Study Of Polarization Coulomb Field Scattering Influfence On The Gate-source And Gate-drain Parasitic Access Resistances Of GaN-based Electron Devices

AlGaN/GaN high-electron-mobility transistor(AlGaN/GaN HEMTs)has several excellent characteristics,such as high critical breakdown electric field,high output power and high saturated electron drift velocity.Moreover,the sheet carrier concentration in the two-dimensional electron gas of the order of 1013 cm-2 at AlGaN/GaN hetero-interface results from the piezoelectric and spontaneous polarization without any doping.Therefore,AlGaN/GaN HEMTs play important roles in high speed and high power integrated circuits.With the deep study of the equivalent circuit for devices,the large-signal parasitic access resistance has been considered to affect the device high speed performance and reliability,especially for the current gain cutoff frequency fT and the extrinsic transconductance gm in AlGaN/GaN HEMTs,restricting the further optimization of thermal noise,on-resistance and transit delay characteristics.In order to understand the origin and mechanism of the parasitic access resistance,we take main scattering mechanisms into consideration,including polarization Coulomb field(PCF)scattering,polar optical phonon(POP)scattering,piezoelectric(PE)scattering and interface roughness(IFR)scattering,and focus on study of the PCF scattering.The PCF scattering is related to gate-source bias,source-drain bias and gate area.So,gate-source and gate-drain parasitic access resistance(RS and RD)are related to gate-source bias,source-drain bias and gate area.Parasitic access resistance in the linear and saturation region of the current-voltage characteristics(I-V)is different.In this paper,for AlGaN/GaN HEMTs,Rs in the linear region and Rs(RD)in the saturation region were explored respectively,the relationship between Rs in the linear region and positive gate-source bias was determined,and the method to determine Rs and RD in the saturation region was proposed.The AlGaN/GaN HEMTs with different gate area and different gate-source distance were fabricated.With the measured RS and the analysis of different scattering mechanisms,the fact that theoretical model calculation shows good agreement with the measured values confirms the parasitic access resistance is affected by PCF scattering.For AlGaN/GaN HEMTs,gate-source and gate-drain parasitic access resistance(Rs and RD)are related to gate-source bias,source-drain bias and gate area.The content of this dissertation is as follows:1.PCF scattering influence on parasitic access resistance Rs in the linear region of the current-voltage characteristicsAfter device processing,the uniform distribution of the polarization charges at AlGaN/GaN hetero-interface is broken by the normal-Ohmic contacts anneal processing and the gate biases,inducing the appearance of the additional polarization charges and resulting in the PCF scattering.The difference of the Ohmic contacts will interfere the study of the gate biases which could change the polarization charges of the AlGaN barrier layer under the gate region because of the converse piezoelectric effect.In order to reduce the influence of the Ohmic contact difference,rectangular HEMTs samples were fabricated,in which a pair of devices shared a common source region.So,gate area and gate bias influence on Rs could been studied accurately.Under the Ohmic contact regions,the diffused metal atoms weaken the piezoelectric polarization of the AlGaN barrier layer,and the additional polarization charges ??1 is a fixed,negative value with the different VGS.With the positive VGS,in-plane additional tensile stress in the AlGaN barrier layer results in the increase of polarization charges under the gate region.The positive,changing Aa3 under the gate region and negative,fixed ??1 under the Ohmic contact region determine the PCF scattering potential together.With an increase of VGS,the positive,increasing Aa3 offsets the effect of the negative,fixed ??1,eventually becomes the dominant factor for the PCF scattering potential.For the AlGaN/GaN HEMTs devices shared a common source region on a single sample,the Rs gate probe measurements were at the same gate bias range in order to ensure the ??3 under the gate region is same for each device.For larger gate area and same gate-source distance,the total additional polarization charges increase,enhancing the intensity of the PCF scattering potential and enlarging Rs with an increasing VGS.For larger gate-source distance and same gate area,the total additional polarization charges is same,but the influence of PCF scattering potential is smaller in larger access region with an increasing VGS,decreasing the amplitude of the change of RS.At last,with the PCF scattering theoretical model,we calculated Rs with different gate biases for devices in different sizes,and good agreement between measured and calculated Rs verified that PCF scattering analysis on the origin and mechanism of the parasitic access resistance was reasonable.Rs is closely related to gate-source bias and gate area.2.PCF scattering influence on parasitic access resistance in the saturation region of the current-voltage characteristics in large gate AlGaN/GaN HEMTsThe PCF scattering is an important scattering to influence the device performance.However,for large gate device,the PCF scattering is not taken into account to explore the parasitic access resistance corresponding to each quiescent bias point in the saturation region of the current-voltage characteristics.So,in order to improve the characteristics of devices,it is essential to consider PCF scattering and determine the parasitic access resistance in the saturation region.Unlike the deep submicron gate devices,drain-source electric field cannot make carrier transport in saturation velocity for the large gate devices.So,the channel potential under the gate which is linear distribution for the short gate devices is not fit for the large gate devices.The channel potential distribution needs to be further studied for the large gate devices.Firstly,the values of Rs and RD at each quiescent bias point were determined by using an improved gate probe method.Secondly,with the charge control by a Schottky gate deposited on the large gap semiconductor,the channel potential under the gate is devided into two sections in the saturation region(VDS=8V).For the gradual channel region ? and pinched saturation region ?,the channel potential under the gate is from VC(0)to Vknee and from Vknee to VC(L)respectively.Here,VC(0)(VC(L))is the electric potential at the source(drain)edge of the gate channel and Vknee is the pinch-off electric potential.Then,the PCF scattering is taken into account to analyze the additional polarization charges ?? distribution and determine the additional scattering potential at AlGaN/GaN hetero-interface.Finally,several scattering mechanisms including polar optical phonon scattering,interface roughness scattering,piezoelectrioc scattering and polarization Coulomb field scattering were taken into consideration to calculate the values of Rs and RD.The calculated values show good agreement with the measured values,verifing the accuracy of the theoretical calculation.For the device in sample 3,under the Ohmic contact regions,??1 is a fixed,negative value with the different VGS.VGS are negative at each quiescent bias point,additional polarization charges ??3 under the gate region is negative.The negative ??1 and ??3 determine the PCF scattering potential together.With gate bias changes from-3V to 0V,the decreasing ??3 and the fixed ??1 weaken the intensity of the PCF scattering potential,making RS and RD decrease.Besides,for other scattering mechanisms,electron tempreture Te and n2D at ungated region determine the intensity of its scattering.It has been verified that there is no signifinant hot-phonon effects and self heating effects in the channel for the large gate device in sample 3 because of the small current and electric field.Moreover,at room tempreture,the value of n2D in gate-source channel is fixed with the different VGS.So,the difference of Rs and RD among quiescent bias points can only result in the difference of the PCF scattering potential.These results confirm that the parasitic resistance RS and RD of AlGaN/GaN HEMTs in the saturation region are related to gate-source and gate-drain bias because of the influence of the PCF scattering.The method of determining the parasitic resistance Rs and RD of the AlGaN/GaN HEMTs device is correct and feasible.