Research On The Breakdown Theory And Novel Structure Of AlGaN/GaN HEMT On Si Substrate

GaN is gaining increasing attention for its attractive performance in high power,high current and high-temperature applications resulting from its wide bandgap and high critical electric field.Besides,profiting from the polarization-induced two-dimension electron gas（2DEG）,the typical GaN-based lateral power device,AlGaN/GaN High Electron Mobility Transistor（HEMT）with silicon substrate,could achieve high breakdown voltage（BV）and low specific on-resistance(R_on,sp)simultaneously.However,limited by the 2D coupling effect of the lateral structure and the traps introduced by heteroepitaxy,the BVs of AlGaN/GaN HEMTs are far away from the theoretical limit of GaN material.The electric field crowding effect appears when the device sustains high voltage,and the overhigh electric field peak would trigger the device breakdown and reliability degeneration.Therefore,electric field homogenization is the key to enhancing the BV and the reliability of the device.Based on the electric field homogenization,the dissertation aims at developing the theory of breakdown mechanism and the techniques to enhance BV and reliability,thus removing the barrier resulted from the ununiform electric field distribution.Under the guidance of the breakdown theory,the device structure is developed and optimized,contributing to the AlGaN/GaN HEMTs with high performance and high reliability.The main contributions of this work are as follows:（1）The breakdown theory of the Si-substrated AlGaN/GaN HEMTs with field plates is developed.The Equivalent Potential Method（EPM）is employed in GaN-based power device modeling.The space charges in the deplete region are equivalented as boundary conditions,thus simplifying the modeling progress.The EPM is suitable for the devices with complicated stacked epitaxial layers.Based on the EPM,the analytical study of the breakdown characteristics of AlGaN/GaN HEMT is conducted.The analytical models of the electric field distribution,the potential distribution,the breakdown voltage and the Baliga’s Figure of Merit（BFOM）are developed.These analytical models are verified and calibrated by numerical simulation to model the relationship between device characteristics and structure parameters accurately.The breakdown of the device is limited by the lateral breakdown and the vertical breakdown simultaneously.Effective guidance could be offered by the analytical model to obtain the optimized breakdown performance.（2）The novel structure,AlGaN/GaN HEMT with step-doped channel（SDC-HEMT）,based on the 2D electric field modulation effect is proposed.The step-doped channel technique is proposed based on the 2D Gaussian Box analysis to homogenize the electric field with the ununiform charge distribution in channel layer.The BV,R_on,sp and impact ionization rate of the device with different channel thicknesses and doping concentration variation rates are explored by numerical simulation.The electric field distributions and potential distributions of different device structures are explored by the numerical simulation and the analytical model.This gives physical insights in the device breakdown and offers effective guidance for BV improvement.The BV,R_on,sp and BFOM of the SDC-HEMT improved 59.4%,7.0%and 168%respectively compared with the conventional HEMT.（3）The novel structure,AlGaN/GaN HEMT with width-modulated drift region（WMD-HEMT）,based on 3D electric field modulation effect is proposed.The drift region width modulation technique is proposed based on the 3D Gaussian Box analysis to homogenize the electric field with the drift region pillars.The drift region pillars widen from the gate electrode to the drain electrode to form the equivalent concentration linearly increased in the 2D cross section.The WMD-HEMTs are fabricated with optimized process and the electrical characteristics of the device are explored by device test.The numerical simulation reveals the mechanism of the device’s BV enhancement.The electric field peak at the gate electrode is mitigated with the employment of the drift region width modulation technique.Compared with the conventional HEMT,the BV and BFOM of WMD-HEMT improved 34.8%and66.3%respectively.（4）The thermal and electrical reliability of WMD-HEMT is explored.The selt-heating effect and the kink effect of the WMD-HEMTs are explored by pulsed measurements and direct current measurements.The thermal and electrical coupling effects induced characteristics degeneration is explored by the numerical simulation and device test.The WMD-HEMT could ease the electric field peak at the gate electrode in on-state and the R_on,sp degeneration rate is mitigated from 9.2%to 1.1%compared with the conventional HEMT.In addition,with the employment of the drift region width modulation technique,the R_on,sp degeneration rate of the device decreased from 40.7%to 29.9%when operates in high temperature condition.Apart from the performance improvement in on-state,the current collapse and dynamic R_on,sp degeneration also improve significantly owing to the more uniform electric field distribution in off-state.With the employment of the drift region width modulation technique,the collapsed current recovered from 90.3%to 94.6%and dynamic R_on,spdegeneration rate mitigated from 32.8%to 27.3%.