Base-collector design optimisation of InGaP/GaAs DHBTs

In this paper we present the design and realisation of DHBTs based on InGaP/GaAs and compare the performance of these devices with the standard single HBT (SHBT). Optimisation of these devices in terms of base/collector electron collection efficiency is also discussed using the base and collector transit times, τ_B and τ_C , deduced from the phase and magnitude of the common-base current gain, α(ω), which itself was directly extracted from measured S-parameter data. The method is applied to InGaP/GaAs single and double HBTs. A smaller cut-off frequency in the latter device is attributed to τ_B and τ_C due to two effects: trapping of electrons in the conduction band triangular barrier existing at the base-collector (B-C) heterojunction and smaller saturation velocity of electrons in InGaP as compared to GaAs. Finally, a new B-C design of InGaP/GaAs DHBTs is proposed to partially compensate the transit time effects. Numerical simulation of the cut-off frequency demonstrates the superiority of the proposed structure for high frequency applications