Heat Distribution Calculation For HEMT Devices

The key structure of GaN High Electron Mobility Transistor(GaN HEMT)is a heterojunction formed by GaN material and a ternary compound Al GaN with a higher band gap.The heterostructure can generate two-dimensional electron gas with high density and high mobility at the heterojunction surface due to the polarization effect.However,it is also because of the existence of its high-density two-dimensional electron gas that a large amount of heat will accumulate in the channel during operation,causing a sharp rise in the temperature of the device and affecting the performance of the device.The traditional drift-diffusion model has gradually been unable to meet the modeling requirements of smaller and smaller semiconductor devices.Therefore,in this thesis,the Monte Carlo method,which can describe the carrier transport characteristics more accurately,is used to solve the carrier transport characteristics,and then the heat conduction equation is solved by the finite element method to obtain the thermal distribution state of the device.The main contents of this thesis are as follows:(1)The calculation process of Monte Carlo method in device modeling is analyzed,including a series of processes such as energy band structure,drift motion of carriers,selection and calculation of scattering mechanism.Then the finite element method of Poisson equation and heat conduction equation is analyzed in detail,and the final matrix form of the corresponding partial differential equation is deduced.The matrix form of a series of related calculations,such as the corresponding unit interpolation function,is given to meet the needs of program development and calculation.(2)Develop semiconductor device calculation model based on finite element method and Monte Carlo method in C++ programming language.It includes a series of processes from the construction of the initial device physical model,the dissection of the device,and the data visualization of the final solution results.In this way,the modeling calculation of GaN HEMT devices is realized.(3)Use the self-developed calculation module to calculate and simulate the carrier transport characteristics of the GaN HEMT device,and obtain the current output characteristic curve and transient output current of the GaN HEMT device.The heat distribution of the device is then solved by solving the heat conduction equation using the finite element method.Under the same conditions,the effects of four different substrate materials of diamond,silicon carbide,silicon and sapphire on the thermal distribution state of the HEMT device were calculated and analyzed,which were basically consistent with the experiments.The results show that different substrate materials have important effects on the thermal distribution of HEMT devices.Among them,the diamond substrate has the best heat dissipation and can effectively reduce the temperature in the channel.Next is the silicon carbide material,which shows only a small increase in the device temperature peak compared to the diamond substrate under the same conditions.Sapphire has the worst heat dissipation,resulting in a large temperature increase in the device.The temperature increase of HEMT devices on sapphire substrates is almost twice that of diamond substrates.Therefore,selecting suitable substrate materials is an effective means to reduce device temperature.The accuracy and effectiveness of the calculation module and method are verified,and effective methods and tools are provided for further research on the thermal effect and management mechanism of HEMT devices.