Post on 25-Dec-2015
UNIT I
Power Semiconductor Devices
EE2301-POWER ELECTRONICS
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 CharacteristicsAn important parameter of a power MOSFET is on resistance:
, whereon S CH DR R R R ( )CH
n ox GS T
LR
W C V V
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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Three States:Reverse BlockingForward BlockingForward Conducting
Thyristor: I-V Characteristics
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 tapeEntertainment Power supplies, stereo, radio and televisionAppliance 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
G ate Cathode
J 3
J 2
J 1
Anode
10 cm17 - 3
10 - 5 x 10 cm13 14 - 3
10 cm17 - 3
10 cm19 - 3
10 cm19 - 3
10 cm19 - 3
n+
n+
p-
n–
p
p+
10 m
30- 100 m
50- 1000 m
30- 50 m
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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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1 1
1 1
1 1
1
1
Considering PNP transistor
of the equivalent circuit,
, , ,
,
1 1
E A C C
CBO CBO B B
B A CBO
I I I I
I I I I
I I I
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2 2 2
2 2
2 2
2
2
Considering NPN transistor
of the equivalent circuit,
, ,
2
C C B B E K A G
C k CBO
C A G CBO
I I I I I I I I
I I I
I I I I
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2 1
2 1 2
1 2
From the equivalent circuit,
we see that
1
C B
g CBO CBOA
I I
I I II
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1 2
1 2
Case 1: When 0
1
g
CBO CBOA
I
I II
2 1 2
1 2
Case 2: When 0
1
G
g CBO CBOA
I
I I II
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Turn-on Characteristics
on d rt t t
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Turn-off Characteristi
c
Anode currentbegins todecrease
tC
tq
t
t
Commutationdidt
Recovery Recombination
t1 t2 t3 t4 t5
tr r tgr
tq
tc
V A K
I A
tq=device off tim e
tc=circuit off tim e
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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 100sec.
• 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 50sec.
• 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
MT2 Positive,
Gate Positive
P 1
N 1
N 2
P 2Ig
Ig
M T 2 (+ )
M T 1 ( )G
V(+ )
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Mode-II Operation
MT2 Positive,
Gate Negative
P 1
N 1
N 2N 3
P 2
Ig
M T 2 (+ )
M T 1 ( )G
V
F in a lcon d u ctio n
In itia lcon d u ctio n
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Mode-III Operation
MT2 Negative,
Gate Positive
P 1
N 1
N 4
N 2
P 2
Ig
M T 2 ( )
M T 1 (+ )G(+ )
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Mode-IV Operation
MT2 Negative,
Gate Negative
P 1
N 1
N 4
P 2
Ig
M T 2 ( )
M T 1 (+ )
N 3
G(- )
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Triac Characteristics
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BJT structure
note: this is a current of electrons (npn case) and so theconventional current flows from collector to emitter.
heavily doped ~ 10^15provides the carriers
lightly doped ~ 10^8 lightly doped ~ 10^6
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BJT characteristics
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BJT characteristics
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BJT modes of operation
Mode EBJ CBJ
Cutoff Reverse Reverse
Forward active
Forward Reverse
Reverse active
Reverse Forward
Saturation Forward Forward
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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.
BJT modes of operation
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BJT structure (active)
current of electrons for npn transistor –
conventional current flows from collector to emitter.
BB
CCEE
IIEE IICC
IIBB
--
++
VVBEBE VVCBCB
--
++
++-- VVCECE
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• 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
• NMOS: N-channel Metal Oxide Semiconductor
np-type silicon
oxide insulator n
L
• L = channel length
“Metal” (heavily doped poly-Si)
W• W = channel width
MOSFET
SOURCE
DRAIN
GATE
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• 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.
N-channel MOSFET
n
p
oxide insulatorgate
n
DrainSource
Gate
ID
IG
IS
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N-channel vs. P-channel MOSFETs
• For current to flow, VGS > VT
• Enhancement mode: VT > 0
• Depletion mode: VT < 0
– Transistor is ON when VG=0V
p-type Si
n+ poly-Si
n-type Si
p+ poly-Si
NMOS PMOS
n+ n+ p+ p+
• For current to flow, VGS < VT
• Enhancement mode: VT < 0
• Depletion mode: VT > 0
– 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
p-type Si
n+ poly-Si
NMOS
n+ n+
n-type Si
p+ poly-Si
PMOS
p+ p+
G G
G G
S
SS
S
Body
Body
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• 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
MOSFET Terminals
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VGS
S
semiconductoroxide
G
VDS
+ +
D
always zero!
IG
VGS
The gate is insulated from the semiconductor, so there is no significant steady gate current.
IG
NMOSFET IG vs. VGS Characteristic
Consider the current IG (flowing into G) versus VGS :
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VGS
S
semiconductoroxide
G
VDS
ID
+ +
D
ID
zero if VGS < VT
VDS
Next consider ID (flowing into D) versus VDS, as VGS is varied:
Below “threshold” (VGS < VT): no charge no conduction
Above threshold (VGS > VT): “inversion layer” of electrons appears, so conduction between S and D is possible
VGS > VT
NMOSFET ID vs. VDS Characteristics
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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
NMOSFET Example:ID
IDS = 0 if VGS < VT
VDS
VGS = 1 V > VT
VGS = 2 V
Inversion charge density Qi(x) = -Cox[VGS-VT-V(x)]where Cox ox / tox
oxide thickness 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
2) Saturation Region: VDS > VGS VT
oxnn
TGSn
DSAT
Ck
VVL
WkI
where
2
2
oxnn
DSDS
TGSnD
Ck
VV
VVL
WkI
where
2
process transconductance parameter
“CUTOFF” region: VG < VT
Part I: Bipolar Power TransistorsThe Evolution Of IGBT
• Bipolar Power Transistor Uses Vertical Structure For Maximizing Cross Sectional Area Rather Than Using Planar Structure
EmitterBase
Collector
P
N+
N-
N+
Collector
Base
Emitter
Part II:Power MOSFETThe Evolution Of IGBT
• Power MOSFET Uses Vertical Channel Structure Versus The Lateral Channel Devices Used In IC Technology
n+
P
n-
P
n-
n+
SiO2
Gate
Source
Drain
Gate
Source
Drain
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Lateral MOSFET structure
The Evolution Of IGBT
• Discrete BJT + Discrete Power MOSFET In Darlington Configuration
E
NPN
N-MOSFET
G
B
S
D
C
Part III: BJT(discrete) + Power MOSFET(discrete)
Part IV: BJT(physics) + Power MOSFET(physics) = IGBTThe Evolution Of 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
• IGBT Fabricated Using Vertical Channels (Similar To Both The Power BJT And MOSFET)
n+
n- - drift
p+
p - base
p+ - substrate
Emitter Gate
Collector
E
PNP
NPN
N-MOSFET
G
C
Part IV: BJT(physics) + Power MOSFET(physics) = IGBT
Device Operation• Operation Of IGBT Can Be Considered Like A PNP Transistor With Base Drive Current Supplied By The MOSFET
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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 Hewlett-Packard 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.