Design considerations for millimeter-wave power HBTs based on gain performance analysis

Critical design issues involved in optimizing millimeter-wave power HBTs are described. Gain analysis of common-emitter (CE) and common-base (CB) HBTs is performed using analytical formulas derived based on a practical HBT model. While CB HBT´s have superior maximum-gain at very high frequencies, their frequency limit is found to be determined by the carrier transit time delay. Thus, to fully exploit the potential gain in a CB HBT, it is essential to maintain a high f_T even at high collector voltages. The advantage of using CB HBT´s in a multifingered device geometry is also discussed. Unlike CE HBTs, CB HBTs are capable of maintaining a high gain even if the device size is scaled up by increasing the number of emitter-fingers. Moreover, it is found that reducing the wire parasitic capacitance allows emitter ballasting resistance to be used without affecting the gain. Fabrication of HBTs based on these design considerations led to excellent power performance in a CB unit-cell HBT at 25-26 GHz, featuring output power of 740 mW and power-added efficiency of 42%