Study On GaN-based Impact-ionization-avalanche-transit-time Devices In Millimeter Wave Band

With the development of millimeter wave and terahertz wave technology,the requirement for the solid-state source of terahertz semiconductor devices is becoming higher and higher.In the lower Terahertz domain,the electronics based devices viz.,Gunn Diode,IMPATT diode,Resonant Tunneling Diode and Nanometer Field Effect Transistor（Plasma wave FET）are widely investigated.As the most powerful one,IMPATT diode has become the most compelling research object in power output field.In recent years,as an excellent representative of the third generation of semiconductor materials,gallium nitride（GaN）material,because of its wide band gap,high critical electric field,high electron mobility,saturation velocity,high thermal conductivity and other superior properties,becomes a high-profile candidate for semiconductor devices fabrication nowadays.Based on the consideration of materials and devices,GaN based IMPATT devices naturally become the object of research and development.Because there are still many technical problems to be solved,the research on GaN based IMPATT diodes nowadays is still in the stage of theoretical research and computer simulation.It can be predicted that in the operating frequency,output power and conversion efficiency,GaN based IMPATT diodes have better performance than other materials.Based on this background,try to find the advantage and operating potential of GaN based IMPATT diodes;study and find its disadvantage and problems;discuss the mechanism causing these problems and try our best to solve them,these make up the research purpose of this dissertation.The main results of this dissertation are as follows:The modeling process of GaN based IMPATT diode is carried out in chapter 2,based on the existing experimental data,numerical model and Monte Carlo data,the dissertation determines the relationship of GaN electron velocity-field used in the simulation.Using Silvaco-Atlas platform,the structure of GaN IMAPTT diode is designed,the models are embedded,and the static breakdown results are gotten.By using the Atlas-mixedmode platform,the large signal simulation of GaN based IMPATT diode is carried out.Through adjusting the parameters,the reasonable output characteristics of devices are obtained.The chapter 3 researches the five most influential factors on the output performance of GaN based high-low doping structure IMPATT devices:the doping concentration of avalanche junction p-type region,the doping concentration of avalanche junction n-type region,the width of avalanche junction n-type region,the doping concentration of drift region and the width of the drift region.The effects of these factors on the breakdown voltage,the electric potential distribution,the RF frequency,the RF power and the conversion efficiency are studied in detail.The performance of GaN based IMPATT devices operating between 200 and 300 GHz mm band is studied.It is found that operating on the high-low structure IMPATT diodes,the efficiency of the device increases with the concentration of p-region increasing.The doping concentration and width of n-region in the avalanche region mainly affect the performance of power and efficiency,but not the frequency.The doping concentration of the drift region mainly affects the performance of power and efficiency,while the width mainly affects the frequency performance.These results provide theoretical guidance for the design of GaN based IMPATT diodes.This part of the research also shows that for the manufacturing of Ga N based IMPATT didoes based on PN junction,the 1×10¹99 cm^-3 free holes concentration of P-type GaN is needed,which is difficult for the current manufacturing process.In chapter 4,the enhancement effect on the performance of IMPATT devices by the strong negative differential mobility of GaN material is carried out.It is shown that the negative differential mobility（NDM）characteristics of GaN coupled with the space charge effect acting as a self-feedback system can markedly increase the drift velocity of injection carriers,and thereby enhance diode performance under appropriate external RF voltage.The behavior of traveling electrons in the transit zone is investigated in detail.It is found that the IMPATT diode with a punch-through structure operating in the NDM mode exhibits superior characteristics compared with the equivalent diode operating in the Si-like constant mobility mode.In particular,the NDM-mode diode can tolerate a larger RF voltage swing than that operating in constant mobility mode.Numerical simulation results reveal that the highest efficiency of 26.6%and maximum RF power of 2.29 W can be achieved for the NDM-mode diode at a frequency of 225 GHz.A highest efficiency of19.0%and maximum RF power of 1.58 W are obtained for the diode with constant mobility.The mechanism of the enhancement of the device performance is explained from the details of the electron motion inside the device.At the same time,the large signal simulation of GaN IMPATT is carried out by using Schottky junction instead of PN junction.The results show excellent performance.The chapters 5,the anisotropy effects on the performance of Ga N IMPATT diodes are carried out.Due to the fabrication technology,there is no known experimental data of GaN IMPATT diodes.Therefore,this work provides a prediction for the GaN IMPATT didoes fabrication.By comparing large signal simulation of the GaN diodes along the c-axis（Г-A direction）and in the basal plane（Г-M direction）,it is found that the results are very sensitive to the material anisotropy.The performance of theГ-M direction diode is better at high frequency,which is mainly reflected in the higher output power,conversion efficiency and stronger negative resistance,while the performance in theГ-A direction is better at low frequency.The chapter 6,the effect of series resistance on GaN based IMPATT devices is carried out.The mechanism is discussed.The results show that at 40%voltage modulation and the100 kA/cm² DC bias current density,when the ohmic specific contact resistances reach the order of¹0^-5,the operation of IMPATT diode in theГ-A direction is cut off.When it reaches the order of¹0^-6Ω·cm²,the operation of IMPATT diode in theГ-M direction is cut off.At present,the specific contact resistance of n-GaN can easily meet the requirements.However,most specific contact resistances of the p-GaN are of¹0^-4Ω·cm².It is obvious that the high specific contact resistance of p-GaN is one of the most important reasons that limits the fabrication of IMPATT diodes based on GaN PN junction.The article also analyzes the mechanism about how the contact resistance affects the performance of GaN IMPATT diode,it does not reduce the current and voltage amplitude,but decreases the phase delay of IMPATT diodes operation,thus making the RF performance degradation.The conclusion of this chapter indicates that in the study of PN junction GaN based IMPATT devices,we cannot only focus on improving the hole concentration of p-GaN to improve the current intensity,but also to improve the p-GaN ohmic contact resistance.It is considered that the fabrication of GaN based IMPATT diode with Schottky contact has a good potential for application.