Transcript MOSFET: drain characteristics

MOSFETs: Drain Voltage Effects
on Channel Current
Prof. Paul Hasler
Origin of Drain Dependencies
Increasing Vd effects
the drain-to-channel
region:
• increases barrier
height
• increases depletion
width
Drain Characteristics
Current versus Drain Voltage
Current versus Drain Voltage
Current versus Drain Voltage
Not flat due to Early effect
(channel length modulation)
Current versus Drain Voltage
Not flat due to Early effect
(channel length modulation)
In BJTs --- Base Modulation Effects
Current versus Drain Voltage
Not flat due to Early effect
(channel length modulation)
Id = Id(sat) (1 + (Vd/VA) )
or
Id = Id(sat) e
In BJTs --- Base Modulation Effects
Vd/VA
Current versus Drain Voltage
Not flat due to Early effect
(channel length modulation)
Id = Id(sat) (1 + (Vd/VA) )
or
Id = Id(sat) e
Vd/VA
Iout
Id(sat)
In BJTs --- Base Modulation Effects
GND
Rout
Early Voltage Length Dependence
Width of depletion region
depends on doping, not L
Might expect Vo to linearly
vary with L
Full Range of Effects
7
Punchthrough
Current (nA)
6
Slow / Exponential Increase:
Channel Modulation /
DIBL Effect
5
4
3
2
1
Other (faster) effects:
Current (nA)
0
100
Punchthrough --- depletion
regions converge
(Punchthrough voltage = 2 * VA)
10
Avalanche Breakdown --impact ionization
Exp model
VA = 5V
1
0
1
2
3
4
5
6
Voltage (V)
7
8
9
10
Cause of DIBL
Drain Induced Barrier Lowering
Data taken from a popular
1.2mm MOSIS process
Data taken from a popular
2.0mm MOSIS process
Different Manifestations of DIBL
Different Manifestations of DIBL
Drain Voltage Effects
• Channel Length Modulation / Early Effect
• Exponential Modeling
• Drain Induced Barrier Lowering (DIBL):
Source of Exponential I-V dependence
• Punchthrough: Highest Drain-Source
voltage available