1.5.1 High-Voltage Power MOSFEt
In.certain.applications,.minimizing.the.volume,.weight,.and.of.course.the.cost.of.transformers.and.
other.inductive.devices.is.aimed.at,.which.is.done.by.increasing.the.operation.or.switching.frequency.of.
power.devices..SMPS,.as.used.in.many.consumer.or.information.technology.appliances,.are.one.exam-ple.for.these.applications..Here,.switching.frequencies.between.30.and.300.kHz.are.common.today.with.
a.blocking.capability.of.the.switches.between.200.and.1000.V..Operation.of.IGBTs,.for.example,.at.these.
frequencies.is.possible,.but.result.in.rather.high.dynamic.losses..Power.MOSFETs.have.higher..on-state.
resistance.compared.to.IGBTs.with.the.same.chip.area,.but.they.also.have.no.charge.plasma.resulting.in.
much.lower.turn-off.losses..On.the.other.hand,.the.on-state.resistance.per.chip.area.increases.with.the.
blocking.capability.as.sketched.in.Figure.1.21.
The.reason.behind.this.is.that.higher.blocking.voltages.require.thicker.voltage-sustaining.layers.with.
lower.doping..Since.the.load.current.flows.directly.through.this.voltage-sustaining.layer,.it.forms.a.series.
resistance.that—at.least.for.high-voltage.power.MOSFETs.above.200.V—dominates.by.far.the.overall.
resistance.of.the.device.
Selecting.a.500.V.MOSFET,.for.example,.with.low.on-state.resistance.for.a.given.application,.would.
result. in. a. device. with. a. correspondingly. large. and. expensive. chip. area.. A. large. chip. area. exhibits.
large.stray.capacities,.which.have.to.be.charged.and.discharged.during.each.turn-on.and.turn-off.of.
the.device,.resulting.in.comparative.slow.switching.transients..These.slow.switching.transients.help.
to.reduce.electromagnetic.interference.(EMI).and.ringing,.mainly.in.disadvantageous.layouts..But.on.
3
Gate
Ch1 200 V Ch2 400 V M 1.00 μs Ch3 500 mV 5.00 V
Ch3
C3 Max 10.0 V
C2 Max 2.400 kV
C1 Max 1.224 kV
C1 High 1.224 kV
1+ I
Voltage
Current
FIGURE.1.20. Overcurrent.(twice.the.rated.current).turn-off.behavior.of.a.1200.A.1200.V.module.with.a.DC.link.volt-age.of.900.V,.an.estimated.worst-case.stray.inductance.of.400.nH.at.an.ambient.temperature.of.125°C..(Data.from.Laska,.T..
et.al.,.Field.stop.IGBTS.with.dynamic.clamping.capability—A.new.degree.of.freedom.for.future.inverter.designs,.EPE.
2005,.11th European Conference on Power and Electronics Applications,.Dresden,.Germany,.CD-ROM,.2005.)
the.other.hand,.slow.switching.transients.waste.switching.energy.and,.due.to.the.rather.high.switch-ing..frequency,.impair.the.efficiency.of.a.design..Worse.efficiency.directly.leads.to.oversize.the.power.
switches.and.heatsinks.to.solve.thermal.issues.
A.professional.system.designer.will.strive.for.an.EMI.conform.layout.and.using.fast-switching.devices.
to.get.an.efficient.and.compact.solution.for.the.application.at.least.cost.
The.major.tasks.for.the.device.manufacturer.are.to.provide.high-voltage.power.transistors.with.less.
switching.losses,.ergo.smaller.parasitic.capacitances.and.lower.on-state.resistance.per.chip.area..Both.
tasks.can.be.accomplished.by.reducing.the.area-specific.on-state.resistance,.since.smaller.chips.with.the.
same.nominal.on-state.resistance.also.have.smaller.parasitic.capacitances.
Practically.a.higher.doping.of.the.voltage-sustaining.layer.will.lead.to.more.carriers.available.for.cur-rent.transport..On.the.other.hand,.such.a.device.would.lead.to.a.lower.blocking.voltage.capability..The.
way.out.of.this.dilemma.was.the.introduction.of.a.doping.of.the.opposite.type.close.to.the.doping.of.the.
current.bearing.path.[34,35].to.have.local.high.conductivity.and.global.low.doping.due.to.the.compensa-tion..It.took.several.years.until.the.first.devices.were.commercially.available.that.successfully.used.this.
approach.[36,37]..Since.then,.the.doping.compensation.devices.were.the.development.path.for.improved.
high-voltage.MOSFETs.using.different.manufacturing.approaches.and.optimization.goals.
Beyond.all.these.developments.lies.the.same.basic.structure.as.in.the.right.part.of.Figure.1.22,.which.
is.compared.to.a.conventional.power.MOSFET.(left.part)..Both.devices.basically.have.the.same.struc-tures.on.the.chip.front.side.with.a.gate.controlling.an.inversion.channel.
Here,.the.voltage-sustaining.layer.consists.of.donor.(n−).and.acceptor.(p− ).doping.situated.in.two.indi-vidual.regions..For.blocking.operation,.the.difference.of.donor.and.acceptor.doping.determines.the.blocking.
voltage..This.net.doping.is.comparable.to.the.very.low.doping.of.the.voltage-sustaining.layer.for.a.conven-tional.power.MOSFET..The.donor.doping.of.the.most.modern.devices.can.be.increased.by.a.factor.of.15.or.
more.compared.to.standard.MOSFETs.and.thus.the.on-state.resistance.is.reduced.by.a.factor.of.7.5.or.more.
Since.the.blocking.characteristic.is.determined.by.the.difference.of.a.comparatively.high.donor.and.
acceptor.doping.the.control.of.this.net.doping.becomes.the.most.challenging.task.
When.building.up.a.blocking.voltage.at.closed.channel,.a.space–charge.region.starts.extending.from.
the.folded.pn-junction.into.the.p.compensation.columns.and.into.the.n.current.path..The.width.of.this.
insulating.region.grows.with.rising.blocking.voltage.as.sketched.in.Figure.1.23..Already,.at.rather.low.
blocking.voltages.applied.between.drain.and.source.compared.to.the.blocking.capability,.almost.the.
whole.area.of.current.path.and.compensation.column.is.depleted.
One.major.advantage.is.the.reduction.of.the.parasitic.capacitances.of.compensation.devices,.espe-cially.at.higher.drain-source.voltages.as.depicted.in.Figure.1.24..This.leads.to.lower.control.power.needs.
0 10 20 30 40 50 60 70
0 200 400 600 800 1000
Blocking voltage [V]
Ron*A [Ω mm2]
FIGURE.1.21. Dependence.of.the.series.resistance.of.a.MOSFET.of.the.blocking.voltage.
Voltage sustaining layer pn+
D n+(Substrate)
G
Gate oxide Channel
Compensation column Current path
n+
FIGURE.1.22. Left:.Cross.section.of.a.conventional.vertical.n-channel.power.MOSFET:.The.load.current.is.con-trolled.by.the.gate.and.flows.from.the.n.source.on.the.front.surface.of.the.chip.towards.the.drain.on.the.rear.surface.
through.the.low.doped.voltage.sustaining.layer..Right:.Cross.section.of.a.super.junction.vertical.power.MOSFET..
During.on-state.the.load.current.flows.through.the.n- -doped.current.path.while.during.off-state.the.doping.is.com-pensated.by.an.adjacent.p.column.leading.to.a.low.net.doping.serving.as.voltage.sustaining.layer.
pensation colum
n Current path
p
D G S
Space–charge region p
D G S
Com
pensation colum
n
n+
p– n–
Current path
n+ (Substrate) n+
n+ (Substrate) n+
FIGURE.1.23. Left.to.right:.Increasing.blocking.voltage.and.growth.of.the.space.charge–region..For.a.device.with.
600.V.blocking.capability.the.virtually.fully.depleted.case.at.the.right.is.already.reached.below.100.V.
1.5.2 Low-Voltage Power MOSFEt
Low.voltage.power.MOSFETs.are.widely.used.as.switching.transistors,.for.example,.in.AC/DC.converters.
or.DC/DC.converters..Especially.in.the.latter.case,.they.are.operated.at.high.frequencies.above.0.5.MHz.
making.their.parasitic.capacitances.more.important.as.for.high-voltage.power.MOSFETs..Also,.the.control.
losses.are.of.higher.importance.since.the.output.voltage.is.only.a.few.volts.higher.as.the.control.voltage.and.
thus.the.relation.between.switched.output.power.to.needed.control.power.is.much.smaller..The.parasitic.
capacitances.therefore.are.crucial.for.low.voltage.power.MOSFETs.
To.compare.the.performance.of.different.low.voltage.power.MOSFETs,.the.figure-of-merit.on-state.
resistance.multiplied.by.gate.charge.or.multiplied.by.total.charge.are.used—depending.on.the.focus.on.
control.losses.or.switching.losses.
Stray.inductances.as.well.as.inductances.on.the.application.board.and.the.transistor.package.are.of.
high.importance,.because.of.the.rather.low.voltage.used..The.fast.switching.leads.to.significant.voltage.
drops.across.small.stray.inductances,.which.will.influence.the.device.behavior.
Compared.to.high-voltage.devices.where.the.losses.are.dominated.by.the.conduction.losses.in.the.volt-age-sustaining.layer,.low.voltage.power.MOSFETs.have.more.leveled.distribution.to.the.on-state.losses..
The.impedance.level.in.total.must.be.lower.in.total,.which.leads.to.a.different.approach.for.the.cell.design.
The.on-state.resistances.for.the.conducting.inversion.channel.and.the.voltage.drop.over.the.voltage-sustaining.layer.are.in.the.same.order.of.magnitude..Additionally,.the.stray.resistance.of.the.package.
and.interconnections.play.an.important.role.leading.to.new.package.concepts.with.less.parasitics;.also.
stray.inductances.are.reduced.
Low.voltage.power.MOSFETs.use.trench.gates.most.frequently.(see.Figure.1.25).compared.to.planar.
gate.structures.dominating.high-voltage.MOSFETs..Trench.cells.allow.a.denser.packaging.of.the.cells,.
higher.channel.widths.and.thus.lower.channel.resistances..The.area.of.the.gate.electrode.opposite.to.the.
drain.electrode.is.smaller,.leading.to.a.smaller.gate-drain.capacitance,.thus.less.feedback.(Miller.effect).
and.faster.switching..The.area.of.the.source.electrode.opposite.to.the.drain.electrode.also.is.smaller,.
leading.to.smaller.output.capacitances.and.less.switching.losses.
10,000
1,000
100
10 0 50 100 150 200 250 300
VDS [V]
Capacitance [pF]
VDS CGS
VGD
600 V/190 mΩ Standard-MOS CoolMOS C3
FIGURE.1.24. Comparison.of.standard.MOSFETs.and.Infineon.CoolMOS.both.with.190.mΩ.on-state.resistance.
Future. developments. for. low. power. MOSFETs. will. focus. on.
using.finer.structures.to.improve.the.figure.of.merit.Ron.×.Qtotal.