Study Of Polarization Coulomb Field Scattering In AlGaN/GaN/AlGaN/GaN Double-Heterostructure Field-Effect Transistors

With the excellent properties such as high saturated electron drift velocity,high breakdown electric field,and high output power,AlGaN/GaN heterostructure field effect transistors(AlGaN/GaN HFETs)are widely used for fabrication of high temperature,high frequency,high power,anti-radiation,and high density integration of electronic devices.Due to the spontaneous and piezoelectric polarization effects,the sheet carrier concentration of two-dimensional electron gas(2DEG)up to 1013 cm-2 can be generated at the hetero-interface even without any doping.Therefore,AlGaN/GaN HFETs have an irreplaceable position in aerospace,defense,power,and communications.In the past two decades,the properties of AlGaN/GaN HFETs have been significantly improved due to the improvement of material quality and device fabrication processes.However,with the rapid development of mobile communications and defense electronic communications applications,conventional AlGaN/GaN HFETs have many problems,such as threshold voltage drift,buffer layer leakage,current collapse effects,softly turn-off phenomena and so on.In order to solve these problems,it is considered to insert a back barrier layer between the GaN channel layer and the buffer layer,so that the 2DEG can be well confined in the channel to improve the limited domain of carriers in recent years.Correspondingly,the breakdown voltage of the device will be significantly improved,the off-state leakage current will be greatly reduced,the current collapse effect will also be weakened,and the reliability of the device will be significantly enhanced.In particular,as the gate length of the device decreases,the advantages of GaN-based double-heterostructure field-effect transistors(DHFETs)will become more apparent under high-temperature,high-voltage,high-power operating conditions.The application of AlGaN/GaN/AlGaN/GaN DHFETs is one of them.Many years of research have shown that there is a very important scattering mechanism in theAlGaN/GaN HFETs-polarization Coulomb field scattering(PCF scattering).However,this scattering mechanism has not been studied in AlGaN/GaN/AlGaN/GaN DHFETs containing a back barrier layer.Therefore,it is extremely important to consider PCF scattering in the study of AlGaN/GaN/AlGaN/GaN DHFETs.This dissertation will focus on PCF scattering mechanism in AlGaN/GaN/AlGaN/GaN DHFETs by comparing the 2DEG electron mobility,gate-source channel resistance RchS and transconductance gm between AlGaN/GaN/AlGaN/GaN DHFETs and conventional AlGaN/GaN HFETs.The content of this dissertation is as follows:In this dissertation,the central gate devices of AlGaN/GaN/AlGaN/GaN DHFETs and AlGaN/GaN HFETs with source-drain spacing LSD of 100?m and gate length LG of 80?m,60?m,40?m and 20?m and AlGaN/GaN/AlGaN/GaN DHFETs with source-drain spacing LSD of 20?m and gate length LG of 16?m,12?m,8?m,and 4?m were fabricated.1.PCF scattering influence on the carrier mobility in AlGaN/GaN/AlGaN/GaN DHFETsWe tested the capacitance-voltage(C-V)and output characteristic curve of AlGaN/GaN/AlGaN/GaN DHFETs and AlGaN/GaN HFETs with source-drain spacing LSD of 100?m and gate length LG of 80?m,60?m,40?m,and 20?m and AlGaN/GaN/AlGaN/GaN DHFETs with source-drain spacing LSD of 20?m and gate length LG of 16?m,12?m,8?m,and 4?m respectively.By the PCF scattering model we fitted and calculated the carrier mobility in each device.For the large-size AlGaN/GaN/AlGaN/GaN DHFETs and AlGaN/GaN HFETs,we found that as the gate-source bias VGS increases from negative bias to zero bias,POP scattering is relatively closer to the total carrier mobility,which is more significant than other scattering mechanisms.As the gate length decreases,only the PCF scattering changes with the gate length LG.When the gate length LG is large,POP scattering plays a decisive role,and the corresponding carrier mobility is basically the same as the total carrier mobility.When the gate length LG is small,the PCF scattering influence is all enhanced.For the same size devices,compared the large-size AlGaN/GaN/AlGaN/GaN DHFETs with AlGaN/GaN HFETs,the carrier mobility of AlGaN/GaN/AlGaN/GaN DHFETs decreases more slowly as the gate-source bias VGs increases.Moreover,with the decreases of the gate length LG,the carrier mobility of the AlGaN/GaN/AlGaN/GaN DHFETs gradually increases as the gate-source bias VGs increases.There is only the PCF scattering changes more obviously with the change of the gate length LG and the carrier mobility corresponding to the PCF scattering gradually increases as the gate-source bias VGs increases.Therefore,the PCF scattering in AlGaN/GaN/AlGaN/GaN DHFETs is stronger.In order to further verify this conclusion,we presented the fitting results of carrier mobility for various scattering mechanisms.We found that for the same size AlGaN/GaN/AlGaN/GaN DHFETs and AlGaN/GaN HFETs,the carrier mobility corresponding to the PCF scattering in AlGaN/GaN/AlGaN/GaN DHFETs is lower and closer to the total carrier mobility.So the PCF scattering is more strongly in AlGaN/GaN/AlGaN/GaN DHFETs.For the reasons of this phenomenon,we give the following explanation.The conventional AlGaN/GaN single heterostructure forms a triangular well at the AlGaN/GaN heterostructure,but the barrier on the side of the GaN is low,which causes the 2DEG overflow the well and enter the GaN buffer.If an AlGaN back barrier layer is inserted on the side of the GaN,a back barrier will be formed below the channel,thereby blocking the leakage of the 2DEG to the buffer layer and improving the 2DEG confinement.In this way,2DEG will be closer to the hetero-interface.Due to the polarized charge distributed at the hetero-interface,the 2DEG will be closer to the polarized charge in the vertical channel direction(z-direction).For the PCF scattering,the z in the additional scattering potential V(x,y,z)is smaller,resulting in a greater PCF scattering potential and a relatively stronger influence of PCF scattering.Therefore,the PCF scattering in AlGaN/GaN/AlGaN/GaN DHFETs is stronger than in AlGaN/GaN HFETs.Compared AlGaN/GaN/AlGaN/GaN DHFETs for different source-drain spacing LSD but the same ratio of gate length to source-drain spacing LG/LSD,we found that small-size devices have lower carrier mobility corresponding to PCF scattering and will closer to actual carrier current.So the polarization of the Coulomb field scattering is stronger in small devices.This shows that the shorter the channel,the shorter the PCF scattering distance and the stronger the PCF scattering effect.2.PCF scattering influence on gate-source channel resistance in AlGaN/GaN/AlGaN/GaN DHFETsWe tested the Schottky diode ?-? curve and gate-source channel resistance RchS of AlGaN/GaN/AlGaN/GaN DHFETs and AlGaN/GaN HFETs with source-drain spacing LSD of 100?m and gate length LG of 80?m,60?m,40?m,and 20?m and AlGaN/GaN/AlGaN/GaN DHFETs with source-drain spacing LSD of 20?m and gate length LG of 16?m,12?m,8?m,and 4?m respectively.By the results,the gate-source channel resistance RchS of each device is calculated.For the large-size AlGaN/GaN/AlGaN/GaN DHFETs and AlGaN/GaN HFETs,we found that for the devices with LG=40?m,60?m.and 80?m,the gate-source channel resistance RchS increases with increasing gate-source bias VGS.For the devices with LG=20?m,the gate-source channel resistance RchS is almost constant with increasing gate-source bias VGS.Comparing the large-size AlGaN/GaN/AlGaN/GaN DHFETs with AlGaN/GaN HFETs,we found that the gate-source channel resistance RchS of AlGaN/GaN/AlGaN/GaN DHFETs is larger than that of AlGaN/GaN HFETs.In our opinions,compared to AlGaN/GaN HFETs,2DEG is closer to the additional polarization charge at the hetero-interface in the AlGaN/GaN/AlGaN/GaN DHFETs,so the z in the additional scattering potential V(x,y,z)is smaller,the larger the PCF scattering potential and the stronger the PCF scattering,which results in a relatively large value of the gate-source channel resistance RchS.Moreover,with the gate-source bias VGs increases,the growth rate of the gate-source channel resistance RchS in AlGaN/GaN/AlGaN/GaN DHFETs is greater than in AlGaN/GaN HFETs,and the growth trend is more obviously as the increases of the gate length LG.This is because relative to AlGaN/GaN HFETs,the influence of the PCF scattering is larger in the AlGaN/GaN/AlGaN/GaN DHFETs,resulting in the gate-source channel resistance RchS being relatively more affected by the PCF scattering.However the PCF scattering is greatly affected by the gate bias VGS,so the increase rate of the gate-source channel x resistance RchS with the gate-source bias VGS is greater and the growth trend is more obviously as the increases of the gate length LG.Compared AlGaN/GaN/AlGaN/GaN DHFETs for different source-drain spacing LSD but the same ratio of gate length to source-drain spacing LG/LSD,we found that as the increase of the gate bias VGS,the growth rate of gate-source channel resistance RchS for small-sized device is greater than large-size devices,and the growth trend is more obvious.We believe this is due to the fact that the relative influence of polarized Coulomb-field scattering in a small-sized device is stronger,resulting in a relatively larger influence on gate-source channel resistance RchS.The influence of the gate bias VGS is larger,resulting in a larger growth rate of the gate-source channel resistance RchS as the gate bias VGS increases and the trend of increase is more pronounced.3.PCF scattering influence on transconductance in AlGaN/GaN/AlGaN/GaN DHFETsWe tested the transfer characteristics of AlGaN/GaN/AlGaN/GaN DHFETs and AlGaN/GaN HFETs with source-drain spacing LSD of 100?m and gate length LG of 80?m,60?m,40?m,and 20?m and AlGaN/GaN/AlGaN/GaN DHFETs with source-drain spacing LSD of 20?m and gate length LG of 16?m,12?m,8?m,and 4?m respectively.By the results,the extrinsic transconductance gm of each device is calculated.For the large-size AlGaN/GaN/AlGaN/GaN DHFETs and AlGaN/GaN HFETs devices,we have found that the transconductance of devices gradually rises up to the peak value without decreasing as the gate-source bias VGS increases.Compared the large-size AlGaN/GaN/AlGaN/GaN DHFETs with AlGaN/GaN HFETs,we find that the extrinsic conductance gm peak value for AlGaN/GaN/AlGaN/GaN DHFETs is smaller than that for AlGaN/GaN HFETs.We believe this is bacause the 2DEG is closer to the additional polarization charge in the hetero-interface for the AlGaN/GaN/AlGaN/GaN DHFETs,so the smaller the z in the PCF scattering potential V(x,y,z)is,the larger the PCF scattering potential is,and the stronger the relative influence for the PCF scattering is,resulting in a relatively large value of the gate source channel resistance RchS,and a smaller extrinsic transconductance gm peak value.Compared AlGaN/GaN/AlGaN/GaN DHFETs for different source-drain spacing LSD but the same ratio of gate length to source-drain spacing LG/LSD,we found that the large-sized devices do not show a decrease after the transconductance reaching the peak but the transconductance of the small-sized device do.