Effect of surface recombination and channel on P-N junction and transistor characteristics

The current flowing in a semiconductor junction may be divided into four components according to the location of the recombination and generation of electrons and holes. These are: 1) the bulk recombination-generation or the diffusion current, 2) the bulk recombination-generation current in the transition region, 3) the surface recombination-generation current in the transition region, and 4) the surface channel current. The current-voltage relationship for these four current components may be approximated by
I = I_{s} \exp (qV/mkT)
, if
V > 4kT/q
. Analysis shows that the reciprocal slope
m
for the current components 1) to 3) lies between 1 and 2, while for the surface channel current component 4)
m
is greater than 2 for silicon junctions and may be more than 4 for large channels. The theoretical expressions for these four current components are tested with extensive experimental data taken on silicon junctions and found to account for all of the observed current-voltage characteristics. The importance of surface recombinations and surface channels on the current gain of silicon transistor is also demonstrated experimentally. The experimental data are in accord with the theoretical prediction based on the transistor current equations which include the carrier recombination in the emitter junction and the carrier generation in the collector junction.