Generation mechanism of gate leakage current due to reverse-voltage stress in i-AlGaAs/n-GaAs HIGFET's

Device degradation characterized as an increase in the gate leakage current due to continuous reverse-voltage stress was investigated for a 0.35-mu m WSi gate i-AlGaAs/n-GaAs doped channel HIGFET (heterostructure insulated-gate field-effect transistor). The gate leakage current, which was dominated by a hole current generated by impact ionization, was found to increase after the application of a gate-to-drain voltage of -6 V for a certain period. The occurrence of the impact ionization was evidenced by the generation of a substrate current and by the negative temperature coefficient of the gate current. The degradation was retarded at an elevated temperature, indicative of hot-carrier-related degradation. The degraded device also showed an ohmic-like gate leakage current. Subsequent annealing at temperatures above 300 degrees C significantly restored the current-voltage (I-V) characteristics. From these observations, a degradation model was developed in which hot holes generated by impact ionization are trapped in the insulator/semiconductor interface, contracting the surface depletion region and thereby increasing the electric field near the gate-edge. A surface treatment using CF4 plasma was used to suppress the degradation. An FET fabricated using this treatment showed a remarkable decrease in degradation.