METAL-OXIDE-SEMICONDUCTOR CAPACITOR

Metal-oxide-semiconductor (MOS) is a special case of the generic metal- insulator-semiconductor (MIS) structure. Because thermally grown oxide on silicon is the only high-quality oxide-semiconductor interface, MOS implies a silicon substrate with thermal silicon dioxide. Even though the name assumes a metal gate, other gate materials such as poly-silicon and silicide are also referred to by the same name.

The MOS structure was first proposed by Moll and Pfann and Garrett in 1959 as a varactor (voltage-dependent capacitor) to be a contender for the p-n junction varactor. The first practical MOS device was fabricated by Ligenza and Spitzer in 1960 using high-pressure steam oxidation. This consequently led to the first MOSFET (Kahng and Atalla) whose success critically relied on the quality of the MOS capacitor. The development of the MOS capacitor was also pushed by researchers as a powerful tool to investigate semiconductor surface and oxide properties (Terman, Nicollian and Goetzberger, Kuhn, Snow et al.), as surface passivation for junction diodes and bipolar transistors (Atalla et al.), as an effective diffusion mask, and for electrical isolation (Frosch and Derick). It also laid the foundation for the invention of the CCD in 1970 (Boyle and Smith).

The oxide layer is usually grown thermally for good interface and oxide. It is common practice to introduce a trace of chlorine, such as in HC1, during oxidation to minimize sodium contamination which is a main source of mobile charge. A metal gate can be deposited in vacuum by evaporation or sputtering. A poly-Si gate is commonly used, deposited by LPCVD and subsequently doped by diffusion or ion implantation. After the gate is deposited, interface quality can be further improved by low temperature annealing (»500oC) in a gas ambient containing hydrogen.

The imperfections of an MOS capacitor due to oxide charges and interface traps are ignored for now and will be included later.. Under this condition, the semiconductor band bending (or surface potential) is zero, and the applied flat-band voltage corresponds to the difference in work functions. Under different biases, the MOS capacitor is driven into different regimes. The Fermi level remains flat under all bias conditions because of zero current flow. The main effect caused by the bias is to modulate the net carrier concentration at the semiconductor surface (the classical field effect), accompanied by a change of depletion width. At the onset of strong inversion, the depletion width reaches its maximum equilibrium value.