Direct extraction and numerical simulation of the base and collector delay times in double heterojunction bipolar transistors

A new method is presented to evaluate the base and collector transit times, τ_B and τ_C in heterojunction bipolar transistors (BBT´s) 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 HBT´s. A smaller cutoff 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 DNBT´s is proposed to partially compensate the transit time effects. Numerical simulation of the cutoff frequency demonstrates the superiority of the proposed structure for high-frequency applications