First Demonstration Of 4H-SiC RF Bipolar Junction Transistors On A Semi-insulating Substrate With fT/fMAX Of 7/5.2 GHz
Boulder Advanced Technology Center, Microsemi Corporation
Abstract — 4H-SiC RF BJTs on a semi-insulating (>105 O-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.
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 18.6- dB at 500 MHz. These are, to the best of our knowledge, the highest values published to date for any SiC bipolar transistor.
Copyright (c) 2005 IEEE. Presented at IEEE MTT-S 2005 International Microwave Symposium (IMS).
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*Also of Department of Electrical and Computer Engineering, University of Colorado