Electrical behavior of an NPN GaAlAs/GaAs heterojunction transistor

This paper deals with a theoretical and experimental study of GaAlAs/GaAs heterojunction transistors fabricated by liquid phase epitaxy.Starting from the general transport equations in a structure of variable composition, the expressions for the injection efficiency and for the transfer characteristic (I_C, V_BE) are first established. Using reasonable simplifying assumptions, allowed by the physical characteristics of the base and emitter regions, the major role played by the zone including the conduction band spike is clearly shown. In particular, it appears that the presence of such a spike entails a substantial decrease of the injection efficiency and also modifies, over a wide bias range, the (I_C, V_BE) characteristic which then follows a non ideal law.The experimental behaviour of devices fabricated by liquid phase epitaxy are described. The measurements were carried out at temperatures ranging from 77 to 316°K. The results obtained are compared with those predicted by the theoretical model.