First demonstration of 4H-SiC RF bipolar junction transistors on a semi-insulating substrate with fT/fMAX of 7/5.2 GHz

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 f_T of 7 GHz and an f_MAX of 5.2 GHz biased in common-emitter configuration at J_E = 10.6 kA/cm² and V_CE = 20 V. These are the highest RF figures of merit reported to date for any SiC bipolar transistor. The calculated maximum available power gain (G_MAX) 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 L-band applications.