On the current mechanism in reverse-biased amorphous-silicon Schottky contacts. II. Reverse-bias current mechanism

Abstract

The physical mechanisms that determine the current transport in reverse-biased Schottky diodes on undoped ‘‘device-grade’’ hydrogenated amorphous silicon (a-Si:H) are elucidated. The current-voltage (J-V) curves for several Schottky diodes up to reverse-biases of 40 V have been measured at temperatures between 40 and 180 °C. The reverse currents generally increase approximately exponentially with reverse bias. The decrease of the apparent barrier height as obtained from internal photoemission experiments is in good agreement with the decrease of the thermal activation energy with reverse bias. Extra information on the current transport mechanism can be obtained from the bias dependence of the prefactor in the Arrhenius plot. A theoretical model is presented which gives a semiquantitative fit to all the features observed in the experimental data. The model involves quantum-mechanical tunnelling of a thermal distribution of carriers through an image-force lowered triangular potential shape. At low reverse bias, the apparent barrier height decreases due to image-force lowering alone and the prevailing carrier transport mechanism is drift/diffusion or thermionic emission over the barrier, which can be determined from the bias dependence of the conduction prefactor in the Arrhenius plots. At higher fields, the apparent barrier height decreases faster than the image-force lowering. This is due to tunnelling of carriers through (the top of) the potential barrier and the apparent barrier becomes approximately equal to the mean energy at which the carriers move through the barrier. This energy is lowered with increasing reverse bias. The conduction prefactor from the Arrhenius plot now decreases with increasing applied bias and gives an indication of the effective tunnel probability.