Study Of Improving The Breakdown Voltage Of GaAs MESFET

GaAs metal-semiconductor field-effect transistor (MESFET) is very important for developing high performance-microwave power devices and integrated circuits. The gate drain breakdown voltage (BV^gd) of GaAs MESFET is one of the most important factors limiting maximum output power. The BV^gd of domestic GaAs MESFET is about 10V and obviously too low to meet the need of high power output. So, it's necessary to improve the breakdown voltage of GaAs MESFET.In this paper, the effects of the device structure and surface treatment on the breakdown characteristics were studied. A novel GaAs MESFET was designed and fabricated with a GaAs cap layer grown by molecular beam epitaxy (MBE) at low temperature. The function of the LT GaAs is two aspects. Firstly, it can limit the surface current. Secondly, the deep-level in LT GaAs can capture electrons, this can lead to the increase in surface negative charge and decrease in peak value of electric field. So the breakdown voltage is improved effectively. The BV^gd, defined as a gate current density of 100 μ A/mm, was evaluated to be 22 V.At the aspect of improving breakdown voltage by surface treatment, we treated GaAs MESFET with (NH4)SX solution. The breakdown voltage was improved obviously after the treatment. We thought that the sulfur passivation can reduce the ratio N_d/N_a in the surface of GaAs, where N_d is the concentration of donor-type defects and N_a is the concentration of acceptor-type defects. The increase of acceptor states in the surface can increase the density of negative charge. The negative charge in the surface can reduce the density of electric field lines near the gate edge on the drain side, then the electric field decreased with the increase of negative of charge in the surface.The breakdown characteristics of GaAs MESFET can be improved greatly by sulfur passivation technique, but the passivation effect is unstable. We performed plasma-enhanced chemical vapor deposition(PECVD) SiN_x passivation after sulfur passivation under high temperature, low teperature and low/high temperature conditions. The results of this experiment show that a stable passivation effect can beachieved by the second PECVD SiNx passivation, but the breakdown voltage decreased simultaneously. The passivation effect under low/high temperature condition is better than others, these samples have high instability and high voltage. So, if the growth conditions of PECVD SiNx passivation is optimized further, the breakdown voltage would be still high while the instability was maintained.