Ultrafast operation of heterostructure bipolar transistors resulting from coherent base transport of minority carriers

The author has discussed coherent base propagation effects that can be obtained in stepwise graded and linearly graded bandgap base structures of heterostructure bipolar transistors. These effects open up one or several bands of frequencies above f_T, where the transistor is active. Physically, the active behavior of a coherent transistor results from the phaseshift between the collector current and voltage, acquired during the minority-carrier transit across the base