Study On Low Temperature Characteristics Of GaN-based HEMT

The band gap of GaN is 3.45eV, which is a wide band gap semiconductor. In addition to high electron saturation velocity, the thermal properties of GaN material is also very good, which makes GaN promising in microwave poweraplications. Currently, GaN-based AlGaN/GaN hetero structure HEMT devices are the main representative of microelectronic devices, which has been widely used in the aerospace field. Since the device is always long-term working in high temperature and cryogenic environments alternately, the characteristics at high and low temperature conditions are very important. Currently most researches are about the high reliability of the device in high temperature conditions, and the mechanisms are almost mature. It is not enough just studying high-temperature reliability in the field of aerospace environment, the knowledge of low-temperature characteristics is also very necessary. In this paper, we study the DC characteristics of AlGaN/GaN HEMT devices at low temperature, analysis the DC characteristics of HEMT devices in both theoretical and experimental aspects.At first it is necessary to measure the DC characteristics of HEMT devices at room temperature to ensure that the devices are used with good performance at room temperature. Then we measure the DC characteristics of HEMT devices at cryogenic temperature, by comparing the DC characteristics curves of room temperature and cryogenic temperature, to find out the DC characteristics differences, and analysis the mechanisms. We test the Schottky gate characteristics at both room temperature and cryogenic temperature. By observing the transfer curve, find out the transconductance value at low temperature of the devices increases, which indicates that the drain output current can be better controlled by the gate voltage. There is also analysis of Schottky barrier height and ideality to assess the quality of the Schottky contact. At the same time we extract the gate reverse leakage current value at different gate voltage variation with the temperature, also find out the gate reverse leakage current values at low temperature of the devices are larger than those at room temperature. This is partly because the Schottky barrier height decreases with decreasing temperature, leading to increase of electrons tunneling probability, so that the gate reverse leakage current values increase. On the other hand, there are many defects existing in the material, the defects can cause the so-called trap-assisted tunneling current by assisting electrons tunneling, which increases the gate reverse leakage current values at low temperature.Then according to the comparing of drain output current curves at both room temperature and low temperature, we can observe that the drain output current values at low temperature are larger than those at room temperature at different gate voltage. According to the DC output characteristics of the devices, it can be concluded that this phenomenon is as the temperature changes, factors includuing the threshold voltage, 2DEG density and mobility affecting the saturation drain output current vary. The threshold voltage reduces at low temperature, which is due to both the Schottky barrier height decreases and the conduction band differences ΔEc increases with the temperature changing. In addition, from the given curves of 2DEG density and mobility changes with temperature, it is clear to see the 2DEG density increasing a small margin with decreasing temperature. The increasing value is so small that it has little impact to the drain output current value of the devices. The 2DEG mobility has a substantial increasing at low temperature, which is the main reason for increasing of the saturation drain output current value. The 2DEG mobility increases at low temperature, the result is mainly due to the influence of various scattering mechanisms at low temperature.It is also available to analyze the characteristics of HEMTs by C-V,pulse and noise measurement to complete the analysis of devices.