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UNIT I
Power Semiconductor
Devices
Introduction
•
What are Power Semiconductor Devices (PSD)?
They are devices used as switches or rectifiers in
power electronic circuits
•
What is the difference of PSD and low-power
semiconductor device?
 Large voltage in the off state
 High current capability in the on state
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Classification
Fig. 1. The power semiconductor devices family
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Important Parameters
•
•
•
•
Breakdown voltage.
On-resistance.
Trade-off between breakdown voltage and
on-resistance.
Rise and fall times for switching between on
and off states.
Safe-operating area.
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Power MOSFET: Structure
Power MOSFET has much higher current handling capability in
ampere range and drain to source blocking voltage(50-100V)
than other MOSFETs.
Fig.2.Repetitive pattern of the cells
structure in power MOSFET
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Power MOSFET: R-V Characteristics
An important parameter of a power MOSFET is on resistance:
Ron  RS  RCH  RD , where RCH 
L
W nCox (VGS  VT )
Fig. 3. Typical RDS versus ID characteristics of a MOSFET.
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Thyristor: Structure
• Thyristor is a general class of a four-layer pnpn
semiconducting device.
Fig.4 (a) The basic four-layer pnpn structure.
(b) Two two-transistor equivalent circuit.
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Thyristor: I-V Characteristics
Three States:
Reverse Blocking
Forward Blocking
Forward Conducting
Fig.5 The current-voltage
characteristics of the pnpn device.
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Applications
Power semiconductor devices have widespread
applications:
Automotive
Alternator, Regulator, Ignition, stereo tape
Entertainment
Power supplies, stereo, radio and television
Appliance
Drill motors, Blenders, Mixers, Air conditioners
and Heaters
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Thyristors
• Most important type of power
semiconductor device.
• Have the highest power handling
capability.they have a rating of 1200V /
1500A with switching frequencies ranging
from 1KHz to 20KHz.
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• Is inherently a slow switching device
compared to BJT or MOSFET.
• Used as a latching switch that can be
turned on by the control terminal but
cannot be turned off by the gate.
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Different types of Thyristors
•
•
•
•
Silicon Controlled Rectifier (SCR).
TRIAC.
DIAC.
Gate Turn-Off Thyristor (GTO).
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SCR
Symbol of
Silicon Controlled
Rectifier
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Structure
Cathode
Gate
+
n
J3
19
10
-3
+
cm
-
p
n
17
10
cm
-3
J2
–
n
J1
p
+
p
13
10
17
10
19
10
14
-5 x 10
cm
-3
-3
cm
19
10
-3
cm
Anode
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-3
cm


10m
30-100m


50-1000m
30-50m
Device Operation
Simplified model of a
thyristor
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V-I
Characteristics
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Effects of gate current
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Two Transistor Model of SCR

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Considering PNP transistor
of the equivalent circuit,
I E 1  I A , I C  I C1 ,   1 ,
I CBO  I CBO1 , I B  I B1

I B1  I A 1  1   I CBO1    1
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Considering NPN transistor
of the equivalent circuit,
I C  I C2 , I B  I B2 , I E2  I K  I A  I G
I C2   2 I k  I CBO2
I C2   2  I A  I G   I CBO2     2 
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From the equivalent circuit,
we see that
 I C2  I B1
 2 I g  I CBO1  I CBO 2
 IA 
1  1   2 
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Case 1: When I g  0
IA 
I CBO1  I CBO2
1  1   2 
Case 2: When I G  0
 2 I g  I CBO1  ICBO 2
IA 
1  1   2 
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Turn-on
Characteristics
ton  td  tr
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VAK
tC
tq
t
IA
Anode current
begins to
decrease
Commutation
di
dt
Recovery
t1 t2
Recombination
t3
t4
t5
t
tq=device
off time
tc=circuit
off time
tgr
trr
tq
tc
Turn-off
Characteristi
c
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Methods of Thyristor Turn-on
•
•
•
•
•
Thermal Turn-on.
Light.
High Voltage.
Gate Current.
dv/dt.
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Thyristor Types
•
•
•
•
•
Phase-control Thyristors (SCR’s).
Fast-switching Thyristors (SCR’s).
Gate-turn-off Thyristors (GTOs).
Bidirectional triode Thyristors (TRIACs).
Reverse-conducting Thyristors (RCTs).
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• Static induction Thyristors (SITHs).
• Light-activated silicon-controlled rectifiers
(LASCRs).
• FET controlled Thyristors (FET-CTHs).
• MOS controlled Thyristors (MCTs).
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Phase Control Thyristor
• These are converter thyristors.
• The turn-off time tq is in the order of 50 to
100sec.
• Used for low switching frequency.
• Commutation is natural commutation
• On state voltage drop is 1.15V for a 600V
device.
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• They use amplifying gate thyristor.
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Fast Switching
Thyristors
•
•
•
•
Also called inverter thyristors.
Used for high speed switching applications.
Turn-off time tq in the range of 5 to 50sec.
On-state voltage drop of typically 1.7V for
2200A, 1800V thyristor.
• High dv/dt and high di/dt rating.
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Bidirectional Triode
Thyristors (TRIAC)
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Mode-I
Operation
MT (+)
2
P1
N1
P2
Ig
N2
MT1 ()
G
V
(+)
MT2 Positive,
Gate Positive
Ig
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Mode-II
Operation
MT (+)
2
P1
Initial
conduction
Final
conduction
N1
P2
N3
N2
MT1 ()
G
MT2 Positive,
Gate Negative
V
Ig
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Mode-III Operation
MT 2 ()
N4
P1
N1
P2
N2
MT1 (+)
G
(+)
MT2 Negative,
Gate Positive
Ig
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Mode-IV Operation
MT2 ()
N4
P1
N1
N3
P2
MT 1 (+)
G
(-)
MT2 Negative,
Gate Negative
Ig
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Triac Characteristics
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BJT structure
heavily doped ~ 10^15
provides the carriers
lightly doped ~ 10^8
lightly doped ~ 10^6
note: this is a current of electrons (npn case) and so the
conventional current flows from collector to emitter.
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BJT characteristics
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BJT characteristics
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BJT modes of operation
Mode
EBJ
CBJ
Cutoff
Forward
active
Reverse
active
Saturation
Reverse
Forward
Reverse
Reverse
Reverse
Forward
Forward
Forward
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BJT modes of operation
Cutoff: In cutoff, both junctions reverse biased. There is very little current flow, which
corresponds to a logical "off", or an open switch.
Forward-active (or simply, active): The emitter-base junction is forward biased and the
base-collector junction is reverse biased. Most bipolar transistors are designed to afford the
greatest common-emitter current gain, βf in forward-active mode. If this is the case, the
collector-emitter current is approximately proportional to the base current, but many times
larger, for small base current variations.
Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the
forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the
emitter and collector regions switch roles. Since most BJTs are designed to maximise
current gain in forward-active mode, the βf in inverted mode is several times smaller. This
transistor mode is seldom used. The reverse bias breakdown voltage to the base may be an
order of magnitude lower in this region.
Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates
current conduction from the emitter to the collector. This mode corresponds to a logical
"on", or a closed switch.
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BJT structure (active)
current of electrons for npn transistor –
conventional current flows from
collector to emitter.
IE
E
-
IC
VCE +
C
-
+
VBE
VCB
IB
+
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B
MOSFET
• NMOS: N-channel Metal
Oxide Semiconductor
W
• L = channel length
• W = channel width
GATE
L
“Metal” (heavily
doped poly-Si)
DRAIN
SOURCE
• A GATE electrode is placed above (electrically insulated
from) the silicon surface, and is used to control the
resistance between the SOURCE and DRAIN regions
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N-channel MOSFET
Gate
Source
IS
IG
Drain
gate
oxide insulator
n
ID
n
p
• Without a gate-to-source voltage applied, no current can
flow between the source and drain regions.
• Above a certain gate-to-source voltage (threshold
voltage VT), a conducting layer of mobile electrons is
formed at the Si surface beneath the oxide. These
electrons can carry current between the source and drain.
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N-channel vs. P-channel
MOSFETs
NMOS
PMOS
n+ poly-Si
p+ poly-Si
n+
n+
p+
p-type Si
p+
n-type Si
• For current to flow, VGS > VT
• For current to flow, VGS < VT
• Enhancement mode: VT > 0
• Enhancement mode: VT < 0
• Depletion mode: VT < 0
• Depletion mode: VT > 0
– Transistor is ON when VG=0V
– Transistor is ON when VG=0V
(“n+” denotes very heavily doped n-type material; “p+” denotes very heavily doped p-type material)
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MOSFET Circuit Symbols
G
NMOS
G
n+ poly-Si
n+
n+
S
S
p-type Si
PMOS
Body
G
G
p+ poly-Si
p+
p+
S
n-type Si
Body
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S
MOSFET Terminals
• The voltage applied to the GATE terminal determines whether
current can flow between the SOURCE & DRAIN terminals.
– For an n-channel MOSFET, the SOURCE is biased at a lower
potential (often 0 V) than the DRAIN
(Electrons flow from SOURCE to DRAIN when VG > VT)
– For a p-channel MOSFET, the SOURCE is biased at a higher
potential (often the supply voltage VDD) than the DRAIN
(Holes flow from SOURCE to DRAIN when VG < VT )
• The BODY terminal is usually connected to a fixed potential.
– For an n-channel MOSFET, the BODY is connected to 0 V
– For a p-channel MOSFET, the BODY is connected to VDD
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NMOSFET IG vs. VGS Characteristic
Consider the current IG (flowing into G) versus VGS :
IG
G
S
D
oxide
semiconductor
VGS +

IG
VDS
+

The gate is insulated from the
semiconductor, so there is no
significant steady gate current.
always zero!
VGS
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NMOSFET ID vs. VDS Characteristics
Next consider ID (flowing into D) versus VDS, as VGS is varied:
S
VGS +

ID
G
D
oxide
semiconductor
ID
VGS > VT
zero if VGS < VT
VDS
VDS
+

Above threshold (VGS > VT):
“inversion layer” of electrons
appears, so conduction
between S and D is possible
Below “threshold” (VGS < VT):
no charge  no conduction
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The MOSFET as a Controlled Resistor
• The MOSFET behaves as a resistor when VDS is low:
– Drain current ID increases linearly with VDS
– Resistance RDS between SOURCE & DRAIN depends on VGS
• RDS is lowered as VGS increases above VT
oxide thickness  tox
NMOSFET Example:
ID
VGS = 2 V
VGS = 1 V > VT
VDS
IDS = 0 if VGS < VT
Inversion charge density Qi(x) = -Cox[VGS-VT-V(x)]
where Cox  eox / tox
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ID vs. VDS Characteristics
The MOSFET ID-VDS curve consists of two regions:
1) Resistive or “Triode” Region: 0 < VDS < VGS  VT
VDS 
W 
ID
VGS  VT 
VDS


L 
2 
where k n   n Cox
 k n
process transconductance parameter
2) Saturation Region:
VDS > VGS  VT
kn W
VGS  VT 2
I DSAT 
2 L
where kn   nCox
“CUTOFF” region: VG < VT
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The Evolution Of IGBT
Part I: Bipolar Power Transistors
•
Bipolar Power Transistor Uses Vertical Structure For
Maximizing Cross Sectional Area Rather Than Using Planar
Structure
Base
Emitter
Collector
N+
P
Base
N+
N-
Emitter
Collector
The Evolution Of IGBT
Part II:Power MOSFET
•
Power MOSFET Uses Vertical Channel Structure Versus
The Lateral Channel Devices Used In IC Technology
Gate
Source
Drain
SiO2
n+
n+
P
P
n-
Gate
n-
Source
Drain
Lateral MOSFET structure
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The Evolution Of IGBT
Part III: BJT(discrete) + Power MOSFET(discrete)
•
Discrete BJT + Discrete Power MOSFET In Darlington
Configuration
C
N-MOSFET
D
S
NPN
B
E
G
The Evolution Of IGBT
Part IV: BJT(physics) + Power MOSFET(physics) = IGBT
•
More Powerful And Innovative Approach Is To Combine
Physics Of BJT With The Physics Of MOSFET Within Same
Semiconductor Region
•
This Approach Is Also Termed Functional Integration Of
MOS And Bipolar Physics
•
Using This Concept, The Insulated Gate Bipolar Transistor
(IGBT) Emerged
•
Superior On-State Characteristics, Reasonable Switching
Speed And Excellent Safe Operating Area
The Evolution Of IGBT
Part IV: BJT(physics) + Power MOSFET(physics) = IGBT
•
IGBT Fabricated Using Vertical Channels (Similar To Both
The Power BJT And MOSFET)
E
Gate
Emitter
n+
p - base
p+
NPN
n- - drift
N-MOSFET
PNP
p+ - substrate
Collector
C
G
Device Operation
•
Operation Of IGBT Can Be Considered Like A PNP
Transistor With Base Drive Current Supplied By The
MOSFET
DRIVER CIRCUIT (BASE / GATE)
• Interface between control (low power electronics) and (high power) switch.
• Functions:
– amplifies control signal to a level required to drive power switch
– provides electrical isolation between power switch and logic level
• Complexity of driver varies markedly among switches. MOSFET/IGBT drivers
are simple but GTO drivers are very complicated and expensive.
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ELECTRICAL ISOLATION FOR DRIVERS
• Isolation is required to prevent damages on
the high power switch to propagate back to
low power electronics.
• Normally opto-coupler (shown below) or high
frequency magnetic materials (as shown in
the thyristor case) are used.
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ELECTRICAL ISOLATION FOR DRIVERS
•
Power semiconductor devices can be categorized into 3
types based on their control input requirements:
a) Current-driven devices – BJTs, MDs, GTOs
b) Voltage-driven devices – MOSFETs, IGBTs, MCTs
c) Pulse-driven devices – SCRs, TRIACs
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CURRENT DRIVEN DEVICES (BJT)
• Power BJT devices have low current gain due to
constructional consideration, leading current than would
normally be expected for a given load or collector current.
• The main problem with this circuit is the slow turn-off time.
Many standard driver chips have built-in isolation. For
example TLP 250 from Toshiba, HP 3150 from HewlettPackard uses opto-coupling isolation.
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ELECTRICALLY ISOLATED DRIVE CIRCUITS
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EXAMPLE: SIMPLE MOSFET GATE DRIVER
• Note: MOSFET requires VGS =+15V for turn on and 0V to
turn off. LM311 is a simple amp with open collector
output Q1.
• When B1 is high, Q1 conducts. VGS is pulled to ground.
MOSFET is off.
• When B1 is low, Q1 will be off. VGS is pulled to VGG. If
VGG is set to +15V, the MOSFET turns on.
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