Transition from the bipolar to the bulk-barrier transistor

In a conventional bipolar transistor the current is limited by diffusion in the neutral base region. In the bulk-barrier transistor the current is determined by diffusion or thermionic emission over a potential barrier. This type of operation occurs in bipolar transistors if the width of the neutral base is below a definite value. The limit between bipolar- and bulk-barrier mode is defined. The current voltage-characteristic as well as the small signal equivalent circuit are given.