-parameter measurements were performed on a 4-finger device with 3 µm emitter stripe width and 150 µm finger length. Both, the current gain and unilateral power gain, were calculated from the measured s-parameters, yielding an fT of 7 GHz and an fMAX of 5.2 GHz biased in common-emitter configuration at JE = 10.6 kA/cm2 and VCE = 20 V. These are the highest RF figures of merit reported to date for any SiC bipolar transistor. The calculated maximum available power gain (GMAX) is 18.6-dB at 500 MHz and 12.4-dB at 1 GHz, demonstrating the potential of 4H-SiC BJTs for both UHF and Lband applications. Index Terms — 4H-SiC, RF BJTs, semi-insulating substrate, fT, fMAX, GMAX, UHF, L-band. II. DEVICE DESIGN AND FABRICATION Emitter Base Collector SiO2 passivation n-emitter p-base Isolation mesa n-collector n-buffer layer Semi-insulating 4H-SiC substrate Fig. 1. The schematic cross-sectional structure of a 4H-SiC BJT on a semi-insulating substrate. I. INTRODUCTION * Previously PowerSicel, Inc. 0-7803-8846-1/05/$20.00 (C) 2005 IEEE Emitter pad Collector fingers Collector pad Base pad Emitter pad 150 µm Collector “Bridge” (Ti/Au) Collector pad 4H-SiC bipolar junction transistors (BJTs) are promising RF power devices for operation up to 1 GHz with the ability to handle large power [1, 2] and to operate at a large collector voltage [3]. More specifically, compared to its silicon counterparts, SiC devices can be operated at 10 times the voltage, for a given drift region thickness, due to the 10 times larger breakdown field of SiC [4]. The attainable power density is also higher due to the excellent thermal conductivity of SiC and its wide energy band-gap. Previously, a 4H-SiC BJTs was reported with up to 4 GHz fT and up to 1.8 GHz fMAX [5, 6, 7]. In this work we have improved fMAX almost threefold. Recently, high purity semi-insulating 4H-SiC wafers were developed [8, 9] and are now commercially available. Devices on semi-insulating substrates have been demonstrated [10, 11] with improved RF performance due to the reduction of parasitic components. In this paper, we report the first 4H-SiC RF BJTs fabricated on a semi-insulating substrate with an fT/fMAX of 7/5.2 GHz, and GMAX of 12.4-dB at 1 GHz and 18.6dB at 500 MHz. These are, to the best of our knowledge, the highest values published to date for any SiC bipolar transistor. 50 µm Fig. 2. Micrographs of a 4-finger RF BJT with front-collector“bridge” contacts and on-wafer RF pad layout. The epitaxial structures (n-p-n) were grown by Acreo AB 5 (Sweden) on a 2-inch 4H-SiC semi-insulating substrate (>10 Ω-cm). The nominal thickness and doping density of each epilayer are listed in Table I. Bipolar transistors were fabricated using a triple mesa-etch and an interdigitated emitter-base finger structure. Each device was completely isolated by etching the isolation mesa into the semi-insulating substrate. A cross-sectional drawing of a single finger structure is shown in 2 Fig. 1. The emitter mesa area is 3×150 µm and the base mesa 2 area is 11×150 µm . The surface was passivated with a thin layer of thermal oxide followed by a layer of deposited oxide. Ni/Cr was used for the emitter and collector contacts, Ti/Al for the base contacts, and Ti/Au for the wiring and pads as well as the front-collector-“bridge” contacts as shown in Fig. 2. The fabrication process has been discussed in detail elsewhere in the literature [12]. TABLE I NOMINAL DOPING DENSITY AND THICKNESS OF EPI-LAYERS Wafer (S.I.) n-contact n-emitter p-base n-collector n-buffer layer Substrate Thickness 40 nm 100 nm 140 nm 1000 nm 700 nm 300 µm Doping 19 -3 9×10 cm 19 -3 3×10 cm 18 -3 8×10 cm 15 -3 8×10 cm 19 -3 1×10 cm 18 -3 ~10 cm Dopant Nitrogen Nitrogen Aluminum Nitrogen Nitrogen Vanadium h21 = U= III. RESULTS AND DISCUSSION A DC characterization for the common-emitter configuration was performed to qualify the RF transistors as well as to identify the proper DC bias points for the small signal measurements. A typical I-V is illustrated in Fig. 3. The maximum DC current gain βmax is 11 and decreases at higher bias due to the self-heating. The maximum emitter current 2 density JE is 10.1 kA/cm at VCE = 20 V with a corresponding 2 DC power dissipation of 200 kW/cm normalized to the emitter mesa area. The breakdown voltage is greater than 100 V in spite of the 1 µm collector drift region. 250 IC (mA) IB =20 mA 150 100 2 mA/step 50 IB =2 mA 0 0 5 10 15 VCE (V) 20 2 ⋅ s 21 s12 ⋅ s 21 + (1 − s11 ) ⋅ (1 + s 22 ) s 21 / s12 − 1 (1) 2 (2) 2 ⋅ k s 21 / s12 − 2 ⋅ Re( s 21 / s12 ) (3) GMAX = s 21 / s12 ⋅ (k − k 2 − 1) The DC current-voltage (I-V) properties of the transistors shown in Fig. 2 were measured with an HP 4155C. The onwafer small signal RF measurements were performed using an Agilent E5071B network analyzer with GSG probes. All measurements were done at room temperature. The network analyzer was calibrated using Short-Open-Load-Thru (SOLT) standards. The s-parameters were measured from 8 MHz to 8 GHz at a collector-emitter voltage (VCE) of 20 V and at multiple emitter bias current densities (JE). 200 The high frequency performance is characterized by onwafer small signal s-parameter measurements with the remaining parasitic components de-embedded based on a widely used correction procedure [13]. The AC common emitter current gain |h21|, the unilateral power gain U, and the maximum available power gain GMAX were calculated from measured s-parameters using the following formulae [14, 15]: 25 Fig. 3. I-V characteristics of a 4-finger RF transistor. IB = 2, 4, …, 18 and 20 mA. 2 k= 2 1 − s11 − s 22 + s11 ⋅ s 22 − s12 ⋅ s 21 2 s12 ⋅ s 21 2 (4) Where k is the Rollett stability factor. The de-embedded frequency dependence of |h21|, U and GMAX from a 4-finger 4H2 SiC transistor biased at VCE = 20 V and JE = 10.6 kA/cm are presented in Fig. 4. fT was extrapolated from the fitted line of |h21|. fMAX was obtained from U and GMAX at the frequency where the gain has decreased to 0-dB. The values of fT and fMAX are 7 GHz and 5.2 GHz respectively, the highest numbers reported to date for any SiC bipolar transistor. The improvement of fT and fMAX is due to the reduction of parasitic components achieved by the use of the semi-insulating substrate. A more detailed discussion of this can be found elsewhere [12]. The maximum available power gain GMAX was calculated to be 12.4-dB at 1 GHz and 18.6-dB at 500 MHz, showing the potential of SiC RF bipolar devices for UHF and L-band applications such as radar, broadcast and wireless communication. It is worth noticing that the calculated U and GMAX follow the expected 20-dB/decade slope, however, the slope of the fitline to |h21| is not 20-dB/decade, but 14-dB/decade. The latter is explained by the back-injection current flowing from base to emitter in homojunction bipolar transistors, resulting in the small signal emitter injection efficiency significantly lower than one [14]. With the increase of back-injection effects, the slope of |h21| versus frequency decreases from 20-dB/decade to 10-dB/decade. Current Gain |h 21| (dB) 30 25 Fit-line The extracted fT and fMAX from the measured s-parameters of two 4-finger RF transistors biased at various emitter current densities are summarized in Fig. 5, with the calculated results based on an ideal small signal transit-time model [16]: 20-dB/dec 20 1 = τ EC = τ E + τ B + τ SC + τ C 2πf T 15 10 VCE = 20 V JE = 10.6 kA/cm2 5 = fT =7 GHz VT (C j , BE + C j , BC ) JE 0 0.01 0.1 1 Frequency (GHz) Fit-line (20-dB/dec) 20 fMAX =5.2 GHz VCE = 20 V JE = 10.6 kA/cm2 0 0.1 1 Frequency (GHz) 10 2 µ n , BVT + x dep , BC 2v sat + RC C j , BC fT 8πR B C j , BC (5) (6) 8 fT 6 4 fMAX 2 0 100 (b) Maximum Available Gain G M AX (dB) 2 10 Frequency (GHz) Unilateral Power Gain U (dB) 40 10 wB' Where Cj,BE and Cj,BC are the base-emitter and base-collector junction capacitance. The electron mobility in the base region, µn,B, is 185 cm2/V-s [12] and the saturation velocity vsat is 2×107 cm/s [17]. (a) 30 f MAX = 10 + 1,000 10,000 100,000 2 Emitter Current Density (A/cm ) 20 16 18.6-dB@500 MHz Fig. 5. fT and fMAX extrapolated from s-parameter measurements at different JE for two 4-finger RF transistors (open and filled symbols) at two different locations on the wafer. Solid lines represent the calculated values of fT and fMAX using the transit-time model. Fit-line (20-dB/dec) 12 12.4-dB@1 GHz 8 IV. CONCLUSION VCE = 20 V JE = 10.6 kA/cm2 4 fMAX =5.2 GHz 0 0.1 1 Frequency (GHz) 10 (c) Fig. 4. (a) Current gain |h21|; (b) unilateral power gain U; and (c) maximum available power gain GMAX calculated from the measured sparameters versus frequency of a 4-finger 4H-SiC BJT. 4H-SiC RF BJTs on a semi-insulating substrate were designed, fabricated and tested for the first time. On-wafer small signal s-parameter measurements show a 7 GHz fT and a 5.2 GHz fMAX. With the improvement of fT and fMAX, the maximum available power gain GMAX is 12.4-dB at 1 GHz and 18.6-dB at 500 MHz, showing the potential of these devices for UHF and L-band applications. ACKNOWLEDGEMENT This work was funded in part by a NIST/ATP grant. REFERENCES [1] I. Perez-Wurfl, J. Torvik, and B. Van Zeghbroeck, “Analysis of power dissipation and high temperature operation in 4H-SiC 2 bipolar junction transistors with 4.9MW/cm power density handling ability,” Materials Science Forum, vols. 457-460, pp. 1121-1124, 2004. [2] C. F. Huang, I. Perez-Wurfl, F. Zhao, J. Torvik, R. Irwin, K, Torvik, F. Abrhaley and B. Van Zeghbroeck, “215W pulsed class A UHF power amplification based on SiC bipolar technology,” 2004 IEEE DRC Dig., Late News Papers, 2004. [3] S. H. Ryu, A. K. Agarwal, R. Sing, and J. Palmour, “1800V npn bipolar junction transistors in 4H-SiC,” IEEE Electron Device Let., vol. 22, no. 3, pp. 124-126, March 2001. [4] M. A. Capano and R. J. 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