An MOS capacitor can be used as a varactor. Compared to other versions of varactors such as the p-n junction and Schottky barrier, it has much improved leakage and a wider capacitance range. It is also the “heart” of a MOSFET or CCD. As a powerful analytical tool for semiconductor surfaces and dielectrics, the following properties can be extracted
1. Semiconductor band bending and depletion width.
2. Semiconductor avalanche breakdown field.
3. Semiconductor doping profile.
4. Semiconductor minority-carrier lifetime.
5. Semiconductor dielectric constant.
6. Semiconductor surface recombination velocity.
7. Oxide thickness or oxide dielectric constant.
8. Oxide breakdown field.
9. Oxide charges.
10. Hot-carrier trapping in oxide.
11. Interface trap density distribution.
12. Interface trap capture probability.
13. Work function difference between semiconductor and gate material.
14. Tunneling in semiconductor and oxide.
15. Quantum effects in the inversion layer.
An MOS capacitor can be used as a fixed-value capacitor by doping the substrate heavily or by restricting the voltage range within the constant capacitance regime. A very important application is the storage capacitor in a DRAM cell. In this particular example, the surface area is to be maximized for large capacitance, and different approaches such as using trenches have been applied.
