This document summarizes key concepts about MOS capacitors: 1. A MOS capacitor consists of a metal gate separated from a semiconductor substrate by an insulating layer. Applying different gate biases results in accumulation, depletion, or inversion of charge carriers near the semiconductor surface. 2. In depletion mode, a positive or negative gate bias depletes the semiconductor surface of holes or electrons. At the threshold inversion point, the surface is inverted from p-type to n-type or vice versa. 3. Key parameters that determine the depletion layer thickness and threshold voltage include acceptor doping concentration, permittivity of the semiconductor, and surface potential.

1. ETE411 :: Lec16MOS Dr. MashiurRahman Copyright: Semiconductor Physics And Devices 3rd ed. - J. Neamen

2. Basic MOS capacitor structure

3. How it works? A parallel-plate capacitor showing the electric field and conductor charges A corresponding MOS capacitor with a negative gate bias showing the electric field and charge flow The MOS capacitor with an accumulation layer of holes

4. MOS capacitor with a moderate positive gate bias Induced space charge region Electric field and charge flow

5. MOS capacitor with a p-type substrate A negative gate bias Moderate positive gate bias We have inverted the surface of the semiconductor from a p-type to an n-type semiconductor

6. A “large” positive gate bias

7. N-type substrate Positive gate bias Moderate negative gate bias

8. MOS with n-type substrate Positive gate bias Moderate negative gate bias A “large” negative gate bias

9. Depletion layer thickness Acceptor doping concentration Surface potential Permittivity of the semiconductor

10. Maximum space charge width Фs = 2 Фfp eФs = 2e Фfp This condition is known as threshold inversion point Applied voltage is the threshold voltage

11. Quiz 6 Quiz 6 will be held on Tuesday 4 July. Lec 13 Lec 14 Mid 2 (8 August, Saturday) Lec 8 - Lec 15 Votes: 10-1: 0 2-4: 5 person 3-5: 14 person 6-8: 9 person

12. The energy-band diagram in the n-type semiconductor at the threshold inversion point