Structure Design And Reliability Study Of AlGaN/GaN HEMTs

As a new kind of semiconductor material, Gallium Nitride (GaN) gets a lot of attention in the field of semiconductor devices due to its wide band gap (3.39eV), high breakdown electric field (-3.0MV/cm) and high electron mobility (800-1700cm2/Vs). Composed of GaN material, AlGaN/GaN High Electronic Mobility Transistors (HEMTs) also have advantages in band gap, breakdown voltage and electron mobility. Since the Two Dimensional Electron Gas (2DEGs) formed by the AlGaN/GaN heterojunction structure, of which electron density can be1×1013cm-2, AlGaN/GaN HEMTs can have large output current. Considering the devices having high breakdown voltage at the same time, they can output high power in the end. So AlGaN/GaN HEMTs can be widely used in high frequency and high power applications.For example, how to further optimize the surface electric field of the devices in order to improve the devices’ characteristics, and it means to obtain higher voltage with the smaller conduction resistance. Besides, we should also consider the reliability problems of devices in high frequency and high voltage applications. According to these problems, the structure design and the reliability of the AlGaN/GaN HEMTs are studied in this paper. The main contents are:1. Proposing a method to increase the breakdown voltage of the devices, and it means the regfon of substrate which nears the drain is partly doped with the material of opposite conductivity type. With the help of CSuprem and Apsys, the simulation of AlGaN/GaN HEMTs which have source field plate structure is completed in this paper. Due to the modulation of the body electric field to the surface electric field, the breakdowm voltage of the device is increased in this simulation, and the result shows: when the length of the field plate is2μm, the breakdown of devices has increased by55%.2. Proposing an investigation of gate degradation characteristics of AlGaN/GaN HEMTs under PWM stress. The device under test features double source-connected field plates to reshape the surface electric field. A PWM signal with the operation frequency varied from10kHz to1MHz was applied to the gate electrode while the source was shorted to ground. HP4155B was used to measure the gate characteristic of the device by every30minutes. The value of on-resistance shows an increase of12.9%after the device has been under stress for30minutes with the switching frequency of10kHz and the degradation is improved with higher switching frequency, while the threshold voltage and gate leakage current do not change significantly during the test.