Study On The Key Processes And Modeling Of SiC Field-Effect Devices

Silicon Carbide (SiC) has outstanding properties such as high saturated electron drift velocity, high electric breakdown field and high thermal conductivity, and is a very promising wide band gap semiconductor material to fabricate high-temperature, high power and high frequency semiconductor devices. As the most important SiC devices, SiC field-effect transistors such as metal-semiconductor field-effect transistor (MESFET )and metal-oxide-semiconductor field-effect transistor (MOSFET) and the complementary-metal-oxide-semiconductor (CMOS ) inverter using MOSFET have been granted with considerable attention and intensively investigated.In this work, one of the emphases is laid on the modeling of SiC field-effect transistors and CMOS inverter. The main works and contributions are outlined as follows.Firstly, Based on the analysis of its conduction band structure and isotropic relaxation time approximation, a new analytical model of the electron Hall mobility and Hall scattering factor for n-type 6H-SiC with which the impact of different scattering mechanisms on the low field electron transport can be illustrated clearly, is proposed utilizing three ellipsoidal and parabolic constant energy surfaces simplification. Comparisons with experimental data confirm the present calculation over a wide range of temperatures and doping concentrations. In addition, the high-field electron transport properties in 6H-SiC have been investigated by an ensemble monte carlo technique with a single equivalent nonparabolic band model.Secondly, an improved analytical model for the performance of 4H-SiC MESFET has been presented based on the two-dimensional analysis of the distribution under the gate. The model takes into account the effects of parasitic resistances at the source and drain ends of the channel and the incomplete impurity ionization due to the deeper impurity in SiC. The model is used to generate the large signal current voltage characteristics of the device and expressions for the transconductance and output conductance for a small signal model. The model is superior to the empirical models such as Statz and Curtice models in describing the large-signal characterization of short channel SiC MESFET and has less calculations, therefore suitable for CAD(computer aided design)application to predict the behavior of the SiC MESFET prior to actual device fabrication.Thirdly, based on an analytical solution of the Poisson equation and iterativecalculation of the surface potential, a physics-based analytical model which can be used in all the operation regions for the dc I-V characteristics and small signal parameters of n-channel 6H-SiC MOSFET is proposed utilizing charge-sheet approximation. The influences of incomplete ionization of deep-lying impurity state and the interface state charges that have an important effect on SiC MOSFET are considered simultaneously. The comparison between simulations and physical measurements shows a good agreement. The model is simple in calculations and distinct in physical mechanism and can be used to analyze the characterization of SiC devices and circuits. Then, a circuit model which can be utilized to simulate the DC and transient transfer characteristics of SiC CMOS inverter is developed by modifying the material and device model based on the MEDICI device simulator. The results of simulation show that the CMOS inverter can operate stably at higher temperatures and has better voltage transfer characteristics for different supply voltages. In the temperature range from room temperature to 600K, the simulated switching threshold voltage of the inverter demonstrates little change. The rise time, fall time and propagation delay of the dynamic response of the inverter exhibit a pronounced decrease at 573K compared with room temperature.The process technology for SiC device is another important part in this work. Three key processes including ohmic contact, oxidation and dry etching have been investigated under the fabrication ability of our own. The utility processing flow and the layout for this three processes are first designed. Two different methods have been used to fabricate SiC ohmic contact with Ti/Ni/Au and Ti/Pt/Au respectively. With the testing TLM structures, low specific contact resistances(~10'6Q- cm2)have been achieved and secondary ion mass spectrometry (SIMS) is employed to examine the metal-semiconductor interface reaction. From the research on the oxidation characteristics of SiC at high temperatures in both dry and wet oxygen ambience, the dependence of oxide layer thickness on the oxidation time, temperature and ambience has been given. Inductively coupled C^/Ar plasma (ICP) etching of SiC has also been performed in this work. The SiC etch rate has been investigated as a function of Ar concentration in the gas mixtures, inductively coupled plasma coil power, bias power and work pressure. The surface morphology is observed using Atomic force microscopy (AFM) and the results indicate that our etching process does not induce roughness on the etched surface. X-ray photoelectron spectroscopy (XPS) and Auger energy spectra (AES) data have been analyzed to study the surface damage and contamination.