Transient Currents in Semi-Insulating CdTe Characteristic of Deep Traps

Abstract

The transport of charge in single crystals of semi-insulating cadmium telluride has been studied under the influence of deep trapping and subsequent thermal release of carriers from traps. Trapping and detrapping times for electrons and holes are determined directly from the shape of the transient response of surface-barrier devices to alpha particles. Hole-trapping times are 60 nsec; electron-trapping times range from 30 to 90 nsec. The measurement of the detrapping times from −60° to 60° C indicates that an electron trap with an activation energy of 0.59±0.04 eV can exist below the conduction band in semi-insulating material. The present measurement of drift mobilities, trap densities, and trapping cross sections does not require observation of a transit time. Furthermore, pulse analysis is not limited to times less than the transit time, a restriction in previous drift measurements. The theoretical response which accounts for both the trapping and detrapping of charge was derived by solving the kinetic differential equations which represent charge conservation. This extends the small-signal theory of transient currents in insulators to times beyond the transit time. Excellent agreement is found between experimental traces and theoretical shapes using a single-level trap model. Limiting cases which allow convenient measurement of the material parameters are described. In addition, the energy required to form an electron-hole pair is redetermined and found to be 4.9±0.1 eV.