Studies Of Carrier Mobility And Related Device Paramaters In Algan/gan Heterostructure Field-effect Transistors

AlGaN/GaN heterostructure field-effect transistors(HFETs),as an important representative of GaN-based electronic devices,have attracted much attention in the past two decades.On the one hand,AlGaN/GaN HFETs can effectively exert the excellent material properties of GaN,including large bandgap width,high breakdown voltage,high saturation electron drift velocity,superior radiation resistance and corrosion resistance;On the other hand,due to the existence of spontaneous polarization and piezoelectric polarization in AlGaN/GaN heterostructure,two-dimensional electron gas(2DEG)with electron density of?10-3 cm-3 and electron mobility of?2000 cm-2/V·s is induced at the AlGaN/GaN heterostructure interface without any doping,which not only effectively compensates for the low electron mobility in GaN material(?1000 cm-2/V·s)due to the large effective electron mass,but also avoids the effect of ionized donor impurity scattering caused by doping,thereby providing a superior channel transport layer for AlGaN/GaN HFETs.These characteristics of AlGaN/GaN HFETs make it unique in high frequency and high power field,and have a broad application prospects in radar communications,electronic countermeasures,5G applications,small base stations,new communication microsatellites,power transmission,and automotive electronics.Therefore,the study of AlGaN/GaN HFETs is of great significance for improving our military strength and civil equipment performance.Since the first AlGaN/GaN HFET was fabricated in 1993,up to 25 years of research in AlGaN/GaN HFETs has been explored.In the meantime,AlGaN/GaN HFETs have experienced sapphire substrates with moderate performance,low-mismatched and high-conductivity SiC substrates,as well as low-cost and easy-to-integrate Si substrates.Many aspects in terms of device power density,frequency characteristics,breakdown characteristics,and enhancement device fabrication have been studied in various ways,and a series of breakthrough results have been achieved.Although the products based on AlGaN/GaN HFETs have been,put into use in the market,some related issues are still in urgent need of solution,which includes:the physical model of AlGaN/GaN HFETs is in urgent need of completeness;the theoretical research on the electrical characteristics of small size AlGaN/GaN HFETs need to perfect;as the core device of power amplifier,the problem of nonlinear distortion of AlGaN/GaN HFETs needs to be solved urgently.Polaritonai Coulomb field(PCF)scattering,originating from the uneven distribution of polarization charges at the AlGaN/GaN heterostructure interface,is a scattering mechanism related to the strain distribution of the AlGaN barrier layer.The lateral diffusion of ohmic metal atoms during the ohmic annealing process and the inverse piezoelectric effect caused by the gate bias,can change the strain of the AlGaN barrier layer and make the distribution of polarization charges nonuniform,resulting in PCF scattering.It has been determined that PCF scattering has an important influence on the characteristics of AlGaN/GaN HFETs.The PCF scattering was presented in 2007 and the research methods and theoretical systems for PCF scattering has been developed during a more than a decade.However,how to use this scattering mechanism to effectively improve the parametric performance of the device and apply it to the practical application of devices,is an important issue.The aim of this thesis is based on the previous PCF scattering studies,combining the PCF scattering with the current issues of AlGaN/GaN HFETs,focusing on the application of PCF scattering to the actual device analysis and performance improvement,using the PCF scattering theory to accurately determine the device mobility,analyzing the small size device electrical characteristics,exploring to improve device performance by the gate length design and the selection of gate bias,and improving the device linearity.Specific studies include the following:1.A method to accurately determine the carrier mobility in AlGaN/GaN HFETsFirst,based on the two-dimensional scattering theory,the scattering mechanisms in AlGaN/GaN HFETs are introduced.In particular,the formation of PCF scattering and the establishment of theoretical models are described in detail.After that,the limitations of the traditional method to determine the carrier mobility in AlGaN/GaN HFETs were analyzed.The conventional method to determine the 2DEG electron mobility under the gate region is based on the principle that the electron mobility under the gate region is modulated by the gate bias,and the gate-source channel resistance(Rs)and the gate-drain channel resistance(RD)is constant.However,due to the presence of PCF scattering,the strain of the AlGaN barrier layer can be changed by the gate bias and the polarization charge under the gate region will be change accordingly,causing the PCF scattering to be changed with the gate bias.The PCF scattering will affect the 2DEG electron mobility in the gate-source and the gate-drain channel.This will cause Rs and RD to be affected by PCF scattering and to be changed with the gate bias.Therefore the traditional method will no longer be suitable.For this purpose,considering the effects of PCF scattering on Rs and RD,using the measured current-voltage(?-?)output characteristics and capacitance-voltage(C-V)curves,based on the two-dimensional scattering theory and the iterative calculation,a method to accurately determine the 2DEG electron mobility under the gate region was presented.Using this new method,the 2DEG electron mobility for the two samples with source-drain spacing LSD of 20 ?m and gate lengths LG of 4/16 ?m was determined.Compared with that calculated by the conventional method,the electron mobility determined by the new method shows an apparent difference.The PCF scattering on the Rs and RD is stronger for the device with larger gate length,the difference between the new method and the traditional method is larger.Finally,based on the obtained 2DEG electron mobility,the transconductance of the device was calculated,and the correctness of this new method was demonstrated by the comparison between the experimental and calculated transconductance values.2.Study of carrier mobility in 70-nm Gate AlGaN/GaN HFETs.Using T-gate,n+-GaN ohmic contact regrowth,and SiN passivation,AlGaN/GaN HFETs with a gate length of 70 nm,gate-to-source spacings of 300/600 nm and gate widths of 20/40 ?m were fabricated.Based on the measured ?-? output characteristic curves and 2DEG electron density,the 2DEG electron mobility of the 70-nm Gate AlGaN/GaN HFETs were extracted by iterative calculation.It was found that the PCF scattering in the small size AGaN/GaN HFETs was the strongest scattering mechanism and obviously dominated the total electron mobility variation,causing the electron mobility under the gate region to increase with the gate bias.The device with a larger drain-source spacing or a larger gate width,has a lower 2DEG electron mobility.This is mainly due to the increase of the drain-source spacing or gate width,which effectively increases the number of additional polarization charges under the gate-source and gate-drain regions,thereby enhancing the effect of PCF scattering on the 2DEG electrons under the gate region.As the gate bias decreases,the values of Rs and RD was increased.In addition,the increase of Rs and RD is more apparent for the devices with smaller source-drain spacing or larger gate widths.The gate length is constant,and the decrease of the drain-source distance leads to the reduction of the gate source and gate-to-drain spacing.The additional polarization charges under the gate region has enhanced effect on the 2DEG electron in the smaller gate-source and gate-drain channels.And the increase of the gate width can increase the number of additional polarization charges under the gate,and PCF scattering will also be enhanced.The enhanced PCF scattering can makes the Rs and RD have a bigger increase with the decrease of the gate bias.This study shows that PCF scattering has an important influence on the carrier mobility of the small size AlGaN/GaN HFETs,and weakening the PCF scattering is of great significance for improving the performance of small size AlGaN/GaN HFETs.3.Studies of influence of different gate lengths and gate biases on the polarization Coulomb field scattering potential and the source access resistance Rs in AlGaN/GaN HFETsThe AlGaN/GaN HFETs with a source-drain spacing of 20 ?m and gate lengths of 4/10/16 ?m were fabricated.Using the measured ?-? output characteristics and C-V curves,by choosing different 2DEG electron wave functions and PCF scattering potentials,two methods to calculate PCF scattering were presented.In combination with other scattering mechanisms,the drain-source channel resistances for the three samples were calculated.Comparing with measured values,it is found that different gate lengths and gate biases have different effect on the PCF scattering potential.For the device with a small gate length,the 2DEG electron wave function in the gate-source and gate-drain regions,and the polarization charge in the under-gate region,should be selected for PCF scattering calculations.For the device with a large gate length,the 2DEG electron wave function under the gate region and the additional polarization charges in the gate-source and gate-drain regions should be selected for PCF scattering calculations.For the device with a large gate length,the drain-source channel resistances calculated by the two methods have differences from the measured values.Because the theoretical model of PCF scattering is established based on the perturbation theory.When the perturbation(namely,the PCF scattering potential)is much smaller than the Hamiltonian of the system,the calculated results determined by the theory model of PCF scattering is valid.In addition,as the gate bias is decreased toward more negative,PCF scattering is enhanced,and the PCF scattering potential is increased,which also reduces the accuracy of PCF scattering theoretical calculations.Based on this study,it is know that different gate lengths and gate biases can affect the PCF scattering potential.Only the system wave function corresponding to the small PCF scattering potential is chosen to meet the applicability of the perturbation theory,the accuracy of the PCF scattering theoretical model can be assured.The AlGaN/GaN HFETs with gate-source spacing of 3?m,gate-drain spacing of 6?m,and gate lengths of 3/6/10?m were fabricated.The source access esistances Rs of the AlGaN/GaN HFETs were measured using the gate probe method.The measured values of Rs showed different resistance values under different gate bias or different gate lengths.With the increase of the gate bias,the Rs of the device with 3 ?m gate length decreases monotonously,the Rs of the device with 6 ?m gate length drops first and then rises,and the Rs of the device with 10 ?m gate length monotonically rises.By the analysis of different scattering mechanisms and polarization charge distribution,the variation of Rs is thought to come from PCF scattering.There is an interaction between the positive additional polarization charges under the gate region and the negative additional polarization charges near the ohmic contact region.The increase of the gate bias or gate length can increase the positive additional polarization charges under the gate region,and changes the PCF scattering,thus affecting the Rs variation.This result was well verified by theory calculations and makes it possible to change the device gate length design or select the appropriate gate bias to effectively improve the RS,and thus increase the device linearity.4.Effect of polarization Coulomb field scattering on the device linearity in AlGaN/GaN HFETsThe AlGaN/GaN HFETs with a gate length of 100 nm and a drain-source spacing of 2/4 ?m were fabricated.Two types of direct current quiescent points(DCQPs)under the drain-source voltage VDs =10V(a-g)and the drain-source voltage variation(A-G)were selected.The power gain were obtained by the measured small signal S-parameters under two types of DCQPs.The gain of A-G is more flat than that of a-g.That means the device under A-G has better linearity.The gain difference between the two types of DCQPs is more apparent for the device with smaller drain-source spacing.The increase of the gate bias or the decrease of the drain-source bias can weaken PCF scattering,and the weakened PCF scattering can effectively compensate the enhanced POP scattering caused by the increase of the gate bias.This offset can contribute to decrease the variations of Rs,gm,and gain as the current is increased,leading to a better linearity performance.The AlGaN/GaN HFETs with a gate length of 1 ?m,a drain-source spacing of 6?m,and a gate width of 546/780 ?m were fabricated.The on-wafer RF power performances were tested by using a single-tone continuous-wave signal at 2.7 GHz.At a drain voltage of 20 V,the device matching was optimized for the maximum output power.A distinct improvement in device linearity was observed in the sample with a larger gate width.The 2DEG electron temperature in the gate-source channel was obtained.The electron temperatures of the two samples are nearly the same,and the Rs determined by the POP scattering is almost the same for the two samples,which is increased with the gate bias.Due to the different gate widths,the number of additional polarization charges under the gate region is different,therefore the PCF scattering is different.The device with larger gate width have more additional polarization charges and have enhanced PCF scattering.The Rs determined by the PCF scattering was decreased with the increase of the gate bias,which can effectively reduce the increase of the Rs caused by the POP scattering.Finally,the total Rs was determined by the theory calculation,and the Rs of the device with a large gate width exhibits a more flat variation.Based on the experimental measurement and theoretical calculation,it is proved that the increase of the device gate width can increase the PCF scattering,then improve the device linearity.The AlGaN/GaN HFETs with gate-source(or gate-drain)spacing of 1?m and gate lengths of 0.5/0.35/0.25 ?m were fabricated.The transconductance and the gain of the device with larger gate length showed a smaller decline.The input power corresponding to the 1-dB compression point(PIN-idB)of the device with larger gate length is higher.This means the device with larger gate length had better device linearity.The total Rs and the Rs determined by specific scattering mechanism were obtained by two-dimensional scattering theory.The device with larger gate length had more additional polarization charges under the gate and caused a stronger PCF scattering,which can effectively improve the device linearity.Based on experimental verification and theoretical analysis above,a method to improve the device linearity of AlGaN/GaN HFETs were presented.The method shows that the increase of PCF scattering by adopting thin AlGaN barrier layer,recessed gate process,and high A1 component of AlGaN barrier layers,increasing gate length/gate-source spacing ratio,increasing gate width,and adopting back-barrier structure,can counteract POP scattering,weaken the variation of Rs,and increase device linearity.This method has the advantages of simplicity,directness,high operability and high integration,and is shown to be effective in achieving the improvement of the linearity performance of GaN power amplifiers at the device level.