Study On Structure And Manufacture Of Terahertz GaN IMPATT Diode

Terahertz technology is a very important cross-cutting science and technology.THz has many unique properties and advantages,and it has attracted the attention of many scholars.The terahertz frequency range is 0.1 THz to 10 THz,which is between microwave and infrared.To obtain the terahertz frequency,a suitable device must be selected as the source of the terahertz wave.Impact-Ionization-Avalanche-Transit-Time(IMPATT)diodes can form microwave oscillations in high-frequency and even terahertz frequency bands.It is the most powerful solid-state microwave frequency source capable of producing the highest continuous wave power output in the millimeter waveband.IMPATT diodes fabricated from Gallium Nitride(GaN)materials inherit the advantages of GaN materials.They have high carrier concentration,high carrier mobility,high operating frequency,high power,and high temperature resistance.Compared with other materials,its conversion efficiency and output power are high,so it has become a hot topic.However,high concentration p-type doping of GaN materials is very difficult,and it is difficult to form good p-type ohmic contacts.For the above reasons,there is currently no mature process to fabricate GaN-based IMPATT diodes.In order to fabricate GaN-based IMPATT diodes,a low-high-low Schottky IMPATT diode structure is proposed in this paper.This structure uses a Schottky barrier instead of the traditional PN junction barrier to generate avalanche breakdown,thereby avoiding GaN-based P-type materials.The advantage of Schottky contact is that it can provide high breakdown current strength,with a very small electrode contact resistance,avoiding the minority carrier storage effect of the PN junction diode.In the low-high-low structures,each region has a different doping concentration.Highly doped avalanche termination region can well limit the avalanche multiplication to the avalanche region.The lowly doped avalanche region can concentrate the high field in this area,which improves the working efficiency of the device.This dissertation mainly studies the structure and process of GaN-based terahertz IMPATT diodes.The research contents are as follows:In the second chapter,firstly,the classification and static characteristics of IMPATT devices are introduced.The working principle of the devices is described.The effects of injection phase delay and transit time on the negative resistance of IMPATT devices are discussed.Define the power and efficiency of IMPATT devices and analyze them.After that,it introduced Silvaco-Atlas simulation software and briefly explained the numerical calculation process of simulation.Finally,the electron velocity field relationship and collision ionization model of the GaN materials used in the simulation are illustrated.In the third chapter,the influence of the doping concentration and size of each region of the low-high-low doped GaN-based IMPATT on the DC characteristics of the diodes is firstly analyzed.The doping concentration in the avalanche region affects the electric field distribution in the drift region.The higher the electron concentration in the avalanche region,the smaller the electric field in the drift region.The doping concentration of the avalanche termination layer determines the rate of decline of the electric field strength in the avalanche termination layer.The doping concentration in the drift region has a greater influence on the electric field distribution in the drift region.The higher the doping concentration in the drift region,the larger the absolute value of the electric field slope in the drift region.The smaller the length of the avalanche region,the higher the electric field in the avalanche region and the drift region.The length of the avalanche termination layer determines the decrease in the electric field strength at the avalanche termination layer,which in turn adjusts the magnitude of the electric field in the drift region.The electric fields of the avalanche termination region,avalanche region,and drift region in the different drift region widths are the same,and the electric field is prolonged according to the original trend only in the portion where the device size extends.After that,a large-signal simulation is performed on the IMPATT diode in the Mixed-Model,and a stable diode oscillation waveform was obtained by constantly adjusting the circuit parameters.By performing fast Fourier transform analysis on the oscillating waveform,it is found that the diode has high conversion efficiency and RF power density in the range of 190 to 280 GHz.The optimum operating frequency of GaN-based IMAPTT diode is 220 GHz.At this frequency,when the diode AC voltage modulation factor reaches33.9%,the conversion efficiency of the device is 4.06%,and the RF power density is 19.4mW/?m~2.At a frequency of 220 GHz,the effects of different AC voltage amplitudes on device operating efficiency and power are analyzed,and it is found that the operating efficiency and power increase with increasing AC voltage amplitude.At a frequency of 220GHz and an AC voltage amplitude of 40 V,the electron concentration distribution at a fixed time interval(0.3ps)in one cycle was analyzed,and the movement of charge packets during the operation of the device was described.Finally,the effects of carrier concentration on the electric field strength at three different times in one cycle are discussed.The simulation results show that the space charge effect will reduce the electric field of the IMPATT diode in the punch-through structure.The fourth chapter of this paper mainly studies the Schottky IMPATT diode technology of GaN-based low-high-low structure.Firstly,the high-frequency test transmission line layout of GaN-based IMPATT devices was designed.Secondly,epitaxial layer growth and metal electrode fabrication were performed.Then,ohmic contact and Schottky contact performance in the device fabrication process were tested.Two methods of extracting ohmic contact resistivity,LTLM and CDTLM,are introduced.The ohmic contact resistivity is calculated based on the data from the LTLM test.Two methods for extracting Schottky barrier heights are introduced,I-V method and C-V method.I-V characteristics and C-V characteristics of the Schottky test device were performed at room temperature.According to the test results,the Schottky barrier height,the ideal factor and the doping concentration of the semiconductor surface were extracted.Finally,according to the DC test results of this article,the problems in the device design and manufacturing process are analyzed,and an improvement scheme is proposed.