EFFECTS OF QUASI-BALLISTIC BASE TRANSPORT ON THE HIGH-FREQUENCY CHARACTERISTICS OF BIPOLAR-TRANSISTORS

High-frequency transport in bipolar transistors with quasi-ballistic b ase widths (on the order of a minority-carrier scattering length) is e xamined by using the approach of Grinberg and Luryi to solve the Boltz mann transport equation (BTE). By considering the phase angle of the d ynamic distribution function in wave-vector space, it is shown that th e ballistic mechanism of decay in the common-base current gain becomes important even for base widths in the quasi-ballistic regime. Simple expressions, which correctly yield both the magnitude and phase of all the forward characteristics, as predicted by the BTE, up to the intri nsic transit frequency, are found by combining the results from a one- flux approach with the wen-known expressions of Thomas and Moll. Expre ssions for the reverse small-signal parameters are also found by apply ing a ''moving boundary condition'' to the basic one-flux equations of Shockley.