Dual emitter InGaP/GaAs HBT for MMIC power amplifier applications

Abstract

Design of small size of power transistor while keeping the power performance is one of the key issues for power amplifier applications. In order to achieve high power drivability, the transistor``s breakdown voltage and current density are also needed to be high enough with maintaining small size. Because of GaAs-based HBT is superior to field-effect transistor in these characteristics, the HBT is most promising device in power amplifier applications. A new structure of InGaP/GaAs HBT for MMIC power amplifier application was devised in this work. The HBT which is characterized by dual emitter fingers surrounding a base finger, has been fabricated with a standard $4\mum$ emitter-width process. The performance has been analyzed in the aspect of D.C. characteristics, thermal effect, power density, and small signal gain performance comparing to conventional HBT structure which has single emitter with dual base finger. The dual emitter structure has been simulated and measured in thermal behavior for high power application. The proposed dual emitter structure has been identified to have following key advantages in the power amplifier application compared to the conventional structure. 1) The collector current density can be increased a factor of two compared to the conventional device. 2) The junction temperature of the dual emitter structure is lower than that of conventional device, so the dual emitter structure is kept from current gain collapse without emitter resistance. 3) Because of less degradation due to thermal limitation, the collector current range of optimum $f_T$ and $f_max$ can be doubled. 4) The 1B2E structure has maximum available gain of above 23 dB at 2 GHz, which is enough gain for the power amplifier of 2 stages.