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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!
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