| ?nitride semiconductor represented by GaN is the direct bandgap semiconductor.Due to the high mobility,high breakdown voltage,strong polarization,anti irradiation and high temperature resistance characteristic,it has been widely used in the region of the lighting,and has the great application potential in the region of the photovoltaic,detection,high speed digital circuits,microwave power and power electronic devices.?nitride heterostructures prepared by AlGaN/GaN heterostructure,due to the strong polarization effect inducing a high concentration of two-dimensional electron gas?2DEG?at the interface of the heterojunction,are very suitable for the fabrication of high-electron-mobility transistors?HEMTs?.Generally,?nitride HEMT are depletion mode devices.However,enhancement mode devices have great application value for the power electronic switch,E/D mode and so on.By studying various new material structure,device structure and device process,many enhancement mode device implementation methods have been obtained,such as the notch etching,p-GaN cap layer,non-polar HEMT and so on.In addition,?nitride semiconductor is also suitable for the fabrication of TFETs.Due to the band gap of InN,GaN,AlN and their compound semiconductor can be adjusted from 0.7 eV to 6.2 eV,the narrow band gap region fully satisfies the requirement of tunneling field-effect transistor for material bandgap.In addition,the direct bandgap,polarization characteristics and other advantages makes?nitride semiconductor of great concern in the field of TFET devices.Moreover,it has been reported that TFET devices made from InN,GaN and alloy materials exhibit superior device characteristics,showing the great potential of application in the low power field.However,?nitride semiconductor research is not mature in both enhancement mode HEMTs and TFET devices.In particular,the research on nitride TFET devices is just beginning.Based on the above background,this paper proposed and carried out the research on the new groove-type channel enhancement mode AlGaN/GaN HEMTs and InGaN TFETs.The characteristics of the above two devices wrere studied and analyzed in detail from the perspective of theoretical simulation.In addition,the device fabrication and characteristic analyses of the AlGaN/GaN heterojunction HEMTs with the most easily realized semi-polar sidewall groove were also studied.The main research work and achievements are as follows.1.We put forward a groove-type channel enhancement mode AlGaN/GaN HEMT.It is realized by using etching and regrowth method.Ideally,the groove wall is nonpolar plane,which will not produce the two-dimensional electron gas?2DEG?;other region of the channel is polar plane,where 2DEG exists.The enhancement mode operation of the device is achieved by using the nonpolar plane channel without 2DEG.This method combines the advantages of polar and non-polar materials to realize the enhancement device operation,and satisfies the process compatibility of the enhancement and depletion mode devices on the same chip.The silvaco-atlas simulations show that when the groove etching depth is around 500 nm,the instability of the groove etching depth has little effect on the threshold voltage of the device.By adjusting the parameters of the device,the groove depth and the gate work function,the transconductance of 182 mS/mm and the subthreshold slope of 85mV/dec are obtained.At the same time,the breakdown voltage of the groove-type channel AlGaN/GaN HEMT increases by 78%compared with that of the conventional planar HEMT.2.The influence of the side wall inclination of the groove on the characteristics of the groove type channel HEMT is studied.Firstly,the variation of 2DEG concentration in the semi-polar AlGaN/GaN heterointerface with the crystal surface inclination is calculated.On this basis,the transfer characteristics of the groove type channel HEMTs with different semi-polar crystals plane are studied by silvaco-atlas.The study shows that when the angle between the crystal orientation of the semi-polar plane and the c-axis increases,the 2DEG density at the AlGaN/GaN heterointerface decreases,and the threshold voltage of the device shifts to the direction of the positive voltage.As the angle is 90°,the threshold voltage reaches the peak.3.A semi-polar plane channel HEMT is fabricated.Through comprehensive consideration of the growth difficulty,surface morphology,electron mobility and electron saturation velocity of each crystal plane,{11?0x}plane is selected as the channel crystal plane.The TMAH treatment and low damage ICP etching are used to improve the damage to the lattice of material surface and improve the surface morphology.By optimizing the growth conditions of the barrier layer and the channel layer,the convex channel structure,groove type channel structure,oneside face drain channel structure and oneside face source channel structure are fabricated.It is found that oneside face source channel structure devices have much higher on-state current.By adjusting the barrier thickness,the threshold voltage of+0.5 V is obtained.And the current collapse effect shows that due to the low trap concentration and low electric field at the gate near drain region of the semi-polar plane channel HEMTs,the semipolar plane HEMTs shows lower drain hysteresis than polar plane devices.4.A novel graded drain region source-side channel InGaN TFET?GD-SC-TFET?is put forward.Firstly,the‘In'fraction in the graded drain region can adjust the bandgap of the drain region material,which modulates the energy band near the drain/channel tunneling junction,so as to achieve the effect of reducing device bipolar current.Simulation study shows this structure did achieve the intended purpose.The ambipolar current reduces from2.0×10-8 A/?m of the conventional device to 1.9×10-14 A/?m of the graded drain region TFET.Secondly,the source-side channel using high‘In'fraction to adjust the energy band of source/channel tunneling junction can reduce the tunneling barrier at the on state condition and increase the tunneling rate,therefore improving the drive current of the device.5.A novel high performance gate engineering InGaN dopingless TFET is proposed.The TFETs are theoretically studied from the aspects of ambipolar characteristics,off-state current,switch ratio,on-state current,subthreshold swing and frequency characteristics.The study shows that the on-state current of InGaN DL-TFET is 1.6×104 times higher than that of Si DL-TFET devices,and the subthreshold slope decreases by 51.1%.The characteristics of the device are further improved by the gate engineering method.A high performance gate engineering TFET device is obtained by simulation optimization of the device:the average subthreshold slope of 7.9 mV/dec at the tunneling gate workfunction of 3.5 eV,and on-sate current of 8.02×10-5 A/?m,cut-off frequency of 119 GHz,and EDP of 0.64 fJ-ps/?m.The study shows that InGaN TFET has great potential in low power consumption. |
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