Failure mechanisms and reliability of low-noise gaas fets

The degradation and failure of low-noise GaAs FETs have been accelerated by various stress-aging techniques including storage at elevated temperatures with and without bias, exposure to humid atmospheres with and without bias, and temperature cycling. Several time-temperature-bias-induced catastrophic failure mechanisms have been observed, all involving the Al gate metallization. These mechanisms are Au-Al phase formation, Al electromigration, and electrolytic corrosion. Each of these processes results ultimately in an open gate. Accelerated aging also produces gradual, long-term degradation in both dc and RF characteristics, though the two are not always correlated. In fact, contrary to some expectations, contact resistance may increase almost two orders of magnitude without significant degradation in the noise figure or gain of a low-noise transistor. Besides contact resistance, other mechanisms such as traps in the channel are thought to play a role in the degradation of RF properties. It was found that all the important degradation mechanisms are bias-sensitive and that aging without bias gives erroneously long lifetime projections. The cumulative failure distributions for the mechanisms observed approximate a log-normal relation with standard deviations between 0.6 and 1.4. The relevant degradation or failure processes have activation energies near 1.0 eV, which give rise to projected median lifetimes at 60°C (channel temperature) over 107 hours and corresponding failure rates (excepting infant mortality) under 40 FITs (40 per 109 device-hours) at 20 years of service.