Research On Characteristics Of AlGaN/GaN HEMT Based On Field Plate And LDD Technology

GaN has many advantages over conventional materials such as Si, Ge and Ga As. High breakdown field, high electron mobility and radiation resistance make GaN to be a very promising material for high-power applications. AlGaN/GaN high electron mobility transistors(HEMTs), which are fabricated with AlGaN/GaN heterojunction, are the most promising candidate among microwave power devices, because of its high breakdown voltage, good frequency characteristic and output characteristic. However, the breakdown voltage of present GaN-based HEMTs is still much lower than the theoretical value.Two 2D AlGaN/GaN HEMT models are established. One is with field plates and the other one is with low-density drain(LDD). The influences of the two structures on electric field distribution and breakdown voltage are researched by simulation.Firstly, the mechanism how a gate field plate improves the breakdown voltage is studied. By researching the step field plate and the composite float field plate, it is demonstrated that these two structures can introduce more electric field peaks than ? gate field plate. Furthermore, parameters such as field plate length, plate spacing and passivation thickness are optimized. The breakdown voltage is improved from 660 V to 832 V. The influences of LDD structure on threshold voltage are also studied, which shows that the fixed negative charges under the gate can shield the positive polarization charges. As a result, the two-dimensional electron gas(2DEG) is suppressed. The device is off at VGS = 0 V, which means to be an enhancement mode device. The simulation illustrates that the function of the negative charge region is similar to that of a gate field plate. An electric field peak is introduced at the edge of the region, which redistributes the field and increase the breakdown voltage. In order to cover the shortage of output characteristic loss caused by LDD, fixed positive charges are injected next to the drain. Meanwhile, another electric field peak is introduced. Finally, an enhancement mode HEMT with a breakdown voltage of 1073 V and a specific on resistance of 0.544 m?·cm2 is achieved.In this thesis, the field plate and LDD structure are studied to improve the breakdown voltage of a HEMT, which provides a theoretical basis and foundation to the future work.