Design, analysis and experimental study of RF 4H-silicon carbide npn bipolar junction transistors

4H-SiC bipolar junction transistors (BJTs) are promising RF power devices for operation up to 1 GHz with the ability to handle large bias voltage (>100 V) and large DC (>200 kW/cm2) and RF (>95 kW/cm2) power density. More specifically, compared to its silicon counterparts, the devices can be operated at 10 times the voltage and power density, due to the 10 times larger breakdown field of SiC. However, compared to other SiC devices such as MESFETs and SITs, SiC BJTs have been studied less and have not yet reached the same level of maturity. This thesis therefore focuses on gaining a detailed understanding and obtaining improved RF performance of 4H-SiC BJTs.; The detailed understanding was obtained through the design, fabrication and analysis of three different sets of 4H-SiC BJTs. The first two sets were fabricated on conducting substrates. The device design and fabrication processes were continuously improved during the course of this work, resulting in an increase of the maximum DC current gain, betamax, from 30 to 41, while fT/fMAX was increased from 2.7/0.4 GHz to 4/2.6 GHz respectively at JE = 10 kA/cm2 and VCE = 20 V. A DC device model was constructed to analyze the dependence of beta max on the base thickness and emitter stripe width. A small-signal transit time model was used to extract the individual transit times as well as the minority carrier mobility in the base. A best fit was obtained for a mobility equal to 215 cm2/V-s in a 4 x 1018 cm-3 aluminum-doped p-type base.; A third set of devices was fabricated on a semi-insulating (>10 5 O-cm) substrate to further reduce parasitic elements. A record fT/fMAX of 7/5.2 GHz was obtained for the 4H-SiC BJTs biased at JE = 10.6 kA/cm2 and VCE = 20 V. This result is in excellent agreement with the calculated values based on the small-signal transit time model. The corresponding maximum available power gain (GMAX) is 18.6dB at 500 MHz and 12.4dB at 1 GHz. This result demonstrates the feasibility of using 4H-SiC BJTs for both UHF and L-band applications.; The transit time model was also used to predict the performance of future 4H-SiC BJTs. For a scaled BJT with 0.5mum emitter stripe width and self-aligned base contacts one expects an fT/fMAX of 16/20 GHz. This shows that future 4H-SiC BJTs could also be operated at higher (1.5--5 GHz) frequencies, which enables their use in RF amplifiers for a wider range of applications including wireless.