Improved High Frequency Performance by Composite Emitter AlGaAs/GaInP Heterojunction Bipolar Transistors Fabricated using Chemical Beam Epitaxy

GaInP/GaAs Heterojunction Bipolar Transistors(HBTs) offer significant advantages over AlGaAs/GaAs devices such as large valence band discontinuity and excellent etching selectivity as demonstrated by the authors[1] and other laboratories. Excellent microwave properties have been obtained using GaInP HBT[2] and Chemical Beam Epitaxy(CBE) using TBA/TBP precursors has been reported for material growth of such devices[3]. A common limitation in high speed performance of HBTs has been their relatively large base-emitter capacitance(CBE) which is limited by mobile carrier transport in the emitter region[4]. Mobile carrier transport takes place in traditional HBT designs by diffusion and results in charge accumulation in the emitter and thus increased CBE. To reduce the impact of this effect, a composite AlGaAs/GaInP emitter design was employed as shown in Table 1. A compositionally graded AlGaAs layer forms an electron launcher at the interface with the GaInP layer which injects the electrons at a high kinetic energy towards the remaining part of the emitter, thus resulting in lower free carrier concentration and smaller CBE, especially at high current drives (JC). Although the dynamic resistance of the HBT also increases with JC, the CBE increase in traditional designs plays a predominant role, dominating therefore the emitter time constant(τE). This paper addresses a new emitter design based on composite AlGaAs/GaInP approach which allows significant reduction of CBE and improved high frequency performance.