Study On Junction Temperature Measurements And Temperature Dependent Characteristics Of GaN HEMTs

Al Ga N/Ga N High Electron Mobility Transistors(HEMTs) have superb application for microwave, high power and high temperature fields, and attract much attention during the past decades. It mainly owes to the numerous excellent attributes of Ga N material, such as wide band gap, large breakdown field and high electron saturation drift velocity. Although Al Ga N/Ga N HEMTs have impressive performances, the reliability of devices is still one of the factors which limit their further development. In high temperature applications, the self-heating effect of Al Ga N/Ga N HEMTs is extremely serious, which results in the increase of junction temperature. The measurement of junction temperature plays a pretty important role in the study of thermal reliability of devices, which is because of the big influence of temperature on their electrical performance.Based on the research background mentioned above, this paper conducts research on the junction temperature measurements and modeling. In addition, the reliability issues of Ga N-based HEMTs working at high temperature have also been discussed. A coupled electric-thermal model of Al Ga N/Ga N HEMTs is carried out through Silvaco software with lattice heat flow and heat generation involved, and the Joule heating power distribution under the gate is obtained. The power density is considered as the heat source in the simulation by ANSYS software. The three dimensional finite-element model reveals the temperature distribution and the dependence of junction temperature on dissipated power is also achieved. According to which, the influence of temperature dependent thermal conductivity on the simulation is further discussed, and the option of boundary conditions is illuminated as well.Next, this paper makes a fundamental summary of various junction temperature measurements existed which can be divided into three categories: electrical, optical and physically contacting methods, all of which have their own superiorities and drawbacks in temperature accuracy and spatial resolution. Then two electrical methods are employed in the measurement of junction temperature of Al Ga N/Ga N HEMTs, both of which are based on the change of saturation drain current with temperature. Finally, the measurement results are analyzed in detail, and compared with the simulation result,which verifies the accuracy of these two methods.High temperature characteristics of Al Ga N/Ga N HEMTs including Ohmic contact, schottky contact and DC characteristics are studied systematically, and the possible degradation mechanisms are analyzed. The research on Ohmic contact indicates it has good thermal stability. While with increased temperature, the saturation drain current and peak transconductance degrades more or less, which is mainly due to the decrease in mobility and the reduction in density of 2DEG in channel. This paper puts emphasis on the temperature dependent current-voltage measurement of schottky contact, and summarizes the current transport mechanisms through numerical fitting. It is found that tunneling dominates at room temperature while the thermionic emission(TE) becomes more significant with increased temperature.