Download Electronics (2001) - University of Ottawa

Field-Effect Transistors
Based on Chapter 11 of the textbook
• The concept that forms the family of field-effect
transistors, or FETs is that an external electric field may
be used to vary the conductivity of a channel, causing the
FET to behave either as a voltage controlled resistor or as
a voltage-controlled current source.
• FETs are the dominant family in today’s integrated
electronics, and although these transistors come in several
different configurations, it is possible to understand the
operation of the different devices by focusing on one
type.
1
Classification of Field-Effect Transistors
Figure
11.1
2
Drain Characteristic Curves for a Typical NMOS Transistor with
VT = 2 V and K = 1.5 mA/V2
Figure 11.5
3
The n-Channel Enhancement MOSFET Circuit and Drain
Characteristic
Figure
11.6
4
MOSFET Small-Signal Model
Figure
11.13
5
N-Channel Enhancement Mode MOSFET
6
iD-vDS Characteristics
vGS  Vt (Induced Channel)
vGS Vt (Continuous Channel)
Vt  1 V
W
k 'n 
L

2
  0.5 mA/V  k

7
Operation of the n-Channel Enhancement-Mode
MOSFET
Cutoff Region : i D  0
2
Saturation Region : i D  K vGS -VT 
v DS
1 
VA




 K vGS -VT 
2
8
Large-Signal Equivalent Circuit Model of the n-Channel
MOSFET in Saturation.
9