1. MOS CAPACITOR Department of Materials Science & Engineering
Dae-Hong Ko
Semiconductor Device Physics and Technologies

2. * MOS CAPACITOR ⇒Two terminal structure
⇒Metal Oxide Semiconductor CAPACITOR

3. * MOS CAPACITOR

4. ◎ Energy Band Diagram of a MOS Capacitor with a P-type substrate

5. ◎ Energy Band Diagram of a MOS Capacitor with a N-type substrate

6. * MOS CAPACITOR
◎ Depletion Layer Thickness

7. *n-type substrate ◎ At inversion point (threshold voltage)
*p-type substrate
*n-type substrate

8. ■ Work Function Differences

9. * P+ and N+ POLY SILICON GATE

10. *MOS SUBSTRATE

11. Flat-Band Voltage Flat-band condition In real cases, Vox ≠ 0
1. work function difference
2. trapped charge in the oxide
1. Φms = Φm – Φs ≠ 0
2. Oxide charges
- fixed charge : positive
→ broken or dangling covalent bonds near semiconductor-
oxide interface
→Q’ss close to the oxide-semiconductor interface

12. • p-sub or
Under gate voltage
for flat-band condition

13. * Threshold Voltage
→ Applied gate voltage required to achieve the threshold
inversion point
→ Φs=2Φp for NMOS, p-sub
Φs=2Φn for PMOS, n-sub

14. At inversion point
Space charge width → maximum
Energy band diagram of the MOS system with an applied positive gate voltage

15. * MOS CAP Capacitance-Voltage Characteristics
- Ideal C-V Characteristics
Three operating conditions,
• Accumulation
• Depletion
• Inversion
MOSCAP with p-type sub.
1. Accumulation
If VG < 0
⇒ induce an accumulation layer of holes in the semiconductor
at the oxide-semiconductor interface
⇒ small VG change → Qm and QSD change

16. * MOS CAP 2. Depletion - VG : small positive voltage
⇒ induce a space charge region in the semiconductor
⇒ small differential change in VG induces a differential
change in the space charge width
As VG ↑ → xd ↑
→ C’SD ↓
→ C(depl)↓

17. At high frequency
- different from low frequency case
Two source of electrons in the inversion layer
1. diffusion of minority carrier electrons from the p-type sub
across the space charge region
2. thermal generation of electron-hole pairs within
the space charge region
Both processes generate electrons at a particular rate
⇒ The electron concentration in the inversion layer
CANNOT change INSTANTANEOUSLY.
⇒ The change in the inversion layer charge cannot respond to
the high frequency AC voltage change.

18. Fixed oxide and interface charge effects
two types of charges for C-V characteristics change.
Fixed oxide charge & Interface charge
1. Fixed oxide charge
- VFB shift negatively for positive fixed oxide charges.
- Oxide charge → not a function of VG
→ parallel shift of C-V curve with oxide charge
→ same shape of C-V curve
High-frequency characteristics of a MOSCAP with p-type sub

19. 2. Interface state charge
- At the oxide-semiconductor interface
→ periodic nature of semiconductor is abruptly terminated
→ electronic energy levels exist within the forbidden bandgap
⇒ Interface states
- Charge can flow between the semiconductor and interface states
→ The net charge in the interface state is a function EF

20. Some are acceptor-like and some are donor-like

21. ⇒ change electron occupancy in the states due to the VG charge
⇒ change in C-V characteristics