Semiconductor Devices 2013Semiconductor Devices - 2013

Lecture CoursePart ofPart of

SS Module PY4P03

Dr. P. Stamenov

School of Physics and CRANN, Trinity College,Dublin 2, Ireland,

Hilary Term, TCD8th of Feb ‘13

Process Flow Example – p-n DiodeWafer

Diffused Region

Oxide

Resist

Metallisation

Contacts

Unipolar FETs’ Family Free Si JFET Single Channel

V Groo eJFET

GaAs JFETV- GrooveMultichannel

Si MESFETDiffusedGrownH j i

FET MESFETGaAs MESFET

I P MESFETSingle GateHeterojunction

InP MESFET Multiple GateInterdigitals

Si MOSFET NMOS, PMOSCMOS

MOSFETGaAs MOSFET

HMOSDMOS, DIMOSGaAs MOSFETVMOSSOS, SOI

Remarks on FETs• Essentially unipolar devices – the current flow (either holes or

electrons) bet een t o electrodes (so rce and drain) is controlled belectrons) between two electrodes (source and drain) is controlled by the potential difference to a third electrode (gate).Cl ( l ) t h i b h i Li l L t t l• Close (closer) to ohmic behaviour. Linear or square law. Lower total harmonic distortion and intermodulation distortion.

• Much higher input impedance. Lower input currents. Simpler matching to microwave circuits.

• Negative temperature coefficient. Self-limiting, with a much lower probability for a thermal runaway or breakdown.

• No minority carrier storage effects – higher switching speeds and higher cutoff frequencies.

• Conceptually simpler, with close to equilibrium carrier concentrations, virtually no carrier injection or extraction., y j

• Could have been the first ones to be realized...

...The First One• Part of Lilienfeld’s first

patent This resemblespatent. This resembles a modern MESFET, but was unrealisablewas unrealisable without semiconductors.

• Shockley et al ~ 1948Shockley et al ~ 1948 worked on similar concepts but had toconcepts, but had to abandon patenting them because of Lilienfeld’s because o e e d swork. Reproposed by Mead et al ~1966

• Most semiconductor transistors are nowtransistors are now unipolar.

MEtal Semiconductor - MESFETContact Metal ImplantedMetal

GateRecipe•DrainGate

ImplantedSource

• ...• Grow an epilayer

O idi• Oxidize• Open windows• Implant• Metalize

Semiconductor base • ...FeaturesOxide InsulationEpitaxial LayerFeatures• Building block

id iR l ti l i l k ith • Provides gain• Voltage-control device

• Relatively simple – can work with compound semiconductors (Schottky gate)F hi h bili h l • Unipolar device• Fast – can use high mobility channels

• Can be done in SOI version, as well

Junction Field Effect Transistor - JFETContact Metal ImplantedMetal

GateRecipe•DrainGate

ImplantedSource

• ...• Grow an epilayer

O idi• Oxidize• Open windows• Diffuse junction• Metalize

Semiconductor base • ...FeaturesOxide InsulationEpitaxial LayerFeatures• Building block

id iL i fi it t t • Provides gain• Voltage-control device

• Low noise – finite gate current• Very flexible intermediate frequencies

lifi ( d id l i ) • Unipolar deviceamplifier (used widely in preamps)• Enhancement and depletion modes

Metal Oxide Semiconductor - MOSFETGate Metal ImplantedGate

OxideRecipe•DrainOxide

ImplantedSource

• ...• Grow an epilayer

O idi• Oxidize• Open windows• Form gate oxide• Metalize

Semiconductor base • ...FeaturesOxide InsulationEpitaxial LayerFeatures• Building block

id iV l t t b t h t i • Provides gain• Voltage-control device

• Very low gate current, but somewhat noisy• Can use both bias signs at the gate contact

• Unipolar device• Enhancement and depletion modes• Buried channel devices possible

Idealised JFET

After: G. C. Dacey

d I M Rand I. M. Ross,Proc. I. R. E.41 970 (1953)41, 970 (1953)

Potential Distribution - JFETPoisson’s Equationfor the channel

For n-type channelThreat as in diodes

Depletion depthDepletion depthat source

Depletion depthb i i

Preparing for apurely linear at arb. positionpurely linearapproximation

Build-in potential

Drain Current - JFET

Depletion depthp pat drain

Pinch-off VoltageAt the opposite endof the gate

CurrentD i

of the gate

Drude’s law DensityDrude s law

DrainCurrent

Potential variationRemember that λd is Potential variationin the channel

dvariable in the channel

Saturation Current - JFET

D iPinch off DrainCurrent

Pinch-offcurrenr

SaturationSaturation SaturationCurrent

SaturationBias

Transconductances

Transconductance and Drain Current

DrainConductance

GateTransconductance

ThresholdThresholdVoltage

Approximate Drain CurrentE i REMEMBER!Expression. REMEMBER!

The MOSFET

After: S. M. Sze,Physics of SemiconductorDevices, 2nd edition

Conductivity – MOSFET case

ConductivityConductivity

Conductance

PotentialDistribution

DrainVoltage

Charge Distribution in the Channel

DistributedIntrinsicCharge

IntrinsicCapacitance

DrainCurrentCurrent

ThresholdVoltage

ApproximateExpression VoltageExpressionREMEMBER!