Improvement of Breakdown Characteristics in AlGaN/GaN Power HEMTs

Abstract

Gallium Nitride (GaN) based high electron mobility transistor (HEMT) or heterostructure field effect transistor (HFET) are promising device for high-power switches which has to operate in electrically and environmentally harsh condition. The devices benefits from the material properties GaN offers: high critical field, high carrier mobility and a high saturation velocity of carriers. The breakdown voltage in AlGaN/GaN HEMTs is known to be triggered by gate leakage caused by the concentration of the electrical field at the drain-side edge of the gate electrode. The influence of gate leakage on blocking characteristics is alleviated by reducing the peak intensity of the electric field at the drain–side of the gate. There are two methods to reduce the peak intensity of the electric field: one is to decease the probability of tunneling of electrons into device active area the other is to relieve the crowding of electric field at the drain-side edge of gate. Nickel has been used as a gate electrode of the AlGaN/GaN HEMTs to form the Schottky contact due to its relatively high work function (5.15 eV). In this work, nickel oxide (NiOx) was inserted as the interfacial layer between main gate (Ni) and AlGaN barrier layer for improve the reliability of the AlGaN/GaN HEMTs. NiOx film was formed through the thermal oxidation in furnace. Material property of NiOx film depended on the two main factors: oxidation temperature, density of the film controlled by deposition rate. Only the NiOx film oxidized proper temperature range from 400℃ to 500℃ gave a favorable effect on the device performance. The NiOx film with high atomic density exhibited resistive switching characteristics, which can be used for GaN based memory device. Experiments to verify the effect of NiOx on reverse blocking operation were carried out. At the high temperature reverse bias (HTRB) test, it was found that work function of the NiOx was maintained. Moreover, it played an important role to improve the stable blocking operation. The result of electroluminescence (EL) analysis was consistent with the results obtained from HTRB test. Leakage current of the AlGaN/GaN HEMTs with NiOX interfacial layer measured at 200℃ was lower than that of the conventional device by 3 orders of magnitude. The breakdown voltage of the proposed device was up to 1.5 kV (1480 V). In recent years, improvements of the overall device performance were achieved by adopting metal-insulator-semiconductor (MIS) or metal-oxide-semiconductor (MOS) structure. At the gate region, by insulating gate electrode by means of a dielectric layer, electron injection is suppressed effectively. In this work, improvement of the blocking capability and reliability of AlGaN/GaN MIS-HEMTs employing atomic-layer-deposited (ALD) Al2O3 material was confirmed by experimental results. Mechanism responsible for the leakage current of the proposed device was investigated. Measured Leakage current of the fabricated MIS-HEMT was reduced to the range from sub pA (fA) to few pA. At the HTRB test, MIS-HEMT exhibited proved its thermal stability. Although drain leakage current (IDSS) was increased in proportion to the operational temperature, the leakage current of the proposed device was still lower than that of conventional device by 2 orders of magnitude. Breakdown voltage of the proposed device was up to 2 kV.