EDC Lesson 12: Transistor and FET
Characteristics
Lesson-12: MOSFET (enhancement
Lessonand depletion mode) Characteristics
and Symbols
2008
EDCLesson12- " , Raj Kamal,
1
1. Metal Oxide Semiconductor Field Effect
Transistor (MOSFET)
2008
EDCLesson12- " , Raj Kamal,
2
Types of FETs
The family of FETs may be divided into :
 Junction FET
 Depletion Mode MOSFET
 Enhancement Mode MOSFET
2008
EDCLesson12- " , Raj Kamal,
3
Remember JFET Definition
•JFET is a unipolar-transistor, which acts as a
voltage controlled current device and is a device
in which current at two electrodes is controlled by
the action of an electric field at a reversed biased
p-n junction.
2008
EDCLesson12- " , Raj Kamal,
4
MOSFET Definition
• MOSFET Field effect transistor is a unipolartransistor, which acts as a voltage-controlled current
device and is a device in which current at two
electrodes drain and source is controlled by the
action of an electric field at another electrode gate
having in-between semiconductor and metal very a
thin metal oxide layer .
•
2008
EDCLesson12- " , Raj Kamal,
5
n-channel depletion
Metal--Oxide
Metal
Oxide--Semiconductor FET (MOSFET)
n channel
p
2008
EDCLesson12- " , Raj Kamal,
6
p-channel depletion
Metal--Oxide
Metal
Oxide--Semiconductor FET (MOSFET)
p channel
n
2008
EDCLesson12- " , Raj Kamal,
7
Effect of Insulating SiO2 (metal-oxide)
layer
• MOSFET Very high input impedance due to
SiO2 layer compared to even reverse biased
p-n junction depletion region input
impedance in JFET
2008
EDCLesson12- " , Raj Kamal,
8
n-channel depletion region
2008
EDCLesson12- " , Raj Kamal,
9
n-channel depletion region (normally ON)
2008
EDCLesson12- " , Raj Kamal,
10
Transfer Characteristics of nn-channel
depletion region MOSFET
2008
EDCLesson12- " , Raj Kamal,
11


There is a convenient relationship between IDS
and VGS.
Beyond pinch-off
I DS



  VGS
 I DSS 1  

  VGS ( off )




2
Where IDSS is drain current when VGS= 0 and
VGS(off) is defined as –VP, that is gate-source
voltage that just pinches off the channel.
The pinch off voltage VP here is a +ve quantity
because it was introduced through VDS(sat).
VGS(off) however is negative, -VP.
2008
EDCLesson12- " , Raj Kamal,
12
p-channel depletion MOSFET
2008
EDCLesson12- " , Raj Kamal,
13
p-channel depletion MOSFET (Normally ON at
VGS = 0)
2008
EDCLesson12- " , Raj Kamal,
14
2. Enhancement Mode MOSFETS
2008
EDCLesson12- " , Raj Kamal,
15
n-channel enhancement mode MOSFET
2008
EDCLesson12- " , Raj Kamal,
16
n-channel enhancement mode Normally Off
2008
EDCLesson12- " , Raj Kamal,
17
n-channel
enhancement
mode

p-channel
enhancement
mode
Note that with a n-channel device we apply a +ve gate
voltage to allow source-drain current, with a p-channel
device we apply a -ve gate voltage.
2008
EDCLesson12- " , Raj Kamal,
18
p-channel enhancement MOSFET
2008
EDCLesson12- " , Raj Kamal,
19
p-channel enhancement MOSFET (normally
OFF)
2008
EDCLesson12- " , Raj Kamal,
20
3. Enhancement Mode MOSFETS in detail
2008
EDCLesson12- " , Raj Kamal,
21
Basic MOSFET (n(n-channel) Enhancement mode
When
VGS = 0,
the nchannel
is very
thin and
channel
width
enhances
with
+ VGS
2008
EDCLesson12- " , Raj Kamal,
22
Channel width enhancement with +VGS



The gate electrode is placed on top of a very
thin insulating layer.
There are a pair of small n-type regions just
under the drain & source electrodes.
If apply a +ve voltage to gate, will push away
the ‘holes’ inside the p-type substrate and
attracts the moveable electrons in the n-type
regions under the source & drain electrodes.
2008
EDCLesson12- " , Raj Kamal,
23
Enhancement mode MOSFET


Increasing the +ve gate voltage pushes
the p-type holes further away and
enlarges the thickness of the created
channel.
As a result increases the amount of
current which can go from source to
drain — this is why this kind of
transistor is called an enhancement
mode MOSFET.
2008
EDCLesson12- " , Raj Kamal,
24
Subthreshold region in nn-chennel enhancement mode
ID= k exp (qVGS/kBT)
where T temperature iu in
Kelvin, is Boltzman
constant
2008
EDCLesson12- " , Raj Kamal,
25
Subthreshold region in nn-chennel enhancement mode
ID= k (VGS  VT)2
2008
EDCLesson12- " , Raj Kamal,
26
Above Threshold – ON state
ID= k (VGS 
If ID (on) = 10
mA and
VT)2 =
0.278 (VGS  VT)2 mA
VGS = 8 V, then k
=10 mA/(8V 
2V)2 = 0.278
mA/V2
ID
VT = 2 V
VGS
0
2008
1
2 3
4
5 6
7
8
EDCLesson12- " , Raj Kamal,
27
Above Threshold – ON state
VT = 2 V
If ID (on) = 10 mA and
ID= k (VGS  VT)2
VGS = 8 V, then k =10 mA/(8V  2V)2 = 0.278
mA/V2
ID= k (VGS  VT)2 = 0.278 (VGS  VT)2 mA
2008
EDCLesson12- " , Raj Kamal,
28
Ideal Output Characteristics of MOSFET
2008
EDCLesson12- " , Raj Kamal,
29
Ideal Output linear region before saturation
Characteristics of MOSFET
2008
EDCLesson12- " , Raj Kamal,
30
Ideal Output Saturation formula of enhancement
mode MOSFET
chang
2008
EDCLesson12- " , Raj Kamal,
31
Transfer Characteristics in above threshold region
and saturation region in nn-channel enhancement
mode
7V
6V
5V
4V
3V
VGS = VT = 2V
2008
EDCLesson12- " , Raj Kamal,
32
Transfer Characteristics in Subthreshold region and
saturation region in nn-channel enhancement mode
2008
EDCLesson12- " , Raj Kamal,
33
4. Symbols of depletion and Enhancement
Mode MOSFETS
2008
EDCLesson12- " , Raj Kamal,
34
Symbol of depletion mode MOSFET
n-channel Depletion
Mode MOSFET
2008
p-channel Depletion
Mode MOSFET 35
EDCLesson12- " , Raj Kamal,
Symbol of enhancement mode of MOSFET
n-channel
Enhancement Mode
MOSFET
2008
p=channel Enhancement Mode
MOSFET
EDCLesson12- " , Raj Kamal,
36
Summary
2008
EDCLesson12- " , Raj Kamal,
37
We learnt
• Definitions of MOSFET
• n-channel and p-channel Depletion
MOSFET normally ON, Pinch off on –
VGS in n-MOSFET and + VGS in pMOSFET
• n-channel and p-channel enhancement
MOSFET normally OFF, below Threshold
and above VT on + VGS in n-MOSFET and
- VGS in p-MOSFET
2008
EDCLesson12- " , Raj Kamal,
38
End of Lesson 12
2008
EDCLesson12- " , Raj Kamal,
39