Fabrication, testing and reliability modeling of copper/titanium-metallized GaAs MESFETs and HEMTs for low-noise applications

Today, GaAs based field effect transistors (FETs) have been used in a broad range of high-speed electronic military and commercial applications. However, their reliability still needs to be improved. Particularly the hydrogen induced degradation is a large remaining issue in the reliability of GaAs FETs, because hydrogen can easily be incorporated into devices during the crystal growth and virtually every device processing step. The main objective of this research work is to develop a new gate metallization system in order to reduce the hydrogen induced degradation from the gate region for GaAs based MESFETs and HEMTs. Cu/Ti gate metallization has been introduced into the GaAs MESFETs and HEMTs in our work in order to solve the hydrogen problem. The purpose of the use of copper is to tie up the hydrogen atoms and prevent hydrogen penetration into the device active region as well as to keep a low gate resistance for low noise applications. In this work, the fabrication technology of GaAs MESFETs and AlGaAs/GaAs HEMTs with Cu/Ti metallized gates have been successfully developed and the fabricated Cu/Ti FETs have shown comparable DC performance with similar Au-based GaAs FETs. The Cu/Ti FETs were subjected to temperature accelerated testing at NOT under 5% hydrogen forming gas and the experimental results show the hydrogen induced degradation has been reduced for the Cu/Ti FETs compared to commonly used AuPtTi based GaAs FETs.; A long-term reliability testing for Cu/Ti FETs has also been carried out at 200°C and up to 1000hours and testing results show Cu/Ti FETs performed with adequate reliability. The failure modes were found to consist of a decrease in drain saturation current and pinch-off voltage and an increase in source ohmic contact resistance. Material characterization tools including Rutherford backscattering spectroscopy and a back etching technique were used in Cu/Ti GaAs FETs, and pronounced gate metal copper in-diffusion and intermixing compounds at the interface between the gate and GaAs channel layer were found.; A quantifying gate sinking degradation model was developed in order to extend device physics models to reliability testing results of Cu/Ti GaAs FETs. The gate sinking degradation model includes the gate metal and hydrogen in-diffusion effect, decrease of effective channel due to the formation of interfacial compounds, decrease of electron mobility due to the increase of in-diffused impurities, and donor compensation from in-diffused metal impurity acceptors or hydrogen passivation. A variational charge control model was applied to simulate and understand the degradation mechanisms of Cu/Ti HEMTs, including hydrogen induced degradation due to the neutralization of donors. The degradation model established in this study is also applicable to other Au or Al metallized GaAs FETs for understanding the failure mechanisms induced by gate sinking and hydrogen neutralization of donors and con-elating the device physics model with reliability testing results.