Degradation of d.c. parameters in enhancement mode WNx self-aligned gate GaAsMESFETs under high temperature stress

The effect of thermal stress on the d.c. parameter degradation of enhancement mode tungsten nitride (WNx) selfaligned gate GaAs MESFETs was investigated. Threshold voltage, source-drain current and transconductance were measured during the tests. The physical properties of the device after thermal stress were analyzed by means of uger electron spectroscopy (AES), X-ray diffractornetry to identify the degradation mechanism. The d.c. failure mode consists of an increase in the threshold voltage and a decrease in the current and transconductance of the FETs. The device simulator was also used for analytical understanding of the d.c. parameter degradation. The simulated results showed that d.c. parameter degradation was mainly attributed to the increase in source and drain ohmic contact resistances. From the AES analysis, we found that the increase of contact resistance was due to carrier compensation, which was caused by Ga outdiffusion and Ni indiffusion under the ohmic contact layer. Therefore the thermally activated carrier compensation effects by trap generation are proposed to be the main failure mechanism for d.c. parameter degradation of enhancement mode WNy self-aligned gate GaAs MESFETs. 1999 Elsevier Science Ltd. All rights reserved.