Study Of Split-gate AlGaN/GaN Heterostructure Field-effect Transistors Combined With Polarization Coulomb Field Scattering Effect

GaN is an important representative of the third-generation semiconductors,and AlGaN/GaN heterostructure field-effect transistors(HFETs)are the main form of GaN-based electronic devices.Based on the advantages of GaN materials,AlGaN/GaN HFETs have high breakdown electric field and saturated electron drift velocity.Meanwhile,two-dimensional electron gas(2DEG)with high density and high mobility can be obtained in AlGaN/GaN heterostructure without intentional doping,which avoids the shortcomings of GaN material itself.These advantages make AlGaN/GaN HFETs especially suitable for high-frequency and high-power applications,and have very broad application prospects in military industry,aerospace,smart phones,new energy vehicles,small base stations and other fields.The study on AlGaN/GaN HFETs is conducive to the improvement of device characteristics and accelerating its commercialization process,which not only meets the needs of daily applications,but also has strategic significance at the national level.In order to improve the device performance,many unique structures have appeared in the development of AlGaN/GaN HFETs,such as T-gate structure,in-plane-gate structure,tri-gate structure and so on.Most of the structures have been fully studied,but the split-gate structure has not received sufficient attention.The split-gate structure first appeared in AlGaAs/GaAs high electron mobility transistors(HEMTs).At that time,researchers mainly used this special structure to realize the transformation of device conductive channel from two-dimensional to one-dimensional,so as to study the relevant physical mechanism in this one-dimensional channel.In AlGaN/GaN HFETs,there are less researches related to split-gate devices,and the research ideas are highly similar to that before.Relevant researches have made many important achievements,but the research directions have not been broadened.In a sense,as a tool to realize the dimensional transformation of electron transport channels,split-gate structures have been widely studied.However,as independent electronic devices,there are still no relevant research reports on the working mechanism,performance optimization and practical application of split-gate AlGaN/GaN HFETs.On the other hand,as an important scattering mechanism in AlGaN/GaN HFETs,polarization Coulomb field(PCF)scattering has an important impact on the device performances.In the past researches,PCF scattering has been deeply studied from many aspects,such as device structure,material composition,process conditions,parameter optimization,theoretical modeling and so on,and a relatively perfect theoretical system has been formed.However,these studies are carried out for normal devices and do not involve split-gate devices.Since the quantitative description of PCF scattering is closely related to the device structure,the theoretical model established in normal devices cannot be directly applied to split-gate devices,so it is necessary to establish a new theoretical model.Based on the above problems,this paper combined PCF scattering with split-gate AlGaN/GaN HFETs for the first time,and systematically studied the scattering theory,working mechanism,application advantages and structure optimization of split-gate AlGaN/GaN HFETs from the perspective of independent devices.Firstly,the scattering theory was studied,and a theoretical model of low-field electron mobility based on PCF scattering was established in split-gate AlGaN/GaN HFETs.Then,PCF scattering was associated with device modulation to improve the working mechanism of split-gate devices.Next,the split-gate device with submicron gate length was studied,and its advantages for class-A common-source voltage amplifier were described.Then,a new auxiliary gate structure was proposed to optimize the saturation characteristics of split-gate devices.Finally,a new experiment was designed to determine the open region effective width of the split-gate devices,and the influence of the gate length on the PCF scattering was further studied.The specific contents of this paper are as follows:1.Theoretical model of low-field electron mobility for split-gate AlGaN/GaN heterostructure field-effect transistors based on polarization Coulomb field scattering.Focusing on PCF scattering,the scattering mechanism in split-gate AlGaN/GaN HFETs was studied,and a low-field electron mobility model based on PCF scattering was constructed in split-gate devices.Due to the different gate structure,the polarization charge distribution of split-gate devices was different from that of normal devices.Combining the polarization charge distribution of split-gate devices with the calculation method of PCF scattering in normal devices,the expression related to PCF scattering in split-gate devices was derived.Considering various scattering mechanisms focusing on PCF scattering,the theoretical model of low-field electron mobility of split-gate devices was obtained.Using the idea of self-consistent iteration,the quantitative solution of the above theoretical model was realized by comparing and iterating the theoretical value with the experimental value of open region channel resistance.This study lays a foundation for the quantitative calculation of PCF scattering in split-gate AlGaN/GaN HFETs.2.Working mechanism and application prospect of split-gate AlGaN/GaN heterostructure field-effect transistors.The working mechanism of split-gate AlGaN/GaN HFETs was mainly studied,and the application prospect was briefly analyzed and prospected.Firstly,normal device and split-gate devices with different gate opening widths were prepared.All devices were located on the same AlGaN/GaN heterostructure material,and the other dimensions were the same except the gate opening width.Comparing the current-voltage(I-V)output characteristics of the devices,it could be found that the turn-off voltage of the split-gate device was less than that of the normal device,and the larger the opening width was,the more difficult it was to turn off the device.When the channel 2DEG underneath the gate was exhausted,the normal device was turned off directly,while the open region of the split-gate device could continue to conduct,and the drain-source current(IDS)of the device could still be effectively modulated by the gate-source voltage(VGS),According to the capacitance-voltage(C-V)characteristics of the devices,the influence of the gate fringe electric field on the open region effective width was analyzed,and it was found that the split-gate device could not be turned off only by the effect of the gate fringe electric field.After further excluding other factors,the PCF scattering theory was introduced into the modulation mechanism of split-gate devices,and the low-field electron mobility of channel electrons was quantitatively calculated.It was found that the gate bias could affect the channel electron mobility through PCF scattering,so as to modulate the current in the open region.By analyzing the saturation characteristics,it was found that the saturation mechanism of the open region of the split-gate device was similar to that of the ungated device,which was realized by the virtual gate formed by the surface electron injection.By analyzing the transfer characteristics,it could be found that the split-gate device had two working modes,among which the low transconductance mode was suitable for class-A common-source voltage amplifier with low power consumption.Relevant studies also showed that the threshold voltages of devices with different gate opening width were also different,which provided a simple and effective method for modulating the threshold voltages of devices.This study not only improves the working mechanism of split-gate devices,but also predicts the broad application prospect of split-gate devices in the field of circuits.3.Study of split-gate AlGaN/GaN heterostructure field-effect transistors with submicron gate length.From the perspective of application,the split-gate AlGaN/GaN HFETs with submicron gate length were studied,and the advantages of split-gate devices as voltage amplifiers were emphatically analyzed.Firstly,the AlGaN/GaN HFETs were prepared and the direct-current(DC)characteristics were measured.The split-gate device had the same material and similar size as the normal device used for comparison.By analyzing the DC measurement results,it could be found that when the channel 2DEG underneath the gate was exhausted,due to the comprehensive modulation of PCF scattering and gate fringe electric field,the split-gate device with submicron gate length could still work normally,but the current was much larger than the theoretical prediction.Then,from the perspective of electric field,the theoretical hypothesis of open region effective width expansion was proposed,which qualitatively explained the problem of excessive current.Finally,the characteristics of split-gate devices and normal devices used as class-A common-source voltage amplifier were studied and compared.The research showed that as a class-A common-source voltage amplifier,the split-gate device could effectively amplify a larger input voltage signal,and achieved higher linearity and lower power consumption at the same time.Because the submicron gate length is closer to the device size in practical application,this research has more guiding significance for the application of split-gate devices in practical circuits.4.Study of split-gate AlGaN/GaN heterostructure field-effect transistors with auxiliary gates.The saturation characteristics of split-gate AlGaN/GaN HFETs were studied,and an auxiliary gate structure which can greatly improve the saturation characteristics of split-gate devices was proposed.Firstly,several AlGaN/GaN HFETs with different structures were prepared on the same material,and their C-V characteristics and I-V output characteristics were measured.The pinch-off voltage of the channel underneath the gate could be extracted according to the C-V characteristics,and the saturation characteristics and saturation mechanism of different devices could be compared according to the I-V output characteristics.The research showed that the saturation characteristics of common split-gate devices were usually poor.The current saturation of common split-gate devices without passivation was mainly affected by the surface virtual gate,and after passivation,the current saturation mainly depended on the self-heating effect.After introducing an auxiliary gate with a potential of 0 V between the main gate and the drain,the current saturation could be realized by the channel pinch-off underneath the auxiliary gate,which greatly improved the saturation characteristics of the split-gate devices.Through further analysis,it could be found that the split-gate devices with auxiliary gates had the characteristics and functions of both split-gate devices and normal devices.Using different potential combinations between the main gate and the auxiliary gate could not only reduce the channel current as well as the power consumption of the device,but also modulate the threshold voltage of the device in a great range.These characteristics are difficult to realize by devices with other structures.This research is of great significance to improve the performance and expand the application of split-gate devices.5.Effect of device size on open region effective width and polarization Coulomb field scattering of split-gate AlGaN/GaN heterostructure field-effect transistors.Through newly designed experiments,the open region effective width of split-gate AlGaN/GaN HFETs was studied,and the correlation between gate length and PCF scattering was further analyzed.Firstly,based on the split-gate structure,the ungated devices with special mesa structure and different sizes were designed and prepared,and their I-V characteristics were measured.The analysis of the measurement results showed that the effective width expansion theory based on the channel electric field distribution qualitatively conformed to the experimental law,but the quantitative expression was not strictly valid.Then,based on the experimental results and ignoring the specific physical mechanism,the empirical expression of the open region effective width was summarized,and the effectiveness of the expression was verified by experiments.Finally,through the quantitative calculation of low-field electron mobility in the open region of split-gate devices with different gate lengths,the correlation between PCF scattering and device gate length was studied.It was found that increasing gate length could enhance PCF scattering and reduce channel electron mobility.This study improves some theoretical mechanisms in split-gate AlGaN/GaN HFETs,and is of great significance for the accurate solution of relevant parameters.