Field Effect Transistors (FET)
The bi-polar transistor should have been introduced, the Field Effect
Transistor or FET carries out similar functions.
There are many different types of FETs, we will be concentrating on the
n-channel Metal Oxide Semiconductor Field Effect Transistor, MOSFET
for short, which are used in the construction of CMOS logic gates.
D
G
S
Fig 1 MOSFET symbol.
The MOSFET has three terminals, referred to as the Drain (D), the Gate
(G) and the Source (S).
Current flow through a MOSFET.
As the voltage between the gate and the source (V ) is increased the
GS
curent in the drain (I ) will increase.
D
I
D
D
ID
G
VGS
S
V
GS
Fig 2 Characteristics of a n-channel MOSFET.
1
Unlike the bipolar transistor the current in the drain I D does not depend
upon an input current, but upon a voltage difference between the gate
and source V .
GS
It can be assumed that the gate does not draw any current as the
resistance between the gate and source is greater than10 9 Ω.
Switching Action of a MOSFET.
+Vs
Lamp
(load)
ID
Input
0V
Output
Voltage
V
V
in
GS
Fig 9.3 The MOSFET switch.
When the input voltage (V in) is increased the drain current ID will
increase. The increase in current will increase the voltage across the
load, thus decreasing V .
out
The relationship between the voltage V GS and the drain current ID is
called the transconductance (g m) and has units of AV-1 or S.
gm = ID / VGS
Therefore
I D = g m x VGS
This can be comparable with the DC current gain (h FE) of the bi-polar
transistor.
2
Circuit Analysis.
Example:
The MOSFET shown in Fig 4 has a transconductance (g ) of 0.25S (250mS).
M
Calculate the minimum input voltage needed to make the bulb operate
at the correct current rating.
6V
6V, 0.55A
ID
V
IN
0V
Fig 4
ID = 0.55A
VGS = ID / g
M
VGS = 0.55 / 0.25
=
2.2 volts
3
Student example
The characteristics of the MOSFET are:
PTot
Max
W
VDS
Max
VGS
Max
V
V
70
100
20
g
S
ID
Max
A
r DS
On
Ω
1.75
10
0.5
m
12 V
Relay
12v, 4A
Logic
Systems
Vin
0V
What is the minimum value of Vin to operate the relay at the correct rating ?
A.0.44 V
C.
0.7 V
B.2.3 V
D.
3.0 V
The function of the diode placed reverse bias across the relay is to :
A protect the MOSFET when the relay energizes.
B protect the MOSFET when the relay de-energizes.
C protect the relay when it energizes.
D protect the relay when it de-energizes.
4