1. DEPARTMENT OF EEE 1
KCG
CONSTRUCTION & OPERATION OF POWER MOSFET
Power MOSFET is a metal oxide semiconductor field effect transistor. It is a voltage controlled
device requiring a small input gate voltage. It has high input impedance. MOSFET is operated in
two states viz., ON STATE and OFF STATE. Switching speed of MOSFET is very high.
Switching time is of the order of nanoseconds.
MOSFETs are of two types
Depletion MOSFETs
Enhancement MOSFETs.
MOSFET is a three terminal device. The three terminals are gate (G), drain (D) and source
2. DEPARTMENT OF EEE 2
KCG
DEPLETION MOSFET
Depletion type MOSFET can be either a n-channel or p-channel depletion type MOSFET.
A depletion type n-channel MOSFET consists of a p-type silicon substrate with two highly doped
n+ silicon for low resistance connections. A n-channel is diffused between drain and source.
Figure below shows a n-channel depletion type MOSFET. Gate is isolated from the channel by
a thin silicon dioxide layer.
D
G
S
Oxide
n
n
+
n
+
M etal
Channel
p-type
substrate G
S
D
3. DEPARTMENT OF EEE 3
KCG
ENHANCEMENT MOSFET
Enhancement type MOSFET has no physical channel.
Enhancement type MOSFET can be either a n-channel or p-channel enhancement
type MOSFET.
D
G
S
Oxide
n
+
n
+
M etal
p-type
substrate G
S
D
4. DEPARTMENT OF EEE 4
KCG
When gate is positive (VGS) free electrons are attracted from P-substrate and they
collect near the oxide layer.
When gate to source voltage, VGS becomes greater than or equal to a value called
threshold voltage (VT).
Sufficient numbers of electrons are accumulated to form a virtual n-channel and
current flows from drain to source.
5. DEPARTMENT OF EEE 5
KCG
VI CHARACTERISTICS OF MOSFET
VG S
ID
+ +
G
S
D
V D S
6. DEPARTMENT OF EEE 6
KCG
Drain characteristic gives the variation of ID with VDS for a given value of
VGS .
Figure below shows the drain characteristic. MOSFET can be operated in three
regions
Cut-off region,
Saturation region (pinch-off region) and
Linear region.
V G S1
V G S2
V G S3
L inear
region
Saturation
region
V D S
ID
7. DEPARTMENT OF EEE 7
KCG
Figure above shows circuit to obtain characteristic of n channel
enhancement type MOSFET. Figure below shows the Transfer
characteristics. Drain characteristic gives the variation of ID with VGS
for a given value of VDS .
VG S
ID
VT
8. DEPARTMENT OF EEE 8
KCG
Power MOSFETs are generally of enhancement type. Power
MOSFETs are used in switched mode power supplies.
Power MOSFET’s are used in high speed power converters and
are available at a relatively l
ow power rating in the range of 1000V, 50A at a frequency range
of several tens of KHz
.
9. DEPARTMENT OF EEE 9
KCG
IGBT is a voltage controlled device. It has high input impedance like a MOSFET and
low on-state conduction losses like a BJT.
Figure below shows the basic silicon cross-section of an IGBT. Its construction is
same as power MOSFET except that n+ layer at the drain in a power MOSFET is
replaced by P+ substrate called collector.
n epi
n Bufferlayer
p
p
n
n
Gate Gate
Em itter
Collector
G
E
C
10. DEPARTMENT OF EEE 10
KCG
IGBT has three terminals gate (G), collector (C) and emitter (E). With
collector and gate voltage positive with respect to emitter the device is in
forward blocking mode.
When gate to emitter voltage becomes greater than the threshold voltage
of IGBT, a n-channel is formed in the P-region. Now device is in forward
conducting state.
In this state substrate injects holes into the epitaxial layer. Increase in
collector to emitter voltage will result in increase of injected hole
concentration and finally a forward current is established.
11. DEPARTMENT OF EEE 11
KCG
C
I
CE
V
GE
V
CHARACTERISTIC OF IGBT
Figure below shows circuit diagram to obtain the characteristic of an IGBT.
An output characteristic is a plot of collector current
versus collector to emitter
voltage
for given values of gate to emitter
voltage .
VG
V C C
E
VC E
RG E
IC
G
RS
RC
V G E
12. DEPARTMENT OF EEE 12
KCG
IC
VC E
VG E 1
VG E 2
VG E 3
VG E 4
V V V >V
G E G E G E G E
4 3 2 1
> >
Fig. : Output Characteristics
OUTPUT CHARACTERISTICS OF
IGBT
13. DEPARTMENT OF EEE 13
KCG
IC
V G E
V T
Fig. : Transfer Characteristic
TRANSFER CHARACTERISTICS OF
IGBT
14. DEPARTMENT OF EEE 14
KCG
SILICON CONTROLLED RECTIFIER (SCR)
The SCR is a four layer three terminal device with junctions as shown.
The construction of SCR shows that the gate terminal is kept nearer the
cathode. The approximate thickness of each layer and doping densities
are as indicated in the figure. In terms of their lateral dimensions
Thyristors are the largest semiconductor devices made. A complete
silicon wafer as large as ten centimeter in diameter may be used to make
a single high power thyristor.
