High frequency small signal characteristics of the SCL heterojunction transistor

The space-charge-limited (SCL) heterojunction transistor is represented as a planar unit area structure. The internal small signal admittances of the device are derived as functions of transit angle or frequency and the base width and built-in-field. A practical GaAs-Ge heterojunction transistor is used to illustrate the derived admittance characteristics. It is shown, for the model chosen, that the internal transconductance is controlled predominantly by the collector region.The SCL heterojunction transistor is subsequently represented by an equivalent circuit constructed of the internal admittances. The two port Y parameters of the device are then derived from the equivalent circuit and shown as functions of transit angle or frequency. The unilateral gain is also derived as a function of transit angle or frequency.Finally the f_τ and f_max of the GaAsGe model are calculated and it is shown that the maximum value of f_max of 50 GHz obtainable is limited by collector transit time effects.