Printed in Great Britain Pergamon Press pie

Much progress has been made towards the understanding and elimination of the Fermi level pin-ning problem at compound.semiconductor surfaccs aml interfaces. This progress includes the dis-covery of several new techniques for reducing and controlling the surface st;lie density and the achievement of metal work function dominated barrier heights at metal/(100) Ga As interfaces. This paper will review this work and the relevance of various Fermi level pinning thcoties to these re-sults. IN'FROD!/(."I'I()N The development of comlx, und semiconductor devices has been hampered by the presence of an appreciable density of uncontrolled surface states, who ~ origin is still a topic of controversy. These states tend to "pin " the location of the Fermi level at various kinds of semiconductor interfaces at some characteristic energy. This pinning adver~ly affects the performance of ~)th high speed and optoelectronic devices and circuits. The introduction of the lattice matched heterojunction i 1967 by Woodall and Rupprecht (I) became the basis of a partial solution to the "Fermi level pinning" problem for many optoelectronic devices, e.g. I,F, Ds, DII lasers, solar cells, etc.. and for some high speed devices, e.g. IIEMTs and IIBTs. Nonetheless, the Fermi level pinning pt~blem still hamper