Breakdown Mechanism And New Structure Of Vertical AlGaN/GaN HFETs

With the development of science and technology, Gallium nitride(GaN) has been widely researched owe to its excellent characteristics. In this paper, the properties of GaN materials are introduced, and the working principle AlGa N/GaN heterostructure FETs is discussed. The key parameters, which affect the breakdown voltage of vertical devices, are briefly analyzed. In addition, the summaries of currently fabrication process for vertical devices are listed and intercompared each other. Then three new high-breakdown-voltage vertical structures are investigated, and the new structures achieve much higher breakdown voltages with low on-resistance.Firstly, a GaN vertical structure with p-type buried layers is proposed to improve the breakdown voltage and trade-off between BV and Ron. The p-type buried layer could regulate the electric field within buffer layer and the electric field distribution has been greatly improved, thus a higher breakdown voltage is obtained. A number of key parameters are analysed and optimized, such as the PBL doped concentration of NPBL, thickness TPBL, and length LPBL. The simulated results fully illustrate that PBL could improve the breakdown voltage by more than 50 % and its FOM value is very close to the GaN-limit.Then, a vertical device with composite dielectric CBL is proposed to solve the predicament of CBL. Owe to the discontinuity of electric field within composite dielectrics with different permittivity, it significantly modulates the electric field in buffer layer, and a higher breakdown voltage is achieved. When the buffer layer is 6 μm, the breakdown voltage of conventional CAVET is only 900 V, and that of the new device with the composite dielectric CBL can be as high as 1744 V, and there are no process problems of p-GaN.Finnaly, a new vertical device with charge compensation structure has been carried out to further improve the device performance. The n-GaN could be inverted by utilizing the negative fixed charge, and a high-density hole is induced at this interface, then the depleted region in buffer layer is expanded owe to the induced hole, thus a higher BV is achieved. Though comprehensive analysis about the breakdown principle is accomplished, and it is verified by using Sentaurus. After the simulation and optimization at Tbuf=20 μm, it realizes a breakdown voltage of 4631 V with on-resistance of 1.83 mΩ?cm2, the excellent performance has beyond the GaN one-dimensional limit.