Study Of Polarization Coulomb Field Scattering Influence On The Characteristics Of Recessed-Gate AlGaN/GaN Heterostructure Field-Effect Transistors

AlGaN/GaN heterojunction field-effect transistors?HFETs?have several excellent characteristics,such as large band gaps,high critical breakdown fields,high saturation electron drift velocity,high thermal conductivity,and excellent radiation resistance and corrosion resistance.What's more,due to the spontaneous polarization effect and piezoelectric polarization effect,high areal density(1×1013 cm^-2)2DEG was generated at the AlGaN/GaN heterojunction interface without doping.It's also due to the existence of the high density 2DEG at the heterojunction interface the conventional AlGaN/GaN HFETs is depleted.Depletion-type devices have complex drive circuits,poor energy efficiency and reliability,which severely limit their application and development.Preparing high-performance enhancement devices has become a hot topic in the research field of GaN HFET devices.The combination of recess gate technology and MOS structure is an effective way to fabricate enhanced AlGaN/GaN HFETs.There are evenly distributed polarization charges at the AlGaN/GaN heterojunction interface.After the device process,the lateral diffusion of the metal atoms in the ohmic contact region in the ohmic annealing process and the inverse piezoelectric effect caused by the gate bias will change the strain of the AlGaN barrier layer.This will cause the polarization charge distribution of the AlGaN barrier layer to be non-uniform,and induced Polarization Coulomb Field?PCF?scattering.Since it has been proposed,PCF scattering has established a sound research method and theoretical system,and studies have shown that polarized Coulomb field scattering has a very important influence on the 2DEG electron mobility??_n?and the gate-source on-resistance(R_ch^s).However,the role of PCF scattering in recessed gate AlGaN/GaN HFET devices has not been explored.This dissertation prepared the conventional AlGaN/GaN HFET and recessed gate depletion AlGaN/GaN HFET with different gate lengths and the same source-drain spacing,The effect of PCF scattering on electron mobility??_n?,gate-source on-resistance(R_ch^s)and extrinsic transconductance?g_m?of the recessed gate depletion AlGaN/GaN HFET were investigated.we also prepared recessed gate enhancement AlGaN/GaN HFET to explore the effect of PCF scattering on its characteristics.First,the process was explored to fabricate recessed gate AlGaN/GaN HFET devices.Recessed gate devices are prepared using low-power inductively coupled plasma etching?ICP?technology.In order to reduce the gate leakage current,Al₂O₃ is deposited by atomic layer deposition?ALD?process after etching at a low temperature of 150?.Then treat them with the rapid thermal annealing to improve the contact quality between the dielectric layer and the barrier layer.1.Effect of Polarization Coulomb Field Scattering?PCF?on 2DEG Electron Mobility in recessed gate depletion AlGaN/GaN HFETs.The total 2DEG electron mobility and the the corresponding mobility of each scattering mechanism was obtained of the conventional AlGaN/GaN HFETs and recessed gate depletion AlGaN/GaN HFETs by PCF scattering theory model.For conventional AlGaN/GaN HFETs,when the L_G/LsD ratio is less than 0.5,in several scattering mechanisms,PCF scattering plays a dominant role in the variety of 2DEG electron mobility.With the increase of the gate-bias voltage?VGs?from negative to zero,the compressive strain subjected to AlGaN barrier layer under the gate is gradually reduced,the polarization charge under the gate is continuously decreased,and this reduce the effect of PCF scattering.As V_GS increases,the scattering effect of PCF decreases,and the growth rate of 2DEG electron mobility below the gate gradually decreases.For devices with a L_G/L_SD ratio greater than 0.5,the positively polarization charge ?2 in the gate-source,gate-drain regions has a weaker scattering effect on the 2DEG,and PCF scattering does not lays a dominant role in the several scattering mechanisms.The change of mobility is dominated by POP scattering,so the mobility show a decreasing trend as VGs increases.For recessed gate depletion AlGaN/GaN HFETs,ICP etching introduces positive charge and trap charge at the barrier layer interface.which corresponds to a negative additional polarization charge ?2 in the gate source and gate drain regions.Due to the inverse piezoelectric effect,the positive charge under the gate area decreases and the PCF scattering decreases.For different sizes of depletion AIGaN/GaN HFETs,the larger the device gate length,the weaker the PCF scattering and the corresponding mobility.Compared with conventional AlGaN/GaN HFETs and recessed gate depletion AlGaN/GaN HFETs,when the L_G/L_SD ratio is less than 0.5,the PCF scattering effect of the conventional AlGaN/GaN HFETs is stronger than the PCF scattering effect of the recessed gate depletion AlGaN/GaN HFETs.This leads to conventional AlGaN/GaN HFETs have lower carrier mobility than the recessed gate depletion AlGaN/GaN HFETs.With the increase of V_GS from negative to zero,the difference in intensity of PGF scattering decreases firstly and then reverse increases.When V_GS is zero,the PCF scattering effect of conventional AlGaN/GaN HFETs is weaker than that of recessed gate depletion AlGaN/GaN HFETs.At this time,the total carrier mobility of the conventional devices is lower than the recessed gate depletion AlGaN/GaN HFETs.When the L_G/L_SD ratio is greater than 0.5,the PCF scattering has a weaker effect in both devices,the trend of carrier mobility is dominated by POP scattering.Due to the higher 2DEG areal density in the recessed gate depletion AlGaN/GaN HFETs,the POP scattering is stronger and its carrier mobility is lower than that of conventional AlGaN/GaN HFETs.2.Influence of polarization coulomb field scattering on gate-source channel resistance(R_ch^s)and extrinsic transconductance?g_m?of recessed gate depletion AlGaN/GaN HFETs.?1?Influence of polarized coulomb field scattering on the gate-source channel resistance(R_ch^s)of recessed gate depletion AlGaN/GaN HFETs.Conventional AlGaN/GaN HFETs and recessed?with a gate depth of 9 nm?depletion AlGaN/GaN HFETs were investigated.The gate source channel resistance(R_ch^s)was measured by the gate probe method and Fitted theoretical model.The study found,For conventional AlGaN/GaN HFETs,under a positive gate bias,the tensile strain of the barrier under the gate is enhanced,and a positive additional polarization charge ?3 appears below the gate.As V_GS increases from zero,the positive additional polarization charge ?3 increases,which offsets the negative additional polarization charge ?1 caused by metal diffusion in the ohmic contact region which does not increase with V_GS.The scattering effect of PCF scattering on the carriers in the channels of the gate-source and gate-drain regions is weakened,and the gate-source on-resistance(R_ch^s)decreases.For recessed gate depletion AlGaN/GaN HFETs,ICP etching introduces polarization charge and trap charge at the barrier layer interface under the gate region,resulting in a positive additional polarization charge ?3 is generated under the gate even no gate voltage is applied.With the increase of the V_GS,the tensile strain of the barrier below the gate increases,and the additional positive charge under the gate increases continuously.The effect of the positive additional charge ?3 below the gate completely offsets the negative additional polarization ?1 of the ohmic contact region.Then the charge ?3 becomes the main source of PCF scattering.As V_GS positively increases the PCF scattering enhancement,the gate-source on-resistance(R_ch^s)increases.The addition polarization charge ?3 under the gate region of the conventional AlGaN/GaN HFETs device and the negative charge ?1 in the ohmic contact region offsets each other,weakening the effect of PCF scattering.The positive polarization charge ?3 below the gate region of the recessed gate depletion AlGaN/GaN HFETs completely offsets the negative additional polarization charge ?1 of the ohmic contact region and becomes the main source of PCF scattering.The PCF scattering is large and increases with the increase of the V_GS,so the effect of PCF scattering in recessed gate depletion AlGaN/GaN HFETs is greater than that of conventional AlGaN/GaN HFETs.As the gate length increases,the influence of the PCF scattering on the R_ch^s,is greater,and the difference of gate-source on-resistance(R_ch^s)between the conventional device and recessed gate depletion device is larger.?2?Effect of polarization coulomb field scattering on extrinsic transconductance?g_m?of recessed gate depletion AlGaN/GaN HFETs.Due to the small device size,the saturation mechanism of the device is the saturation drift velocity mechanism.The change of the extrinsic transconductance g_m is due to the change of the gate source channel resistance R_ch^s.We use R_ch^s?PCF?and R_ch^s?POP?to represent the R_ch^s components corresponding to PCF scattering and POP scattering,respectively.When we analyze the influence of PCF scattering on the gate-source channel resistance R_ch^s is analyzed,we use a gate source and a gate-drain as a reference.There is a positive interface charge and an positive anodized charge in the area under the gate of the recessed gate depletion device with zero gate voltage,which is mean the presence of positively-attached polarization charges in the gate region.For recessed gate depletion AlGaN/GaN HFETs,as the gate bias increases from-3 V to 1 V,the positive polarization charge increases below the gate,increasing the additional positive charge,which offers the negatively-charged polarization charge generated by the lateral diffusion near to the ohmic contact region,so that the effect of PCFon the carriers in the channel of gate and drain regions reduced,resulting in the reduction of the gate-source channel resistance RS ch as the gate bias voltage increases.However,POP scattering is the opposite.When the current increases to a certain extent,the electron velocity in the channel increases,the electron energy increases,and the role of scattering with polarized optical phonons increases,resulting in an increase in electron temperature,POP scattering enhanced.The enhancement causes the gate-source channel resistance RSch to increase with the gate bias voltage increases.For devices with different gate length?L_G?and gate-to-source spacing(L_SD)ratios,the larger the gate length,the stronger the PCF scattering and the greater the impact of PCF scattering on the gate-source channel resistance R_ch^s.As the gate bias voltage increases,the R_ch^s?PCF?corresponding to PCF scattering decreases,and the decrease of R_ch^s?PCF?partially cancels R_ch^s?POP?increasing with the gate bias voltage.The larger the L_G/L_SD is,the stronger the cancellation effect is,and the smaller the change of the gate-source channel resistance R_ch^s with increasing L_G/L_SD ratio is,the more gradual the device transconductance will be.3.Effect of Polarization Coulomb Field Scattering on Characteristics of recessed gate enhancement AlGaN/GaN HFETs.?1?Effect of Polarized Coulomb Field Scattering on 2DEG electron mobility in recessed gate enhancement AlGaN/GaN HFETs.The additional polarization charge and 2DEG mobility of different-sized recessed gate enhancement AlGaN/GaN HFETs were obtained by PCF theoretical models.It is analyzed that ICP etching and the deposition of the dielectric layer introduce a large x number of negative charge and trap charge at the barrier layer interface under the gate region,which corresponds to a large amount of positive additional polarization charge?2 in the gate source and gate drain regions.With the increase of V_GS,due to the inverse piezoelectric effect,the negative charge under the gate decreases,the scattering of PCF decreases,and the electron mobility of the 2DEG under the gate of the device increases.The gate-source and gate-drain spacing are the same.The larger the gate length,the smaller the additional positive charge ?2 in the gate-source and gate-drain regions is,the weaker the PCF scattering,and the carrier mobility increases with V_GS.As V_GS increases,its carrier mobility changes less.When the V_GS is the same,the device which has longer gate length has the weaker PCF scattering and the greater carrier mobility.?2?Effect of Polarized Coulomb Field Scattering on gate-source channel resistance of recessed gate enhancement AlGaN/GaN HFETs.The ICP etching and dielectric layer deposition introduce a large number of negative trap interface charges at the barrier layer interface under the gate region.Therefore,there is a large amount of negative additional charges below the gate even when the V_GS is zero.With V_GS increase,the tensile strain of the barrier below the gate region increases,and the negative charge under the gate decreases.The PCF scattering of 2DEG electrons in the gate-source and gate-drain regions weakens,so RS ch decreases with increasing V_GS.Gate-source and gate-drain spacing are the same.the smaller the gate length is,the stronger the effect of the additional polarization charge on the gate source on-resistance is and the smaller the value of gate-source channel resistance R_ch^s.