Self-limiting advancing gates for GaAs metal–semiconductor field effect transistors

Abstract

The threshold voltage of GaAs metal–semiconductor field effect transistors (MESFET’s) can be controlled by the use of buried gates. The controlling mechanism is through the alloying reaction between the gate material and the GaAs substrate. Although this can be done by choosing suitable annealing conditions, the alloying reaction may continue to proceed if the operating temperature of the device is high and/or for long-device field service. To overcome this problem a Ni–W alloy is chosen as the gate metallization. The idea here is to utilize the concentration dependence of the interfacial reactivity between the Ni–W alloy and the GaAs to limit the alloying reaction. Below 400 °C Ni leaches out of the Ni–W alloy to react with the GaAs, and W does not participate in the reaction. As Ni leaches out, the composition of W at the interface increases and this appears to choke off the reaction, resulting in a self-limiting reaction for buried gates. We found that a thin-Pd (20 Å) layer placed at the GaAs/NiW interface is helpful in improving the interfacial reaction uniformity. It was found that the threshold voltage of GaAs MESFET’s can be adjusted to any desired value using this approach. After the desired threshold voltage has been obtained, the mean value of the threshold voltage of 30 field effect transistors (FET’s) is found to stabilize to ±5 mV with a standard deviation of ±27 mV at a temperature of 235 °C for a period of at least 90 h. The activation energies for the change in threshold voltage at different stages of anneal have also been estimated.