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POWER SEMICONDUCTOR DEVICES
 Contents:
1.1 Introduction
1.2 Diode Characteristics
1.3 Reverse Recovery Characteristics
1.4 Power Diode Types- A] General purpose diode
B] Fast recovery diode
C] Schottky diode
1.5 A] Series connection of diodes
B] Parallel connection of diodes
1.6 A] Gate Turn off SCR [GTO]
B] Power Metal Oxide Semiconductor Field
Effect Transistor [MOSFET]
C] Insulated Gate Bipolar Transistor [IGBT]
1.1 Introduction
 Many
applications have been found for
diodes in electronics engineering circuits.
 A diode
acts as a switch to perform various
functions, such as switches in filters,
regulators,etc.
 Power
diodes can be assumed as ideal
switches for most applications but practical
diodes have certain limitations.
1.2 Diode Characteristics

Symbol
Diode Characteristics
Above fig. shows the symbol and characteristics
of pn- junction diode .
 The complete V-I characteristics obtained by
combining the forward and reverse
characteristics of diode.
 Forward
characteristics is the graph of VF verses
IF. During forward bias forward voltage is small
and less than cutting voltage. Therefore IF
through diode is also small.
 The
voltage at which forward diode current start
increasing rapidally is known as cutting voltage
of diode. During the some period forward vtg
becomes greater than cutting vtg.Therefore
current increases rapidally.
Reverse
characteristics is a graph of reverse
voltage verses reverse current. Reverse
saturation current flows due to minority
carriers.
As
reverse voltage is increase reverse
saturation current remains constant.
As
reverse voltage reaches breakdown voltage
value large current flows through diode due to
avalanche effect .Operation of diode in
breakdown region should be avoided because
it may damage diode due to excessive power
dissipation.
1.3 Reverse Recovery
Characteristics
 The
current in a forward-biased junction diode is
due to net effect majority & minority carriers.
 Once
a diode is a forward conduction mode and
then its forward current is reduced to zero the
diode continues to conduct due to minority
carriers that remain stored in the pn-junction and
the bulk semiconductor material. This time is
called the “Reverse Recovery Time” of the
diode.

Figure shows two reverse recovery
characteristics of junction diodes. The softrecovery type is more common.
 The
reverse recovery time is denoted as trr and is
measured from the initial zero crossing of diode
current to 25%of maximum reverse current Irr.
 The
trr consist of two components ta and tb.
Variable ta is due to charge storage in the
depletion region of the junction and represents
the time between the zero crossing and the peak
reverse current Irr. The tb is due to charge storage
in the bulk semiconductor material
 The
ratio tb/ta is known as the softness factor .
For practical purpose ,one needs be concerned
with the total recovery time trr and the peak
value of the reverse current IRR.
trr= ta + tb .
 The
peak reverse current can be expressed in
reverse di/dt as
IRR= ta di/dt
 Reverse
recovery time trr may be defined as the
time interval between the instant the current
passes through zero during the changeover from
forward conduction to reverse blocking condition
and the moment the reverse current has decayed
to 25% of its peak reverse value IRR.
 It
can be noted from the above equations that
reverse recovery time trr & peak reverse
recovery current Irr depend on storage charge
Qrr & reverse di/dt.
 The
storage charge is depend on the forward
diode current If. The peak reverse recovery
current Irr, reverse charge Qrr & SF are all of
interest to the circuit designer, & these
parameters are commonly included in the
specification sheets of diode.
1.4 Power Diode
 Ideally….a
diode should have no reverse recovery
time. However, the manufacturing cost of such a
diode may increase.
 In
many applications, the effects of reverse
recovery time is not significant, and inexpensive
diodes can be used.
 Depending
on the recovery characteristics and
manufacturing techniques, the power diodes can be
classified into the three categories.
Types of Power Diode
General
purpose
diode
1.4(a) General Purpose diode

The general purpose rectifier diodes have relatively high reverse recovery
time, typically 25µs; and are used in low speed applications, where
recovery time is not critical. e.g. Diode rectifiers.

These diodes covers current rating from less than 1A to several thousands
of amperes, with voltage rating from 50 V to 5Kv.

These diodes are generally manufactured by diffusion. However, alloyed
types of rectifiers that are used in welding power supplies are most costeffective & rugged, & their rating can go up to 1500 V, 400 A.
1.4(b) Fast Recovery Diode

The fast recovery diodes have low recovery time, normally less than 5 µs.

They are used in dc to dc & ac to ac converters circuits, where the speed
of recovery is often of critical importance. These diodes covers current
rating of voltage from 50 V to around 3 Kv.

For voltage rating above 400 V, fast recovery diodes are generally made
by diffusion and recovery time is controlled by platinum or gold
diffusion. For voltage rating below 400V, epitaxial diodes provide faster
switching speed than those of diffused diodes.
1.4(c) Schottky Diode

The charge storage problem of a PN junction can be limited in a schottky
diode. It is accomplished by setting up a “Barrier Potential” with a
contact between a metal & semiconductor.

A layer of metal is deposited on a thin epitaxial layer of N type silicon.
The potential barrier simulates the behavior of a pn junction.

The rectifying action depends on the majority carrier only, & as a result
there are no excess minority carriers to recombine. The recovery effect is
due solely to the self-capacitance of the semiconductor junction.
1.5(a) Series connection of diode

There are some application which requires high voltage
(HVDC) transmission but the single power diode cant
satisfy the high voltage requirement. Hence the power
diodes are connected in series.

The fig shows circuit diagram of series connected diode
in reverse biased condition.
Fig. Series connection of Diode
 In
forward biased condition, diodes conduct equal
amount of current & forward voltage.
 But
in reverse biased condition reverse voltage
across each diode is different because of leakage
current (due to minority charge carriers).
 To
avoid this problem resistance is connected across
of each diode as shown in fig.
Fig. Series connection of diode using resistor
 Further
due to transient at the time of switch of load
or initial switching there is problem of unequal
voltage distribution. Hence a capacitor is connected
across diode.
Fig. Modified Series connection of diodes
 If
there is high resistance the capacitor will damage.
So Rs as current limiting resistance.
1.5(b) Parallel Connection of
Diode
 In
certain high power application higher current is
required But single diode can not achieve the current
capabilities. Hence to obtain high current the diodes
are connected in parallel.
Fig. Parallel connection of Diode
 The
fig shows circuit diagram of parallel connected
diode. To increase the high current capabilities.
 Here
the uniform current sharing is achieved by
connecting the resistance in series with the diode.
Further the inductance are connected in each of
diode in series manner.
Fig. Modified Parallel connection of diode
1.6(a) GTO- Gate Turn Off SCR

The major disadvantage of SCR is that once they
are turned on, the gate will not have any control
on their operation. SCRs once start conducting
can be turned off by using external turn off
circuits known as the commutation circuits.

Therefore, a new type of SCR is called gate turn
off (GTO) SCR is developed which can be
turned off by using a negative gate current pulse.

Gate Turnoff SCR Symbol

Basic Structure of GTO
Construction of GTO
 The
basic structure of GTO is as shown in the figure.
It is basically a four layer structure similar to
conventional SCR.
 As
shown in the circuit symbol of GTO, it is three
terminal device. Gate is control terminal.
 Note
the two arrows marked on the gate terminal.
They indicate that the gate current for GTO can be
either positive or negative. (whereas in SCR gate
current is only positive.)
 There are
few significant differences between GTO &
conventional SCR. They are as follows:
1. The gate & cathode terminal of GTO are highly
interdigitated as compared to those in SCRs with various
types of geometrical forms.
2. The cathode areas are usually formed by etching
away the silicon from cathodes so that they appear as
islands.
3. The difference is in anode region. In the P type
anode layer (P1 layer) the n+ regions penetrate at regular
intervals. These n+ region i.e. the base layer n1. this
results in the so called “Shorted Anode Structure” of GTO.
Specifications of GTO
Sr. No.
Description
1.
On-state Voltage (Vt)
Value
3.4 V
2.
Turn On Time
1µS
3.
Turn Off Time
5 to 30 µS
4.
Turn off Gain
3 to 5
5.
Turn on Gain
600
Applications of GTO
1. Inverters.
2. Uninterruptable
power supplies (UPS).
3. DC motor drives.
1.6(b) Power MOSFET
 The
long form of MOSFET is Metal Oxide
Semiconductor Field Effect Transistor.
 A BJT is
a current controlled device. The
collector current is dependent on the base current
hence current is highly dependent on the junction
temperature. This is a serious disadvantage of
power BJT.
 To
overcome this advantage, we can use voltage
controlled device such as a power MOSFET.
Features of power MOSFET
Low input current.
It is current controlled device.
Switching speed is very high.
Switching speed are of the order of
nanoseconds.
They do not have the problem of second
breakdown.
Symbol
of power MOSFET
Structure
of power MOSFET
Construction of a power MOSFET
 The
power MOSFET’s are generally
enhancement type. In order to increase the
voltage rating of the enhancement MOSFET, a
drift layer is included as shown in the figure.
 The
power MOSFET has the vertically
oriented four layer structure of alternate p & n
layer.
 The
symbol of power MOSFET is as shown in the figure.
1. The vertically oriented structure is used to minimize area
of current flow. This reduces on state resistance &
therefore on state loss. The p type middle layer termed
as “body”. Then n- layer is “drift region” & is lightly
doped as compared to drain & source layers. This drift
region determines breakdown voltage of power
MOSFET.
2. The direction of arrow on body indicates that direction
of current flow if body source pn junction were forward
biased by removing the short link between body &
structure. Therefore n channel MOSFET has a p type
body region & the arrow points into MOSFET symbol.
3. The simplified structure of enhancement MOSFET
is as shown in the figure.
4. The gate terminal is not connected directly to the
semiconductor, instead there exists an oxide layer
SiO2 between the metal & semiconductor. The oxide
layer acts as a layer of dielectric between the metal
& semiconductor to form a MOS capacitance at the
input of MOSFET. This MOS capacitance does not
exist in low power JFET. The input capacitance of
MOSFET is large. The SiO2 oxide layer locates the
gate terminal from body layer & gives the device
insulating properties.
Principle of Operation
 With
gate to source voltage VGS=0 the MOSFET is
equivalent to two back to back diodes connected as
shown in figure.
 The basic structure of MOSFET is very much similar
to BJT. The only difference is presence of MOS
capacitor that isolates gate from body region.
 When VDS is applied, MOSFET turns on.
Applications of power MOSFET
1. In the high
frequency
inverters &
choppers.
2. In the
switching
mode power
supplies
(SMPS).
3. In UPS &
ac motor
control.
4. In stepper
motor
controllers &
brushers'
motor drives.
1.6(c) Insulated Gate Bipolar
Transistor
 The
BJT has advantage of low on state power
dissipation, but it cannot be switched at faster
rates due to longer turn-off time.
 The
power MODFET can be switched at much
higher frequency but has a drawback of higher
on state power loss.
 Thus
to develop a new device having best
qualities of both BJT & MOSFET by combining
BJTs & MOSFETs monolithically on same chip.
Feature of IGBT
Low on state voltage drop.
Low on state power loss.
It has higher switching frequency than that of a
power BJT.
IGBT has the best qualities of BJT & MOSFET.
Symbol
Basic
of IGBT
structure of IGBT
Construction of IGBT
 IGBT uses
vertically oriented structure to
maximize the area available for current flow.
 This
will reduce resistance offered to the current
flow & hence on state power loss taking place in
the device.
 IGBT also
uses highly inter digitated gatesource structure in order to reduce possibility of
source/emitter current crowding.
The
doping layers used in different layers of
IGBT are similar to those used in comparable
layers of vertical MOSFET structures except
for body region.
The
main difference in structure of IGBT as
compared to that of MOSFET is existence of
p+ layer that form’s drain of IGBT.
This
device also uses n- type grain drift layer
which improves its breakdown voltage
capacity, this is same as that in case of power
MOSFET’s.
 The
n+ buffer layer is essential for operation of
IGBT & Some IGBTs are made without it.
 This
layer improves operation of IGBT in two
important aspects- 1) It reduces on state voltage
drop across device. 2) It shortens turn-off time.
 Drawback
is that presence of buffer layer reduces
reverse blocking capacity of IGBT.
 The
circuit symbol for n channel IGBT is as shown
in the figure. IGBT his a three terminal device, gate
being control device.
The
symbol shown in fig. is essentially same
as that of n channel MOSFET, but with
addition of an arrow in drain lead pointing
into body of the device indicating an injecting
contact.
The
symbol indicates that IBT is basically a
BJT with MOSFET gate input. This symbol
indicates that the IGBT has output
characteristics similar to power BJT & input
characteristics similar to power MOSFET.
Principle of operation
The
principle of operation of IGBT is
similar to that of a MOSFET. The only
difference is that there is no “conductivity
modulation” of drift layer in MOSFET.
Therefore
the on state resistance & hence
on state power loss is very high in
MOSFET. In IGBT however the
conductivity modulation takes place &
reduces on state voltage across IGBT.
Application of IGBT
Switching
mode
power
supplies
(SMPS)
UPS
system
AC
motor
control
Chopper
Inverters