Space-charge-limited currents and field emission currents in thin-film CdS diodes

In order to understand and to evaluate evaporated thin films of CdS for their use in electronic devices, a number of diode structures have been extensively studied. These diodes were made rectifying by exploiting the differing properties of metal contacts to the CdS. The metals used behaved as predicted by the classical Schottky theory. Experiments performed under forward bias established that space-charge-limited currents were flowing in the CdS films. These currents indicated a mobility roughly ⅓ to ½ that of single crystal material. Under reverse-bias conditions, Schottky high-field emission was observed. To verify the existence of space-charge-limited currents under forward-bias conditions, the theoretically-predicted inverse-cube dependence for the current as a function of electrode spacing was measured. The square-law currents in the space-charge-limited region implied a free-to-trapped charge ratio of the order of 10^-2. The position of the Fermi level in the material indicated, however, that this relatively high free-to-trapped-charge-ratio stemmed from compensation of the deep trapping centers. Free-electron mobilities for the films of up to 100 cm²/volt sec was measured by observing space-charge-limited currents under pulsed conditions. Values for the mobility were checked by using time-of-flight measurements. These same measurements showed a qualitative agreement with the theory of transient space-charge-limited currents. Under reverse-bias conditions, Schottky-emission observations allowed a calculation of the Richardson factor for the Au-CdS junction that was close to the theoretical value of 120 amps cm^-2deg^-2. Capacitance measurements under reverse bias indicated a true Schottky barrier. These measurements could be used to determine the extent of the penetration of the space-charge region into the CdS film, and thereby to establish the correspondence of the theoretical Schottky-emission field with that measured.