1. MOSCAP Non-idealities
Effect of oxide charges
Poly-Si gate depletion effect
VT adjustment
HW9

2. Oxide Charges Within the oxide: At the interface: Trapped charge Qot
High-energy electrons and/or holes injected into oxide
Mobile charge QM
Alkali-metal ions, which have sufficient mobility to drift in oxide under an applied electric field
At the interface:
Fixed charge QF
Excess Si (?)
Trapped charge QIT
Dangling bonds

3. Threshold Voltage Shift
(x is defined to be 0 at metal-oxide interface)
Fixed charge:
Mobile charge:
Trapped charge:

4. Oxide Charge Effect on CV
Mobile ion:
Trapped charge:

5. Gate Depletion and Inversion
Gauss’s Law dictates that Wpoly = eoxEox / qNpoly
n+ poly-Si
Cpoly + + + + + + + +
Cox - - - - - N+ - - - -
p-type Si
Inversion layer thickness:

6. Effective Oxide Capacitance, Coxe

7. VT Adjustment
A relatively small dose NI (units: ions/cm2) of dopant atoms is implanted into the near-surface region of the semiconductor that shifts the threshold voltage in the desired direction.

8. The MOSFET Non-idealities
Velocity saturation
Short channel effect
HW11

9. Velocity Saturation Esat is the electric field at velocity saturation:
Velocity saturation limits IDsat in sub-micron MOSFETS
Simple model:
Esat is the electric field at velocity saturation:
for e < e sat
for e  esat

10. MOSFET I-V with Velocity Saturation
In the linear region:
EE130/230M Spring 2013
Lecture 22, Slide 10

11. Short- vs. Long-Channel NMOSFET
For very short L:

12. The Short Channel Effect (SCE)
i) VT roll-off
ii) DIBL
ii) Degraded SS

13. The Short Channel Effect (SCE)
iv) Punch-through

14. Hot carriers and SD structure
The lateral electric field peaks at the drain end of the channel.
High E-field causes:
Damage to oxide interface & bulk
(trapped oxide charge  VT shift)
substrate current due to impact ionization:
LDD structure:

15. Current and voltage with Parasitic SD ResistanceRS RD S D
For short-channel MOSFET, IDsat0  VGS – VT , so that
 IDsat is reduced by ~15% in a 0.1 mm MOSFET.
VDsat is increased to VDsat0 + IDsat (RS + RD)