The Design And Research Of Novel GaN HFET Structure

In recent years,GaN heterojunction field effect transistor（HFET）has attracted extensive attention in the field of power semiconductors due to their excellent characteristics such as high stability,high breakdown voltage,and high current.Compared with traditional Si-based devices,GaN HFET has shown greater appeal and is considered to be the most promising candidate in the field of next-generation broadband gap complementary logic integrated circuit（IC）and high-voltage power devices.However,due to various reasons,the breakdown of conventional GaN HFET devices occurs in advance,which seriously hinders its application in the field of high voltage.Moreover,due to the low hole mobility,the development of p-Channel GaN HFET lags far behind its n-channel devices.Based on this,this paper focuses on the low on current of p-channel GaN HFET and the low withstand voltage of n-channel GaN HFET,which are mainly divided into the following aspects:（1）Firstly,the significant advantages of GaN materials compared with the previous two generations of semiconductor materials are discussed in detail,and the development status and existing problems of GaN based HFET are described.Then,the polarization effect of AlGaN/GaN heterojunction and the formation mechanism of two-dimensional electron gas（2DEG）and two-dimensional hole gas（2DHG）are studied,and the device structures of n-channel and p-channel GaN HFET are analyzed respectively.（2）A p-Channel GaN/AlGaN HFET with recessed gate structure is proposed to meet the development requirements of GaN-based complementary logic circuits for enhancement-mode P-channel devices.The structure reduces the 2DHG concentration at the heterojunction interface under the gate through the gate depression design to realize the normal-off operation of the device.After optimizing the structure,the threshold voltage of the device is about-1V,and the off-state current is only 1×10^-5m A/mm when the gate is grounded,and the ON/OFF ratio is 10⁵.（3）A pseudo p-Channel AlGaN/GaN HFET with back gate structure is proposed to solve many problems existing in the conventional GaN HFET based on 2DHG conductivity,such as low hole mobility,difficult activation of acceptor impurities and high process complexity.The effects of back gate structure parameters on the current characteristics of the device,including threshold voltage and ON/OFF ratio,are analyzed in detail.And an analytical model is also obtained.The simulation results show that the threshold voltage of the proposed device is about-3.5 V,the ON/OFF ratio is as high as 10¹⁰,and the current density is as high as 0.1A/mm,which is much higher than that of the conventional enhanced-mode p-channel GaN HFET based on 2DHG.（4）An AlGaN/GaN HFET with a stepped GaN channel layer is proposed.The channel layer of this structure adopts a stepped GaN design,and the buffer layer is an AlGaN material with low Al composition,so as to solve the problem of low withstand voltage of GaN HFET.By introducing a new electric field peak at the step,the lateral electric field distribution of the device is effectively improved,thereby increasing the breakdown voltage of the device.After optimization the device parameters,compared with the conventional structure,the proposed device maintains the low on-resistance of 2.5 m?·cm²,while the breakdown voltage is increased by about 72%.