Velosity saturation

this is about mosfet velosity saturation.

1. PRESENTATION
ON
VELOCITY SATURATION

ARVIND DAUTANIYA
B.TECH (ECE)
PRATAP UNIVERSITY

2. VELOCITY SATURATION
 When A strong enough electric field is applied:
 the carrier velocity in the semiconductor reaches a
maximum value saturation velocity
 the carriers lose energy through increased levels of
interaction with the lattice, collisions and by emitting phonons

3. VELOCITY SATURATION
We assumed carrier velocity is proportional to E-field
v = mE
lat
= mV
ds
/L
At high fields, this ceases to be true
Carriers scatter off atoms
Velocity reaches v
sat
Electrons: 6-10 x 10
6
cm/s
Holes: 4-8 x 10
6
cm/s
Better model
E
sat0
0
slope = m
E
lat
n
2E
sat
3E
sat
n
sat
n
sat
/ 2
lat
sat sat
lat
sat
μ
μ
1
E
v v E
E
E
= Þ =
+

4. VELOCITY SATURATION

5. VELOCITY SATURATION

6. VELOCITY SATURATION

7. VELOCITY SATURATION
At high E
l a t
, carrier velocity rolls off
Carriers scatter off atoms in silicon lattice
Velocity reaches v
sat
Electrons: 10
7
cm/s
Holes: 8 x 10
6
cm/s
Better model

8. VEL SAT I-V EFFECTS
Ideal transistor ON current increases with V
D D
2
Velocity-saturated ON current increases with V
D D
Real transistors are partially velocity saturated
Approximate with a-power law model
I
ds
µ V
DD
a

1 < a < 2 determined empirically (≈ 1.3 for 65 nm)
( )
( )
2
2
ox
2 2
gs t
ds gs t
V VW
I C V V
L
b
m
-
= = -
( )ox maxds gs t
I C W V V v= -