Development And Characteristic Analysis Of AlGaN/GaN Heterostructure High Electron Mobility Transistors

High electron mobility transistors(HEMTs) based on AlGaN/GaN heterostructures have wide application for high temperature and high power electronic devices. But the process technology of the HEMTs is not mature, and many key process, such as dry etching, schottky contact, ohmic contact, passivation and air-bridge are needed to improve. More works are needed to accomplish for optimizing structures, improving temperature characteristics and reliability on AlGaN/GaN HEMTs. In this dissertation, the key process technology and the correlations between characteristics and process, structure, temperature, stress are studied. The major achievements are listed.1. The AlGaN/GaN heterostructure material with excellent electrical characteristics is developed successfully. The changes of the material characteristics are studied in different temperatures. The AlGaN/GaN heterostructures with different layer structures are fabricated to obtain the optimal layer structures for improving device characteristics.2. Inductively coupled plasma(ICP) etching on GaN and AlGaN grown by ourselves is studied. In order to control the etching process effectively, the correlations between etching rates and ICP power, DC bias, pressure and composition of etching gases are studied. At certain effective etching rate, the optimal etching profiles are obtained by using optimized etching conditions and SiO2 mask. The etching selectivity of GaN and Alo.27Gao.73N is studied to get a reference for recessed-gate etching of AlGaN/GaN HEMTs. The etching delay produced by surface native oxides is studied and increasing DC bias can reduce the delay effectively. The etching damage is studied by atomic force microscope. Compared to Cl2/Ar, Cl2/N2 can obtain a few etching roughness at the same etching condition.3. Ni/Au schottky contact on AlGaN/GaN heterostructure grown by ourselves is studied, and the excellent performance is obtained to satisfy the requirement of schottky gates of AlGaN/GaN HEMTs. The ideality factor and the barrier height of schottky contact on GaN are calculated byⅠ-Ⅴmeasurement and C-V measurement. Pre-metallization processing on schottky contact to AlGaN/GaN heterostructures with O2 plasma at different conditions and various solution are studied. We obtained the optimal annealing conditions of schottky contact. By combining pre-metallization processing and annealing, a better method to improve schottky contact is obtained and the effect is conformed by C-V at different frequencies. The effects of surface oxides and interfacial layer on schottky contact are studied. The variational rules of schottky contact characteristics on AlGaN/GaN heterostructure are analyzed during 20-200℃.4. A whole process of AlGaN/GaN HEMTs is provided. After the important processes, such as lithography, metal liftoff, metal deposition, passivation, and electroplating are optimized, AlGaN/GaN HEMTs on sapphire and SiC with excellent characteristics are developed. The optimal layer structures with i-AlGaN or i-GaN cap layers are adopted to improve the schottky characteristics of gates. Self-heat in different substrate is studied, and the saturation current of devices on SiC substrate have a little drop at a high drain bias. Several factors in cut-off frequency are studied and the better frequency characteristics can be obtained by minishing gate-length, contact resistance and parasitical capacitance. The multiple gate-fingers devices are designed, and the air-bridge process is investigated to implement air-bridge interlink.5. The temperature stability of AlGaN/GaN HEMT fabricated by ourselves is investigated. The degradation of device transport property is caused by reduced mobility. The reasons of degradation are analyzed by C-V measurement and the transmission line model (TLM) measurement at variational temperatures. The main parameters of the AlGaN/GaN HEMT are studied at different annealing temperature and duration in N2 atmosphere. A optimal annealing condition is obtained for improving schottky characteristics of gates. The reason of characteristics degradation at different bias after high-field stress is investigated. Comparing the characteristics degradation after high-field stress with current degradation of dc sweep, UV illumination can eliminate the current degradation of dc sweep while UV illumination could not renew the degraded devices of high-field stress.In conclusion, On the basis of fabricating AlGaN/GaN heterostructures with excellent characteristics, the key process such as etching, schottky contact, ohmic contact, passivation and air-bridge are optimized and the AlGaN/GaN HEMTs with excellent characteristics are developed successfully. The devices temperature characteristics and reliability after high-field stress are investigated. A lot of meaningful results are obtained.