Slow Capture of Holes and Electrons by Surface States on Germanium and Silicon at Low Temperatures

Field-effect decay curves at temperatures the order of liquid nitrogen are presented for $n$- and $p$-type germanium and for $p$-type silicon. A strongly asymmetric decay is observed, depending upon whether the induced charge is positive or negative. The decay is strongly influenced by illumination, temperature, and magnitude and sign of the field. Injected minority carriers also affect the decay rate. It is suggested that the trapping centers involved are the fast states, or recombination centers at the surface. After a disturbance of the charge in these states the approach to steady state is slow due to the limited supply of minority carriers arriving at the surface. The results observed are compared with a model which takes surface barrier effects into consideration and are found in reasonably quantitative agreement.