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Power MOSFET BasicsBy Vrej Barkhordarian, International Rectifier, El Segundo, Ca.

Breakdown Voltage.........................................

On-resistance..................................................

Transconductance............................................

Threshold Voltage...........................................

Diode Forward Voltage..................................

Power Dissipation...........................................

Dynamic Characteristics................................

Gate Charge....................................................

dV/dt Capability...............................................

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Power MOSFET BasicsVrej Barkhordarian, International Rectifier, El Segundo, Ca.

Discrete power MOSFETsemploy semiconductorprocessing techniques that aresimilar to those of today's VLSIcircuits, although the devicegeometry, voltage and currentlevels are significantly differentfrom the design used in VLSIdevices. The metal oxidesemiconductor field effecttransistor (MOSFET) is basedon the original field-effecttransistor introduced in the70s. Figure 1 shows thedevice schematic, transfercharacteristics and devicesymbol for a MOSFET. Theinvention of the powerMOSFET was partly driven bythe limitations of bipolar powerjunction transistors (BJTs)which, until recently, was thedevice of choice in powerelectronics applications.

Although it is not possible todefine absolutely the operatingboundaries of a power device,we will loosely refer to thepower device as any devicethat can switch at least 1A.The bipolar power transistor isa current controlled device. Alarge base drive current ashigh as one-fifth of thecollector current is required tokeep the device in the ONstate.

Also, higher reverse base drivecurrents are required to obtainfast turn-off. Despite the very advanced state of manufacturability and lower costs of BJTs, theselimitations have made the base drive circuit design more complicated and hence more expensive than thepower MOSFET.

SourceContact

FieldOxide

GateOxide

GateMetallization

DrainContact

n* Drain

p-Substrate

Channel

n* Source tox

l

VGSVT00

ID

(a)

(b)

ID

D

SB(Channel or Substrate)

S

G

(c)

Figure 1. Power MOSFET (a) Schematic, (b) Transfer Characteristics, (c)Device Symbol.

Another BJT limitation is that both electrons and holescontribute to conduction. Presence of holes with their highercarrier lifetime causes the switching speed to be several orders ofmagnitude slower than for a power MOSFET of similar size andvoltage rating. Also, BJTs suffer from thermal runaway. Theirforward voltage drop decreases with increasing temperaturecausing diversion of current to a single device when severaldevices are paralleled. Power MOSFETs, on the other hand, aremajority carrier devices with no minority carrier injection. Theyare superior to the BJTs in high frequency applications whereswitching power losses are important. Plus, they can withstandsimultaneous application of high current and voltage withoutundergoing destructive failure due to second breakdown. PowerMOSFETs can also be paralleled easily because the forwardvoltage drop increases with increasing temperature, ensuring an even distribution of current among allcomponents.

However, at high breakdown voltages (>200V) the on-state voltage drop of the power MOSFET becomeshigher than that of a similar size bipolar device with similar voltage rating. This makes it more attractiveto use the bipolar power transistor at the expense of worse high frequency performance. Figure 2 showsthe present current-voltage limitations of power MOSFETs and BJTs. Over time, new materials,structures and processing techniques are expected to raise these limits.

2000

1500

1000

500

01 10 100 1000

Maximum Current (A)

Hol

doff

Vol

tage

(V

)

BipolarTransistors

MOS

Figure 2 . Current-VoltageLimitations of MOSFETs and BJTs.

DrainMetallization Drain

n+ Substrate(100)

n- Epi Layer

Channelsn+pn+

p+ Body Region p +

Drift Region

G

S

D

Source GateOxide

PolysiliconGate

SourceMetallization

Figure 3. Schematic Diagram for an n-Channel Power MOSFET and the Device.

Figure 3 shows schematic diagram and Figure 4 shows the physical origin of the parasitic components inan n-channel power MOSFET. The parasitic JFET appearing between the two body implants restrictscurrent flow when the depletion widths of the two adjacent body diodes extend into the drift region withincreasing drain voltage. The parasitic BJT can make the device susceptible to unwanted device turn-onand premature breakdown. The base resistance RB must be minimized through careful design of thedoping and distance under the source region. There are several parasitic capacitances associated withthe power MOSFET as shown in Figure 3.CGS is the capacitance due to the overlap of the source and the channel regions by the polysilicon gateand is independent of applied voltage. CGD consists of two parts, the first is the capacitance associatedwith the overlap of the polysilicon gate and the silicon underneath in the JFET region. The second part isthe capacitance associated with the depletion region immediately under the gate. CGD is a nonlinearfunction of voltage. Finally, CDS, the capacitance associated with the body-drift diode, varies inverselywith the square root of the drain-source bias. There are currently two designs of power MOSFETs, usuallyreferred to as the planar and the trench designs. The planar design has already been introduced in theschematic of Figure 3. Two variations of the trench power MOSFET are shown Figure 5. The trenchtechnology has the advantage of higher cell density but is more difficult to manufacture than the planardevice.

Metal

CGS2

CgsmLTO

CGD

RChCGS1

RB BJT

n-

p-

CDS

JFET

REPI

n-

n- Epi Layer

n- Substrate

Figure 4. Power MOSFET Parasitic Components.

BREAKDOWN VOLTAGE

Breakdown voltage,BVDSS, is the voltage atwhich the reverse-biasedbody-drift diode breaksdown and significantcurrent starts to flowbetween the source anddrain by the avalanchemultiplication process,while the gate andsource are shortedtogether. Current-voltagecharacteristics of apower MOSFET areshown in Figure 6.BVDSS is normallymeasured at 250µA draincurrent. For drainvoltages below BVDSS

and with no bias on thegate, no channel isformed under the gate atthe surface and the drainvoltage is entirelysupported by thereverse-biased body-driftp-n junction. Two relatedphenomena can occur inpoorly designed andprocessed devices:punch-through andreach-through. Punch-through is observedwhen the depletionregion on the source sideof the body-drift p-njunction reaches thesource region at drainvoltages below the ratedavalanche voltage of thedevice. This provides acurrent path betweensource and drain andcauses a soft breakdowncharacteristics as shownin Figure 7. The leakagecurrent flowing betweensource and drain is denoted by IDSS. There are tradeoffs to be made between RDS(on) that requires shorterchannel lengths and punch-through avoidance that requires longer channel lengths.

The reach-through phenomenon occurs when the depletion region on the drift side of the body-drift p-njunction reaches the epilayer-substrate interface before avalanching takes place in the epi. Once thedepletion edge enters the high carrier concentration substrate, a further increase in drain voltage willcause the electric field to quickly reach the critical value of 2x105 V/cm where avalanching begins.

Source

Gate

Source

GateOxide

Channel

Oxide

n- Epi Layer

n+ Substrate(100)

Drain

(b)

G SS

Electron Flow

D

(a)

Figure 5. Trench MOSFET (a) Current Crowding in V-Groove Trench MOSFET,(b) Truncated V-Groove MOSFET

ON-RESISTANCE

The on-state resistance of a power MOSFET is made up of several components as shown in Figure 8:

(1)

where:

Rsource = Source diffusion resistanceRch = Channel resistanceRA = Accumulation resistanceRJ = "JFET" component-resistance of theregion between the two body regionsRD = Drift region resistanceRsub = Substrate resistance

Wafers with substrate resistivities of up to20mΩ-cm are used for high voltagedevices and less than 5mΩ-cm for lowvoltage devices.

Rwcml = Sum of Bond Wire resistance, theContact resistance between the sourceand drain Metallization and the silicon,metallization and Leadframecontributions. These are normallynegligible in high voltage devices but canbecome significant in low voltage devices.

Figure 9 shows the relative importance ofeach of the components to RDS(on) over thevoltage spectrum. As can be seen, at highvoltages the RDS(on) is dominated by epiresistance and JFET component. Thiscomponent is higher in high voltagedevices due to the higher resistivity orlower background carrier concentration inthe epi. At lower voltages, the RDS(on) isdominated by the channel resistance andthe contributions from the metal tosemiconductor contact, metallization,bond wires and leadframe. The substrate contribution becomes more significant for lower breakdownvoltage devices.

TRANSCONDUCTANCE

Transconductance, gfs, is a measure of the sensitivity of drain current to changes in gate-source bias.This parameter is normally quoted for a Vgs that gives a drain current equal to about one half of themaximum current rating value and for a VDS that ensures operation in the constant current region.Transconductance is influenced by gate width, which increases in proportion to the active area as celldensity increases. Cell density has increased over the years from around half a million per square inch in1980 to around eight million for planar MOSFETs and around 12 million for the trench technology. Thelimiting factor for even higher cell densities is the photolithography process control and resolution thatallows contacts to be made to the source metallization in the center of the cells.

R R R R R R R RDS(on source ch A J D sub wcml) = + + + + + +

GateVoltage

7

6

5

4

IDS VS VDS LOCUS

3

2

1

0 5 10 150

5

10

15

20

25

(Sat

urat

ion

Reg

ion)

Line

ar R

egio

n

Nor

mal

ized

Dra

in C

urre

nt

Drain Voltage (Volts)

Figure 6. Current-Voltage Characteristics of Power MOSFET

Channel length also affects transconductance. Reducedchannel length is beneficial to both gfs and on-resistance,with punch-through as a tradeoff. The lower limit of thislength is set by the ability to control the double-diffusionprocess and is around 1-2mm today. Finally the lower thegate oxide thickness the higher gfs.

THRESHOLD VOLTAGE

Threshold voltage, Vth, is defined as the minimum gateelectrode bias required to strongly invert the surfaceunder the poly and form a conducting channel betweenthe source and the drain regions. Vth is usually measuredat a drain-source current of 250µA. Common values are2-4V for high voltage devices with thicker gate oxides, and1-2V for lower voltage, logic-compatible devices withthinner gate oxides. With power MOSFETs finding increasing use in portable electronics and wirelesscommunications where battery power is at a premium, the trend is toward lower values of RDS(on) andVth.

DIODE FORWARD VOLTAGE

The diode forward voltage, VF, is theguaranteed maximum forward drop ofthe body-drain diode at a specifiedvalue of source current. Figure 10shows a typical I-V characteristics forthis diode at two temperatures. P-channel devices have a higher VF dueto the higher contact resistancebetween metal and p-siliconcompared with n-type silicon.Maximum values of 1.6V for highvoltage devices (>100V) and 1.0V forlow voltage devices (<100V) arecommon.

POWER DISSIPATION

The maximum allowable powerdissipation that will raise the dietemperature to the maximumallowable when the case temperatureis held at 250C is important. It is giveby Pd where:

Tjmax = Maximum allowable temperature of the p-n junction in the device (normally 1500C or 1750C) RthJC

= Junction-to-case thermal impedance of the device.

DYNAMIC CHARACTERISTICS

Sharp

Soft

ID

BVDSS VDS

Figure 7. Power MOSFET BreakdownCharacteristics

N+

P-BASERSOURCERCH

RA

RJ

RD

RSUB

N+ SUBSTRATE

SOURCE

GATE

DRAIN

Figure 8. Origin of Internal Resistance in a Power MOSFET.

PdT j

R thJC=

-m ax 25(2)

When the MOSFET is used as a switch, its basic function is to control the drain current by the gatevoltage. Figure 11(a) shows the transfer characteristics and Figure 11(b) is an equivalent circuit modeloften used for the analysis of MOSFET switching performance.

The switching performance of a device is determined by the time required to establish voltage changesacross capacitances. RG is the distributed resistance of the gate and is approximately inverselyproportional to active area. LS and LD are source and drain lead inductances and are around a few tens ofnH. Typical values of input (Ciss), output (Coss) and reverse transfer (Crss) capacitances given in the datasheets are used by circuit designers as a starting point in determining circuit component values. The datasheet capacitances are defined in terms of the equivalent circuit capacitances as:

50V 100V 500VVoltage Rating:

Packaging

Metallization

Source

Channel

JFETRegion

ExpitaxialLayer

Substrate

REPI

RCH

Rwcml

Figure 9. Relative Contributions to RDS(on) With Different Voltage Ratings.

Ciss = CGS + CGD, CDS shorted

Crss = CGD

Coss = CDS + CGD

Gate-to-drain capacitance, CGD, is anonlinear function of voltage and is the mostimportant parameter because it provides afeedback loop between the output and theinput of the circuit. CGD is also called theMiller capacitance because it causes the totaldynamic input capacitance to become greaterthan the sum of the static capacitances.

Figure 12 shows a typical switching time testcircuit. Also shown are the components ofthe rise and fall times with reference to theVGS and VDS waveforms.

Turn-on delay, td(on), is the time taken tocharge the input capacitance of the devicebefore drain current conduction can start.Similarly, turn-off delay, td(off), is the timetaken to discharge the capacitance after the after is switched off.

0.0 0.5 1.0 1.5 2.0 2.50.1

1

10

100

TJ = 1500C

TJ = 250C

VGS = 0V

VSD, Source-to-Drain Voltage (V)I S

D, R

ever

se D

rain

Cur

rent

(A

)

Figure 10. Typical Source-Drain (Body) Diode ForwardVoltage Characteristics.

ID

VGS

Slope = g fs

G RG

CGD

LD

D

D'

S'

CDS

LS

S

CGS

C IDBody-drain

Diode

(a) (b)

Figure 11. Power MOSFET (a) Transfer characteristics, (b) Equivalent Circuit Showing Components ThatHave Greatest Effect on Switching

GATE CHARGE

Although input capacitancevalues are useful, they do notprovide accurate results whencomparing the switchingperformances of two devicesfrom different manufacturers.Effects of device size andtransconductance make suchcomparisons more difficult. Amore useful parameter from thecircuit design point of view isthe gate charge rather thancapacitance. Mostmanufacturers include bothparameters on their data sheets.Figure 13 shows a typical gatecharge waveform and the testcircuit. When the gate isconnected to the supply voltage,VGS starts to increase until itreaches Vth, at which point thedrain current starts to flow andthe CGS starts to charge. Duringthe period t1 to t2, CGS

continues to charge, the gatevoltage continues to rise anddrain current risesproportionally. At time t2, CGS

is completely charged and thedrain current reaches thepredetermined current ID andstays constant while the drainvoltage starts to fall. Withreference to the equivalentcircuit model of the MOSFET shown in Figure 13, it can be seen that with CGS fully charged at t2, VGS

becomes constant and the drive current starts to charge the Miller capacitance, CDG. This continuesuntil time t3.

RD

-

+

VDD

VDS

VGS

RG

D.U.T.

-10VPulse Width < 1µµsDuty Factor < 0.1%

(a)

Figure 12. Switching Time Test (a) Circuit, (b) VGS and VDSWaveforms

td(on) tr td(off) tf

VGS

100%

90%

VDS

(b)

Charge time for the Miller capacitance islarger than that for the gate to sourcecapacitance CGS due to the rapidly changingdrain voltage between t2 and t3 (current = Cdv/dt). Once both of the capacitances CGS

and CGD are fully charged, gate voltage (VGS)starts increasing again until it reaches thesupply voltage at time t4. The gate charge(QGS + QGD) corresponding to time t3 is thebare minimum charge required to switchthe device on. Good circuit design practicedictates the use of a higher gate voltagethan the bare minimum required forswitching and therefore the gate chargeused in the calculations is QG

corresponding to t4.

The advantage of using gate charge is thatthe designer can easily calculate theamount of current required from the drivecircuit to switch the device on in a desiredlength of time because Q = CV and I = Cdv/dt, the Q = Time x current. Forexample, a device with a gate charge of20nC can be turned on in 20µsec if 1ma issupplied to the gate or it can turn on in20nsec if the gate current is increased to1A. These simple calculations would nothave been possible with input capacitancevalues.

dv/dt CAPABILITY

Peak diode recovery is defined as themaximum rate of rise of drain-sourcevoltage allowed, i.e., dv/dt capability. If thisrate is exceeded then the voltage across thegate-source terminals may become higherthan the threshold voltage of the device,forcing the device into current conductionmode, and under certain conditions acatastrophic failure may occur. There are two possible mechanisms by which a dv/dt induced turn-onmay take place. Figure 14 shows the equivalent circuit model of a power MOSFET, including theparasitic BJT. The first mechanism of dv/dt induced turn-on becomes active through the feedback actionof the gate-drain capacitance, CGD. When a voltage ramp appears across the drain and source terminalof the device a current I1 flows through the gate resistance, RG, by means of the gate-drain capacitance,CGD. RG is the total gate resistance in the circuit and the voltage drop across it is given by:

(3)

When the gate voltage VGS exceeds the threshold voltage of the device Vth, the device is forced intoconduction. The dv/dt capability for this mechanism is thus set by:

VDD

DID

D

G

SCGS

CDG

SID

TEST CIRCUIT(a)

OGS OGD

GATEVOLTAGE

VG

VG(TH)

t0 t1 t2 t3 t4t

DRAIN CURRENT

DRAINVOLTAGE

VDDID

WAVEFORM

(b)

Figure 13. Gate Charge Test (a) Circuit, (b) Resulting Gateand Drain Waveforms.

V I R R Cdv

dtGS G G GD= =1

(4)

It is clear that low Vth

devices are more prone todv/dt turn-on. Thenegative temperaturecoefficient of Vth is ofspecial importance inapplications where hightemperature environmentsare present. Also gatecircuit impedance has to bechooses carefully to avoidthis effect.The second mechanism forthe dv/dt turn-on inMOSFETs is through theparasitic BJT as shown inFigure 15. The capacitanceassociated with thedepletion region of the bodydiode extending into thedrift region is denoted asCDB and appears betweenthe base of the BJT and the drain of the MOSFET. This capacitance gives rise to a current I2 to flowthrough the base resistance RB when a voltage ramp appears across the drain-source terminals. Withanalogy to the first mechanism, the dv/dt capability of this mechanism is:

(5)

If the voltage that develops across RB is greaterthan about 0.7V, then the base-emitter junctionis forward-biased and the parasitic BJT isturned on. Under the conditions of high (dv/dt)and large values of RB, the breakdown voltage ofthe MOSFET will be limited to that of the open-base breakdown voltage of the BJT. If theapplied drain voltage is greater than the open-base breakdown voltage, then the MOSFET willenter avalanche and may be destroyed if thecurrent is not limited externally.Increasing (dv/dt) capability therefore requiresreducing the base resistance RB by increasingthe body region doping and reducing thedistance current I2 has to flow laterally before itis collected by the source metallization. As inthe first mode, the BJT related dv/dt capabilitybecomes worse at higher temperatures becauseRB increases and VBE decreases with increasingtemperature.

dv

dt

V

R Cth

G GD=

DRAIN

APPLIEDRAMP

VOLTAGE

NPNBIPOLAR

TRANSISTOR

CDB

RB

I2

D

S

SOURCE

CGS

RG

G

CGD

I1

Figure 14. Equivalent Circuit of Power MOSFET Showing Two PossibleMechanisms for dv/dt Induced Turn-on.

GATESOURCE

N+ A

LN+

RS

CDS

DRAIN

Figure 15. Physical Origin of the Parasitic BJTComponents That May Cause dv/dt Induced Turn-on

dv

dt

V

R CBE

B DB=

References:

"HEXFET Power MOSFET Designer's Manual - Application Notes and Reliability Data," InternationalRectifier"Modern Power Devices," B. Jayant Baliga"Physics of Semiconductor Devices," S. M. Sze"Power FETs and Their Applications," Edwin S. Oxner"Power MOSFETs - Theory and Applications," Duncan A. Grant and John Gower