Intrinsic Delay Times in Single and Double HBTs Directly Extracted from Measured S-Parameter Data

A new method is presented to evaluate the base and collector transit times, tB and 'te, in Heterojunction Bipolar Transistors (HBTs) from the phase and magnitude of the common-base current gain, a( co), directly extracted from measured Sparameter data. The method is applied to InGaP/GaAs single and double HBTs. A smaller cut-off frequency in the latter device is attributed to 'tB and te due to the trapping of electrons in the conduction band triangular barrier existing ill the basecollector (B-C) heterojunction and smaller saturation velocity of electrons in InGaP as compared to GaAs, respectively. Finally, a new B-C design of InGaP/GaAs DHBTs is proposed to partially compensate the transit time effects