Study Of Polarization Coulomb Field Scattering Theory Applied In AlGaN/GaN Power Electronic Devices

Semiconductor power electronic devices are mainly used for power equipment of electric power transformation and control circuits,which plays a very important role in the area of the semiconductor devices and the circuit designs.Semiconductor power electronic devices are widely used in commercial and civil applications,industrial control systems and in many other fields such as military equipments.AlGaN/GaN high electron mobility transistors(HEMTs)have attracted more attentions.Due to the wide band width and stable chemical properties of GaN materials,AlGaN/GaN electronic devices have very high critical breakdown electric field,strong anti-radiation ability and strong corrosion resistance.Moreover,due to the spontaneous polarization and piezoelectric properties of GaN materials.AlGaN/GaN heterostructure materials epigrowed on the GaN layer without any doping,a high density two-dimensional electron gas is produced in the AlGaN/GaN heterojunction interface,and the carrier mobility is very high,too.Therefore,it is easier for GaN to obtain lower on-state resistance and higher output current density.Furthermore,the GaN material has the advantages of high temperature resistance,high radiation resistance and good chemical stability,etc.As a result,the AlGaN/GaN HEMTs have strong anti-corrosion and anti-radiation capability,which is suitable for working in more severe environments such as high temperature.For traditional fabrication process of power electronic devices,the surface passivation and the deposition of field plates have become indispensable steps.These steps have undeniable influence on the strain variation in the barrier layer of the devices,and will further change the electrical characteristics of the devices.The strain of the GaN epitaxial layer is usually characterized by high resolution X-ray diffraction(HRXRD).Raman and photoluminescence(PL)measurements.But for the AlGaN barrier layer,due to the thickness of around 20 nm.it is too difficult to obtain accurate strain and polarization distribution information directly through experiments.Most of all.the HRXRD could not provide a precise experimental result of the strain distribution for the fabricated devices,especially the strain distribution of the AlGaN barrier layer after the passivation process in AlGaN/GaN power electronic devices,which is a huge bottleneck for the study of AlGaN/GaN HEMTs after passivation.No report is available in the literature on the strain distribution of the AlGaN barrier layer for the fabricated HEMTs both before and after passivation and the calculation of the polarization charges underneath the passivated regions till now.The irregularly distributed barrier layer strain will induce an irregular distribution of polarization charges,and will induce polarization Coulomb field(PCF)scattering.This scattering is specific for GaN based electron devices.Some studies have shown that this scattering is critical for the low field carrier mobility and device reliability.In such a scenario,the study of the additional strain induced to the AlGaN barrier layer in AlGaN/AlN/GaN power electronic devices becomes utmost important with the view of understanding the electrical properties of fabricated devices with the passivation process and the depositon of the field-plate structure.Based on the discussion above,this dissertation attempts to promote the further development of AlGaN/GaN power electronic devices through corresponding research and analysis,including the following contents:1.A new method to determine the 2EDG density distribution for passivated AlGaN/GaN power electronic devicesThe polarization charges in the AlGaN barrier layer are very important to the performance of the AlGaN/GaN power electronic devices,for which the polarization effect is directly related to the strain in the AlGaN barrier layer.The strain of the AlGaN barrier layer directly influences the characteristics of the 2DEG electrons.But due to the thickness of barrier layer,accompanied by a certain thickness of gate metal blocking,the strain and polarization cannot be measured directly and experimentally.Further in-depth investigation for the strain variation along with the surface trapping states that alter the 2DEG distribution is needed to thoroughly elucidate the working mechanism of the passivated devices.The PCF scattering originates from the nonuniform distribution of the polarization charges.By using the PCF scattering theory,the 2EDG density after passivation was both quantitively and qualitatively determined.For this purpose,rectangular AlGaN/AlN/GaN HFETs were fabricated,the gate length(LG)of which is 20 ?m and the gate width(W)is 100 ?m.Both the gate-source distance(LGS)and the gate to drain distance(LGD)is 40 ?m.Both the direct-current(DC)measurements and the capacitance-voltage(C-V)measurements were carried out at room temperature(300K)before and after passivation.Better transport properties have been observed for the 400nm-Si3N4 passivated devices compared with the same ones before passivation.As an important characteristic of the power electronic device,the value of the specific on-state resistance RON·A was decreased by 12.7%after passivation and the knee point voltage was reduced by 7.1%,which presents better conductance characteristics for the passivated devices.The leakage characteristics and the transfer characteristics before and after passivation were investigated and the PCF scattering potential after passivation was determined.By using the PCF scattering theory,and adoptting a self-consistent iteration method,the distribution of the additional polarization charges at the interface of the AlGaN/GaN structure through the entire drain-source area was determined.According to the gate probe method,the values of the parasitic source access resistance Rs were measured.By comparing the calculated Rs with the measured Rs at positive biases,the validity of the method was verified.2.Determination of the strain distribution for the Si3N4 passivated AlGaN/GaN power electronic devices and the research on the performance of the devices after passivation.The study of the strain distribution of the AlGaN barrier layer in AlGaN/GaN power electronic devices becomes utmost important with the view of understanding the electrical properties of fabricated devices both before and after passivation.However.the strain of the GaN epitaxial layer is usually characterized by high resolution X-ray diffraction(HRXRD).Raman and photoluminescence(PL)measurements.But for the AlGaN barrier layer,due to the thickness of around 20 nm.it is too difficult to obtain accurate strain and polarization distribution information directly through experiments.Most of all,the HRXRD could not provide a precise experimental result of the strain distribution for the fabricated devices,especially the strain distribution of the AlGaN barrier layer after the passivation process in AlGaN/GaN power electronic devices,which is a huge bottleneck for the study of AlGaN/GaN HEMTs after passivation.To determine the strain distribution of the AlGaN barrier layer after the passivation process,AlGaN/GaN power electronic devices of the same size on the two wafer pieces were fabricated,for which the gate length(LG)of the device is 3?m,and the gate width is 100 ?m.The gate-source distance(LGS)is 7 ?m and the gate to drain distances(LGD)is 30 ?m,respectively.A 250 nm thick Si3N4 passivation and a 400nm thick Si3N4 passivation were deposited on the two wafer pieces by PECVD at 250 ?,and the C-V and DC measurements were carried out both before and after passivation.Based on the analysis of test data,the influence of the S13N4 passivation on the surface properties and the PECVD induced trapping states have been excluded for the two samples.The change of the additional strain in the AlGaN barrier layer after passivation of the device is analyzed and discussed.By using the PCF scattering theory,and adoptting a self-consistent iteration method,the distribution of the polarization charges in the AlGaN barrier layer after the passivation of the AlGaN/GaN power electronic devices was obtained.Then combined with the classical theory of the spontaneous polarization and the piezoelectric polarization for GaN heterostructure materials,a new method to determine the strain distribution of the AlGaN barrier layer after the passivation process in AlGaN/GaN power electronic devices is presented.To investigate the influence of the PCF scattering on the electrical properties of the AlGaN/GaN power electronic devices after the Si3N4 surface passivation,two samples were fabricated,for which the gate length and width were 3 ?m and 100 ?m,respectively.The gate-source lengths were 5 ?m and 7 ?m,and the gate-drain lengths were 40 ?m and 30 ?m,respectively.A Si3N4 passivation with the optimized thickness of 400nm was deposited by PECVD at 250,and the DC and C-V measurements were carried out both before and after passivation.For the two samples after passivation,the values of the knee point voltage after passivation were reduced by 15.3%and 10.4%,respectively.The values of the specific on-state resistance RON·A was decreased by 15.3%and 10.4%after passivation.The leakage characteristics,transconductance characteristics and the C-V characteristics of the two devices before and after passivation wre analyzed.By using the PCF scattering theory,and adoptting a self-consistent iteration method,the 2DEG electron mobility after passivation showed a clear numerical rise.This can be attributed to the additional compressive strain induced into the AlGaN barrier layer,which weakened the non-uniform distribution of the polarization charges and eventually lead to the weakening of the PCF scattering.On condition that the calculated electron mobility curves for PE,POP,and IFR basically remain unchanged,the weakening of the PCF scattering is mainly responsible for the apparent numerical rise of the electron mobility and therefore,is the main reson for the decrease of the RON·A and the knee point voltage3.Study of the field plate structure's influence on the performance of AlGaN/GaN power electronic devices by using the polarization coulomb field scattering theory.To investigate the influence of the gate field-plate on the PCF scattering in the AlGaN/GaN power electronic devices,devices with a gate length(LG)of 20 ?m and ta gate width of 100 ?m were fabricated,for which the gate-source length(LGS)and the gate to drain length(LGD)was 40 ?m.The field-plate connecting to the gate electrode was formed on the passivation layer and the length of the gate field-plate(LFP)was 24 ?m.The same DCand capacitance-voltage C-V measurements were carried out both before and after the FP process.Increases in the maximum drain current(IDmax)and the transconductance(gm)of 2.95%and 2.58%are obtained when compared with the device before the gate FP process,revealing better electron conduction characteristics and a better gate control for the FP devices..The RD values herein were determined by using the gate probe method.The increase of the two-dimensional electron gas density and the decrease of the RD values of the device after the deposition field plate are in good agreement.When devices at the working state,according to the inverse piezoelectric effect,the strain of the area below the gate field plate changes with the variation of the gate bias.leading to the variation of the additional polarization charges in the AlGaN barrier layer,and eventually lead to the change of PCF scattering potential.In the case of invariant temperature and other conditions,only the PCF scattering potential changes,and is affected by the distribution of the additional polarization charges.And as a result,the electrical properties of the AlGaN/GaN power electronic devices change.The value of the additional polarization charges induced by the FP structure was determined by the converse piezoelectric effect.The FP induced additional polarization charges are negative when adding negative gate biases,which was a compensation for the positive charges induced by the passivation process before.The un-uniformity of the additional polarization charges underneath the FP region was then alleviated,which induced a reduction in the scattering potential of the PCF scattering,leading to the improved performance of the device.By using the PCF scattering theory,and adoptting a self-consistent iteration method,the 2DEG electron mobility of the gate-drain region as a function of gate biases was obtained.The increase in the electron mobility shows good agreement with the result that the scattering potential of the PCF has been reduced after the FP process.