Study On GaN Based Resonant Tunneling Device In Terahertz Band

Ga N based semiconductor material and device have been developed rapidly in recent years. It has been concerned as the representative of the third generation of wide bandgap semiconductor materials after the first generation and second-generation semiconductor material. Ga N based semiconductor material and device has been a research focus owning to outstanding properties of nitride heterostructures, such as high internal electric field, high two-dimensional electron gas, large band gap, large band offset, high thermal conductivity and high saturation electron velocity.Terahertz technique is an emerging science and technology that have been paid much attention due to the unique characteristics and advantages. The frequency of terahertz ranges from 0.1 THz to 10 THz which is between the microwave and infrared ray. Resonant tunneling diode is an important choice as the terahertz emitting source to generate the terahertz wave. Ga N based resonant tunneling diode is the research hotspot which is mainly due to that it inherits the advantage of the Ga N based compound semiconductor materials, such as high carrier concentration, high operating frequency, large output power and high temperature tolerance, etc.This dissertation studies Ga N based resonant tunneling diode to generate stable and high-power terahertz signals. The study mainly includes the modeling of the structure and material of the resonant tunneling diode, the investigation of the degeneration mechanism of the device, the simulation and design of the new device structure and materials. The relevant work is supported by the National Natural Science Foundation of China(No. 61274092) and the National Natural Science Foundation of China(No. 61076079). The main conclusion is as follows:The first part focuses on the modeling process of the resonant tunneling diode. On the basis of the result of Monte Carlo simulation and FMBC model, the optimal velocity-field mobility models of Ga N, Al Ga N and In Al N are obtained by the method of mathematic fitting. Depends on the parameters of the energy band diagram and transmission coefficient, the negative differential resistance characteristic of Al Ga N/Ga N RTD is analyzed by self-consistently solving the Poisson-Schrodinger equations and non-equilibrium Green’s function with the method of TCAD tool Silvaco-ATLAS. Meanwhile, the impact of quantum well width, barrier thickness, spacer width, emitter region thickness and doping level on the negative differential resistance characteristic of RTD is also investigated. We demonstrate that the output performance of the device can be increased by appropriately adjusting the parameter of the device.The second part focuses on the degeneration mechanism of Ga N based RTD. This phenomenon is the research bottom neck of RTD, both in China and abroad. The degeneration mechanism of Al Ga N/Ga N RTD is analyzed by introducing deep level trapping centers into polar Al Ga N/Ga N quantum well to reveal the theoretical model and electron capture mechanism of the trapping centers in the Al Ga N/Ga N heterostructure. Then, the failure mechanism is investigated by analyzing the inner relationship of trapping density, ionization rate and activation energy of trapping centers. Therefore, we point out that degeneration mechanism of Ga N based RTD is caused by the combined action of the trapping density and activation energy of the trapping centers. In addition, the trapping centers with higher activation energy play a dominant role in the degradation of NDR characteristics. Thus, it is promising that low Al content Al Ga N/Ga N heterostructures and homoepitaxial growth technology can suppress the negative effects of trapping centers on NDR characteristics.The third part focuses on the improvement of nearly lattice-matched In0.17Al0.83N/Ga N heterostructure on Ga N based RTD. On the basis of the velocity-field mobility models of In Al N and the trapping density and activation energy of the In Al N/Ga N heterostructure, the negative differential resistance characteristic of In Al N/Ga N RTD is analyzed by self-consistently solving the Poisson-Schrodinger equations and non-equilibrium Green’s function. Results demonstrated that it is promising to adopt nearly lattice-matched In Al N/Ga N heterostructures to reduce the activation energy level of trapping centers, eliminate or reduce the piezoelectric polarization and suppress the probability of ionization of the trapping centers so as to get the reproducible performance of Ga N-based RTDs in realistic applications.The fourth part focuses on the resonant tunneling mechanism of the Ga N based RTD with an In Ga N sub-quantum-well. At resonant-state, electrons in the In Ga N/In Al N/Ga N/In Al N RTD tunnel from the emitter region through the aligned discrete energy levels in the In Ga N sub-quantum-well and Ga N main-quantum-well into the collector region. Depends on the parameters of the energy band diagram and transmission coefficient, the negative differential resistance characteristic of In Ga N/In Al N/Ga N/In Al N RTD is investigated. The implantation of the In Ga N sub-quantum-well alters the dominant transport mechanism, increase the transmission coefficient and give rise to the peak current and peak-to-valley current ratio. We also demonstrate that the most pronounced negative-differential-resistance characteristic can be achieved by appropriately choosing the In composition of Inx Ga1-x N at around x=0.06.The fifth part focuses on the large signal model of Ga N based RTD with the method of PSPICE. Firstly, we extract the parasitic capacitance and parasitic resistance and other I-V characteristics from the steady-state simulation in order to build an equivalent model, and mount the model to an external parallel RLC resonant circuit so as to form an NDR oscillator. Then, we extract the radio frequency current and voltage waveforms from the anode of the equivalent model of RTD. By using the Fourier transform algorithm, we obtain the fundamental components of the rf current and the rf voltage. Finally, the impact of trapping centers on the RF performance of RTD is analyzed.