Double-Carrier Injection and Negative Resistance in CdS

Light-sensitive, negative-resistance characteristics, arising from double-carrier injection, have been observed in selected CdS single crystals. The double injection, occurring at Ag and In contacts, is accompanied by relatively intense light emission (green edge fluorescence) even at low injection levels (field strengths often less than 10 V/cm) and the emission is observed largely in the neighborhood of the positive Ag electrode. After appropriate light stimulation of these crystals, pronounced and reproducible negative resistance and semiconductor "regimes," as well as "current oscillations," have been observed at 4.2^∘{}K; negative resistance and low-injection-level, space-charge-limited current regions (I∝V²) have been observed at 77^∘{}K. Negative resistance has been observed at low-field strengths (50 V/cm) in optically stimulated samples; no direct evidence of breakdown effects was observed in unstimulated samples up to a few kV/cm. In terms of Lampert's double-injection (semiinsulator) model, injection-level-dependent hole lifetimes are indicated at 4.2^∘{}K; also, optical-stimulation, level-dependent I-V characteristics are indicated at 4.2^∘{}K. The I-V characteristics are discussed in terms of Lampert's theory, as well as a breakdown-induced injection theory due to Steele, Ando, and Lampert. A previously proposed mechanically excited emission model, due to Warschauer and Reynolds, is invoked to explain a possible mechanism for double injection in these unusual crystals.