Injection and Transport of Added Carriers in Silicon at Liquid-Helium Temperatures

Abstract

Single-carrier, space-charge-limited conduction is observed in compensated n-type silicon when used in n+nn+ samples at liquid-helium temperatures. The variation of the traps-filled-limit voltage (VTFL) with sample length follows the familiar square-law relationship until the presence of impact ionization of impurities, which occurs at a constant field, becomes the predominant conduction mechanism. A surface current is observed at voltages below VTFL when the n+nn+ samples are in contact with the liquid and no similar effect is observed on p+pp+ samples of similar dimensions and fabrication. Two-carrier injection in compensated n-type silicon at liquid-helium temperatures is also observed when used in n+np+ diodes. A modification of the Lampert-Ashley theory is proposed to include two distinct deep impurity levels. From these double-injection experiments, information about the neutral capture cross sections for boron and phosphorous as well as about the attractive capture cross sections is obtained. The field dependence of the capture cross sections is also established. A surface current is also observed when the n+np+ samples are in contact with the liquid helium.