Power Transistor -3.docx

8/14/2019 Power Transistor -3.docx

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Power Converters Power Semiconductor DevicesPower Transistors

Power JT Power TransistorPower Transistor is a controlled Device

A power electronic

device usually has

a third terminal

control terminal

to control the

states of the device

C

G

E

A power electronic device

must have at least two

terminals to allow power

circuit current flow through.

Drive

Circuit

Power TransistorTwo types of transistor are extensively used in power switching circuits:bipolar junction transistor (BJT) and Metal Oxide Semiconductor Field EffectTransistor (MOSFET).The BJT consists of a pnpor npn single-crystal silicon structure.It operates by the injection and collection of minority carriers, both electronsand holes, and is therefore termed a Bipolar Transistor. Power TransistorThe MOSFET depends on the voltage control of a depletion width, it istherefore a Uni-polar Transistor.Unlike the BJT, the MOSFET is a majority carrier device and therefore doesnot exhibit minority carrier storage delays, so switching times of theMOSFETs are ultra fast.

Power JT StructurePower BJT can handle high voltage and large current.


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Power JT doping

A typical high-voltage triple-diffused transistor dopingprofile is shown below

Power JTThe n-collector region is the initial high-resistivity silicon material and thecollector n+ diffusion is performed first, usually into both sides.


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One n+ diffusion is lapped off and the p-base and n+ emitter diffusions aresequentially performed.

Power JTThe n-type collector region is an epitaxial layer grown on an n substrate. Thebase and emitter are sequentially diffused into the epitaxy. This approachallows greater control on the depth of the n-type collector region which isparticularly important in specifying device switching and high-voltageproperties.Also the parasitic series collector resistance of the substrate is minimized

without compromising the pellet's mechanical strength as a result of apossible reduction in thickness.

Power JT OperationA simple and qualitative view of the

operation mechanisms and features of thebipolar power switching transistor is givenbelow.Consider an npn bipolar transistor connectedin the common emitter configuration.

In this configuration, injection of electronsfrom the lower n+p junction into the centre

p-region supplies minority carrier electronsto participate in the reverse current throughthe upper np junction.

The n+ region which serves as the source ofinjected electrons is called the emitter andforms the emitter junction with the p-base,

while the n-region into which electrons are swept by the reverse bias npjunction is called the collector and, with the p-base, forms the collectorjunctionTo have a 'good' npn transistor almost all the electrons injected by the emitterinto the base should be collected.


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Thus the p-base region should benarrow and the electron minoritycarrier lifetime should be long toensure that the average electroninjected at the emitter will diffuse tothe collector sc lwithout recombining inthe base.The average lifetime of electrons in thep-base increases as the p-baseconcentration decreases, that is as thehole concentration decreases.The fraction of electrons which make itacross to the collector is called the basetransport factor, bt.If we neglect the saturation current at the collector, component 5 in figurebelow, and such effects as space charge layer recombination, then ic = btiEn

where iEnis the electron component of the total emitter current ie.

Electrons lost to recombination in the p-base must be re-supplied through thebase contact.It is also required that the emitter junction carrier flow should be composedalmost entirely of electrons injected into the base, rather than holes crossingfrom the base region to the emitter.Any such holes must be provided by the base current, which is minimised by

doping the base region lightly compared with the emitter such that an n+pemitter results.Such a junction is said to have a high injection efficiency.Holes swept into the base at the reverse-biased collector junction because ofthermal generation in the collector must also be accounted for by the basecurrent.This base current component is generally very small in high-voltagetransistors when in the on-state since the collector scl electric field is small.In the common emitter configuration, the ratio between the base current Iband the collector current Icis of practical importance.Since the base current is the difference between the emitter and the collectorcurrent.


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The factor , relating the collectorcurrent to the base current, isdefined as the base-to-collectorcurrent amplification factor.

If is near unity, is very large,implying the base current is verysmall compared with the collectorcurrent.In power switching applications atransistor is controlled in two states:the off-state and the on-state.Ideally the transistor should appearas a short circuit when on and anopen circuit when in the off-state.Furthermore the transition time between these two states is ideally zero. Inreality, transistors only approximate these requirements.The two operational states for the power switching transistor are defined asfollows.Cut-off region:In this region the emitter junction is not injecting; hence onlyleakage current flows.The saturation:In traversing from the off-state to the saturated state thetransistor passes through the linear operating region, where the collector

junction voltage changes from a large reverse bias to a forward bias state.Both junctions are forward-biased, termed saturated, and the collector-to-emitter voltage is almost zero, high current is able to flow.This saturated situation represents the switched-on hard mode, and over-saturation exists.The gain is a minimum in the saturated mode since the neutral base widthbetween the two forward-biased scl sis at a maximum.

Power JT CharacteristicsThe typical BJT collector outputcharacteristics are shown below whichillustrates the various BJT operatingregions.


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Power JT Current GainA number of electrical phenomena are of particular importance to the high-

voltage, power switching BJT .The characteristics to be considered are as a result of the device structure andgeometry.The gain of a power transistor falls off at both very low and very high currentlevels.At low currents the gain decreases as a result of generation recombination.Conductivity ModulationAt high currents, as the concentration of excess electrons in the base becomeslarge, the matching excess hole concentration can become greater than thebase doped level.A balance of holes and electrons must occur in order to maintain a neutralbase region.Thus holes in the base are injected into the emitter, countering the converselyinjected electrons, and thus effectively decreasing the emitter injectionefficiency.This effect is called conductivity modulation.

First reakdownThe collector junction supports the off-state voltage and in so doing developsa wide scl.This sclincreases in width with increased reverse bias, penetrating into thebase.It is unusual that a correctly designed high-voltage power switching BJT wouldbreak down as a result of punch-through of the collector sc lthrough the baseto the emitter sc l.Because of the profile of the diffused base, collector junction voltagebreakdown is usually due to the avalanche multiplication mechanism, createdby the high electric field at the collector junction.In the common emitter configuration, the transistor usually breaks down

gradually, but before the collector junction avalanches.This occurs because the avalanche-generated holes in the collector sc lareswept by the high-field into the base.

The emitter injects electrons in order to maintain base neutrality.This emitter junction in turn causes more collector current, creating moreavalanche pairs and causing a regenerative action.Thus the gain mechanisms of the transistor cause collector emitterbreakdown- first breakdown


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Second reakdown

First breakdown need not be catastrophic provided junction temperaturelimits are not exceeded.If local hot spots occur because of non-uniform current density distribution asa result of crystal faults, doping fluctuation etc., second breakdown occurs.

Silicon crystal melting and irrepairable damage results, the collector voltagefalls and the current increases rapidly as shown in figure below

O

ic

UCE

IB=0

IB5

IB4

IB1

IB2

IB3Active region

Second breakdown

Quasi-saturationHard

saturation 1-

Rd

Primary

breakdown

IB5 >IB4. etc.

IB


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Power MOS Simplified representation of theMOSFET structure.

When positive VGSis applied, aconducting n-channel is formedbeneath the gate in the p-region(body).MOSFET turns on when VGSexceeds VT.The gate acts like a capacitor.The gate power is zero.The gate-drive circuit is very simpleas compared to BJT.

Power MOSIf the MOSFET has to operate at high frequencies, the gate capacitance mustbe charged and discharged quickly, therefore, the gate drive circuit shouldhave low source impedance.

When MOSFET is on it acts like a resistor of Value RDS(on).RDS(on)consists of two parts:Conducting channel resistanceResistance of the extended drain region. Which is unique to Power MOS dueto its vertical structure.

Power MOS StructureThe drain contact is on the bottom of the die, rather than on the top as insignal MOSFET.This vertical structure gives maximum area to both drain and source contacts.


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There are P-wellsbetween drain andsource. These wellsare the body regionsof the device.The channel isformed on the surface

of the p-wells justbeneath the gate-oxide.The p-type wells areshorted to the sourceelectrode.

Creation of channel beneath thegate in the p-regions.

Power MOSThe lightly doped n-type drain regionis unique to Power MOS.It will allow the growth of long SCLto block high voltage when the deviceis off.

Power MOSUnder Reverse iased

Following figure illustrate howthe SCL grows with increasing

VDS , pinching off the n- regionbetween the p-wells.


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Long drain region function like the v-region in a Power BJT.This lightly doped drain region is also referred as extended drain region.

When the device is off, SCL grows and pinches off the region between P-wells.The gate electrode acts as a field plate to promote the depletion of the regionbetween p-wells.The voltage just beneath the gate oxide is only 5-10 volts w.r.t. gate eventhough the drain voltage is 200-400V, as a result the gate oxide can be maderelatively thin keeping VTlow.

If viewed from the top, the gate and source contacts look like interleavedpattern of cells as shown in the figure.These cells can be arranged in a square pattern.For a large power device, the cells can be increased over alarge area of the die.This is similar to connecting the Power MOS transistors inparallel on a single die to increase the conduction regionand power.

The p-type wells can also be arranged in hexagonal cellpattern to increase the gate perimeter for a given die area as

shown in the diagram.

This hexagonal structure is a patent of International Rectifiers(IR) for its trademark HEXFET

On state ResistanceThe drain current distribution when thedevice is on, is shown below.The current flows from drain to source whenon.The current focuses in the area between thep-wells called the neck region.

The current further focuses in the thin entrance tothe p-well channels on either sides of the neckregions.


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The n-region just beneath gate is accumulated which makes it muchmore conductive than rest of the n-region.The total RDS(on) can be divided into four components as shown inthe diagram.For high voltage device, Rxd and Rneck are much larger than Raccumand Rch.For low voltage devices, Rch is to of total RDS(on).

Temperature EffectFor high voltage device, RDS(ON)is dominated by extended drain resistance,the temperature dependence of RDS(ON)is the result of temperaturedependence ofeFor temperature range 0-200oC,edecreases with temperature.

2.2T

e

RDS(ON)increases with temperature.

A 100 degree rise produces an increase of aprox. 90%.ody Diode

The connection of the p-wells to the source metal gives the MOSFET an anti-parallel body diode.This body diode is a PIN diode as discussed earlier.It displays the static and dynamic characteristics of a PIN diode.

The cross sectional area ofMOSFET is approximately same asthe body diode.The body diode displays a reverserecovery phenomenon in the orderof 100nS but this is not as short aspossible in separate diode.

Switching CharacteristicsTurn-on transient Turn-off transient

Turn-on delay time td(on) Turn-off delay time td(off)Rise time tr Falling time tf


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s

GF

L

iD

uGS

upiD

+UE

iD

O

O

O

up

t

t

t

uGSuGSPuT

td(on) tr td (off) tf

Switching PerformanceThe speed at which a Power MOS turns on or off is determined by the rate at which itsparasitic capacitance can be charged or discharged.The more current the gate drive circuit can deliver or sink, the faster the device switch.The gate-drive circuit determines the rise and fall times of the drain current and voltage.For instant, if the gate drive is capable to source 1A, and the gate charge is 40nC then;

nSA

nCtf 40

1

40

Commercial Power MOSFET

part number Rated vag current Qg(typical)RonRated max voltage

IRFZ48

IRF510

APT105M25BN

IRF740

MTM15N40E

APT5025BN

APT1001RBNR

60V

100V

100V

400V

400V

500V

1000V

50A

5.6A

28A

75A

10A

15A

23A

11A

0.0180.54

0.3

1.00.25

0.077

0.550.025

IRF540 100V

110nC

8.3nC

171nC

63nC

110nC

72nC

83nC

150nC

IG TInsulated Gate Bipolar TransistorCombination of MOSFET and Power BJT


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BJT: low conduction losses (especially at larger blocking voltages),

longer switching times, current- driven

MOSFET: faster switching speed, easy to drive (voltage- driven),

larger conduction losses (especially for higher blocking

IGBT

IG TIGBT is an integrated Darlington likeconnection of MOSFET and BJT as shownBelow.

The driving is simple like Power MOS.Low forward drop per unit area of BJT.Much smaller area results compared to samepower MOSFET.The two transistors are of opposite polarity (n-channel and PNP), the gate is driven with respectto collector of BJT, therefore, the collector of BJT isdesignated as Emitter of IGBT and Emitter of BJTas collector of IGBT.

IG T StructureStructure of IG TThe structure of IGBT is very much like vertical MOSFET, except that thesubstrate is heavily doped p-type rather than n-type.Integration of two devicesrather than discrete

connection has an advantage:when the IGBT is on, the BJTis also on conductivitymodulating the drift regionand greatly reducing the drainresistance of MOSFET.If the two devices areconnected discretely, the FETun-modulated resistive drop


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would result in a higher collector-base drop resulting higher VCE for theBipolar Transistor.One disadvantage of integration is that the structure forces the BJT with widebase.Another disadvantage is that the BJT has PNP configuration rather thansuperior NPN transistor.A further problem of integration of the two devices eliminates access to the

base terminal of the BJT, preventing the use of negative base current toimprove turn-off.Turn-off can be improved by reducing the transistor gain but at the expenseof on-state drop.The integration of two devices produces a parasitic SCR as a regenerativeconnection of PNP and NPN transistors.Although the base of NPN transistor is shorted to emitter which should keepthis transistor off, however, there is some resistance in this connection.

Structure and Symbol of IG TStructure of IG TIf during operation, the current through this regionbecomes high, the NPN transistor might be turned on, andSCR may latch.Once latched, nothing can be done to turn off the device.If the rate of rise of voltage at turn off is high enough, thecapacitive charging current could trigger the SCR.Even with these problems IGBT has much to offer.This device is well suited to high voltage with moderatefrequencies (1200 V upto 50KHz.)


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IG TSwitching characteristics

t

t

t

current tail

UGEMUGE

90%UGEM

10%UGEM0

0

0

ICMIC

90%ICM

10%ICM

UCE UCEM

UCEon

ton

tfv1

toff

td(on)

tfv2

tfi1 tfi2

tftr td(off)

Commercial IGBTs


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part number Rated avg current tf(typical)VF(typical

Rated max voltage

Single-chip devices

HGTG32N60E2

multiple-chip power modules_

CM400HA-12E

CM300HA-24E

600V

1200V

600V

1200V

32A

30A

2.4V

3.2VHGTG30N120D2

0.62s0.58s

400A

300A

2.72.7V 0.3 s

0.3 s

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Power Transistor -3.docx