UNIT I Power Semiconductor Devices. EE2301-POWER ELECTRONICS Introduction What are Power...

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


Classification

Fig. 1. The power semiconductor devices family


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.


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


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.


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.


Three States:Reverse BlockingForward BlockingForward Conducting

Thyristor: I-V Characteristics

Fig.5 The current-voltage characteristics of the pnpn

device.


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


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.


• 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.


Different types of Thyristors

• Silicon Controlled Rectifier (SCR).

• TRIAC.

• DIAC.

• Gate Turn-Off Thyristor (GTO).


SCR

Symbol of

Silicon Controlled Rectifier


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


Device Operation

Simplified model of a thyristor


V-I Characteristics


Effects of gate current


Two Transistor Model of SCR


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


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


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


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


Turn-on Characteristics

on d rt t t


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


Methods of Thyristor Turn-on

• Thermal Turn-on.

• Light.

• High Voltage.

• Gate Current.

• dv/dt.


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).


• Static induction Thyristors (SITHs).

• Light-activated silicon-controlled rectifiers (LASCRs).

• FET controlled Thyristors (FET-CTHs).

• MOS controlled Thyristors (MCTs).


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.


• They use amplifying gate thyristor.


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.


Bidirectional Triode Thyristors (TRIAC)


Mode-I Operation

MT2 Positive,

Gate Positive

P 1

N 1

N 2

P 2Ig

Ig

M T 2 (+ )

M T 1 ( )G

V(+ )


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


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(+ )


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(- )


Triac Characteristics


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


BJT characteristics


BJT modes of operation

Mode EBJ CBJ

Cutoff Reverse Reverse

Forward active

Forward Reverse

Reverse active

Reverse Forward

Saturation Forward Forward


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

http://en.wikipedia.org/wiki/Proportionality_%28mathematics%29


BJT structure (active)

current of electrons for npn transistor –

conventional current flows from collector to emitter.

BB

CCEE

IIEE IICC

IIBB

--

++

VVBEBE VVCBCB

--

++

++-- VVCECE


• 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


• 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


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)


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


• 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


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 :


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


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


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


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


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.


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.


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


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.


ELECTRICALLY ISOLATED DRIVE CIRCUITS


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.

UNIT I Power Semiconductor Devices. EE2301-POWER ELECTRONICS Introduction What are Power...

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Transcript of UNIT I Power Semiconductor Devices. EE2301-POWER ELECTRONICS Introduction What are Power...