The development of nonvolatile random access memory in 6 hydrogen silicon carbide
Abstract
A novel nonvolatile random access memory (NVRAM) fabricated on 6H-SiC is formed by integrating an npn bipolar access transistor vertically with a pnp storage capacitor. The measured storage time of the pnp storage capacitor is longer than 100 years at room temperature. Since no applied bias is needed during the store state, the memory is nonvolatile. The access bipolar transistor has been studied extensively. The gain is degraded by the series base resistance. An analytical model is proposed to calculate the access transistor efficiency. The effect of thermal processing on polysilicon-gate SiC MOSFETs has been investigated. It is found that thermal processing at temperatures higher than 900$\sp\circ$C after polysilicon deposition degrades the oxide quality by increasing the fixed oxide charge density and the SiO$\sb2$/SiC interface state density. Electron mobility in the inversion layers on thermally oxidized 6H-SiC has been studied at different temperatures and different vertical electric fields. A hybrid NVRAM chip is designed. The chip contains a 256-bit SiC NVRAM cell array and a Si CMOS logic chip. The SiC chip and the Si chip are wire bonded together on a ceramic substrate. The first NMOS digital integrated circuits in 6H-SiC have been demonstrated. The circuits are functional from room temperature to 305$\sp\circ$C. A process integrating NMOS logic circuits with the NVRAM cell has been proposed and a test wafer fabricated. Both the logic circuits and NVRAM cells are functional. A fully monolithic NVRAM contains memory cells and peripheral circuits on the same SiC wafer. A 64-bit monolithic NVRAM is designed and fabricated. The NVRAM, sense amplifier, and ring oscillator work separately. The threshold voltage of the MOSFET is very high ($\sim$7 V) because the top p epilayer was dopped higher than desired. The whole circuit does not work.
Degree
Ph.D.
Advisors
Cooper, Purdue University.
Subject Area
Electrical engineering
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