Study Of The Breakdown Characteristics In GaAs MESFET's

GaAs crystal is a kind of III-V compound semiconductor material with excellent electrical performance. GaAs semiconductor devices and integrated circuits are widely used because of their high speed of information processing, super-high frequency, low power consumption and low noise. GaAs devices and integrated circuits show their importance in microwave communication and military application. MESFET is the most universal and mature device in GaAs circuits. At present there exists a problem in domestic MESFET that gate-drain breakdown voltage decreases obviously and isn't stable after passivation. It affects the output power, efficiency, the ability to anti-burnout and long-term reliability in GaAs MESFET's. The study for the breakdown characteristics in GaAs MESFET's is at initial stage in our country, so it becomes very important to solve the problem of breakdown in GaAs MESFET's. In order to satisfy the need of civil use and national defence, we must solve the problem that gate-drain breakdown voltage decreases obviously after passivation.This paper has studied systematically the breakdown characteristics of GaAs MESFET's. We performed a XPS spectrum test to the surface of a few GaAs epitaxial samples after treated with several kinds of methods. The Ga3d and As3d spectrums were compared for these samples. We also contrast the number ratio of Ga to As atoms in the surface of the samples. The result shows that ammonia solution and (NH4)2SX solution are effective in removing GaAs native oxides. The number ratio of Ga to As atoms is approximately 1:1 in the GaAs surface after treated with UV ozone or ammonia solution. The number ratio of Ga to As atoms is approximately 1.5:1 in the surface after treated with (NH4)2SX solution, that is to say, there are excess Ga atoms in thesurface.We fabricated GaAs MESFETs in the 3 inch production line of the 13th electronic research institute, and we investigated their dynamic and direct current breakdown characteristics. During the test for dynamic breakdown characteristics, we found that breakdown voltage increases as the lengths of the pulses applied to the gate and drain electrodes increase. We think that the relation between breakdown voltage and pulses lengths could be mainly due to the influence ofsurface states.During the test for DC characteristics, we treated GaAs MESFETs using (NH4)2SX solution, acid solution, alkali solution and plasma. We found that the breakdown voltage in GaAs MESFET's increase obviously after treated with (NH4)2SX solution. We think that the sulfur passivation reduces the ratio N/M, in the surface of GaAs, where Nd is the concentration of donor-type defects and Na is the concentration of acceptor-type defects. The increasement of acceptor states in the surface increases the density of negative charge. The negative charge in the surface can reduce the density of electric field lines at the drain-side edge of the gate electrode,the electric field at the drain-side edge of the gate decreases with the increasing of negative charge density in the surface, so the breakdown voltage will increase in GaAs MESFET's. For the decrease of the breakdown voltage in GaAs MESFET's after annealed under vacuum, we think that As-S bonds are unstable and decompose under high temperature, then the concentration of acceptor-type defects induced by As-S bonds decrease and the negative charge density in the surface decreases, so the breakdown voltage in GaAs MESFET's decreases. We didn't find that the breakdown voltage changes obviously in the GaAs MESFET's after treated with acid solution, alkali solution and plasma.On the basis of the experimental results, we think the most important factor affecting the breakdown characteristics in GaAs MESFET's is the density of negative charge induced by acceptor-type defects in the surface. The electron field strength at the drain-side edge of the gate will be weakened as the density of negative charge increases in the surface, so the breakdown voltage in GaAs MESFET's will increase.