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, τB and τC, in heterojunction bipolar transistors (HBT's) from the phase and magnitude of the commonbase current gain, α(ω), which itself was directly extracted from measured Sparameter data. The method is applied to InGaP/GaAs single and double HBT's. A smaller cutoff frequency in the latter device is attributed to τB and τC due to two effects: trapping of electrons in the conduction band triangular barrier existing at the basecollector (BC) heterojunction and smaller saturation velocity of electrons in InGaP as compared to GaAs. Finally, a new BC design of InGaP/GaAs DHBT's is proposed to partially compensate the transit time effects. Numerical simulation of the cutoff frequency demonstrates the superiority of the proposed structure for highfrequency applications. © 1999 IEEE.