High voltage buried junction vertical silicon carbide field effect transistors

Power MOSFETs are one of the most widely used power devices today due to their superior switching characteristics. However due to the high on-resistance at higher voltage ratings, the use of Silicon power MOSFETs has been mostly limited to breakdown voltages below 200V. The superior material characteristics of Silicon Carbide (SiC) make it an excellent candidate for the fabrication of power MOSFETs and other unipolar devices. However, due to the various technological and fabrication constraints, the Silicon device designs cannot be directly duplicated on SiC. Novel device structures are required to overcome these constraints so that devices with excellent characteristics can be fabricated on SiC. In this work, three new buried junction SiC FET structures, namely the BJ-MOSFET, BJ-MESFET and BJ-JFET are proposed, analyzed and fabricated. The purpose of this research is to study the device physics of the three new FETs through analytical models, two-dimensional numerical simulations and to experimentally demonstrate the operation of these devices. Extensive simulations were performed to optimize the device structures, the breakdown voltage and the on-state characteristics using the device simulator MEDICI.; The device fabrication technology for SiC is not yet well developed and little information is available about the various fabrication processes. Hence a complete process with eight masking levels was developed for the first time at NCSU to fabricate these SiC buried junction FETs. The fabricated devices were electrically characterized and found to have excellent characteristics.; The fabricated BJ-MOSFETs had a blocking voltage of 350V and a low specific on-resistance of 18 m{dollar}Omega{dollar}-cm{dollar}sp2{dollar} which is just 2.5 times the ideal specific on-resistance of the drift region. These devices had excellent gate control, current saturation and almost square FBSOA. The fabricated devices could operate at high current densities ({dollar}sim{dollar}400A/cm{dollar}sp2){dollar} and had a positive temperature coefficient for the on-resistance. It was shown that the issue of high electric field in the gate oxide which has been a major obstacle in the development of SiC UMOSFETs can be solved by reducing the gap (W{dollar}sb{lcub}rm Jch{rcub}){dollar} between the P{dollar}sp{lcub}+{rcub}{dollar} layers in the BJ-MOSFET structure. Simulations indicate that the BJ-MESFET devices have excellent on-state and blocking characteristics, a square forward biased safe operating area (FBSOA) and reverse biased safe operating area (RBSOA). The fabricated MESFETs had extremely low on-resistances as predicted, but had poor blocking capability due to processing related issues. The effect of N-channel dose on the I-V characteristics of the BJ-MESFET and JFET devices was analyzed using numerical simulations and was experimentally confirmed. BJ-JFET devices which could operate at extremely high current densities ({dollar}>{dollar}1000A/cm{dollar}sp2){dollar} with a square FBSOA were experimentally demonstrated. Of the three new device structures studied in this dissertation, the BJ-MOSFET had the best overall performance with low specific on-resistance, low gate voltage (5V) operation, and good blocking characteristics. It also has a closer to ideal (2.5X) specific on-resistance than any SiC power MOSFET that has been reported to date.