heterostructures, hbts and thyristors : exploring the "different"

HeterostructuresHeterojunction Bipolar Transistor

Thyristors

Heterostructures, HBTs and ThyristorsExploring the “Different”

Shuvan Prashant

June 16, 2014

as part of PHY 1001 Physical Electronics Coursework.

Shuvan Prashant Heterostructures, HBTs and Thyristors


Thyristors

Outline

1 HeterostructuresIntroductionHomojunctionHeterojunction

2 Heterojunction Bipolar TransistorStructureApplication

3 Thyristorspnpn JunctionDiac



Thyristors

IntroductionHomojunctionHeterojunction

Like and Unlike

Homojunction

Semiconductor material is homogeneous through out the structure.

Heterojunction

Two different SC materials form junction

Different Energy Band Gaps

Energy Band Discontinuity at the junction interface

Narrow Band gap to wide band gap – Abrupt Junction

Lattice Constant matching must be done



Thyristors


Energy Band Diagram Construction

Assumption

There are negligible number of traps or generation-recombinationcenters at the interface of two dissimilar SCsValidity: SCs have matched Lattice Constants

Requirements

The Fermi Level must be same on both sides of the interfaceThe vacuum Level must be continuous and parallel to the bandedgesDiscontinuity in band edges is unaffected because of dopingMaterials Used III-V Compound SemiconductorsGaAs Eg = 1.42eV Lattice Constant = 5.6533 Ao

AlxGa1−xAs where x can vary from 0 to 1Eg = 2.17eV Lattice Constant = 5.6605 Ao



Thyristors


Different Possibilities

Band Engineering

The three possibilities are

Straddling

Staggered

Broken Gap

Types of Junction

Where dopant changes atjunction– Anisotypee.g nP, PnWhere dopant doesn’t change –Isotypee.g nN, pP



Thyristors


Energy band Edge Picture

Band Edge energies

F The band edge energiesrelative to the Vacuum Refare the property of SC

F Electron Affinity,χ, CB endto Vacuum Ref

F Energy Gap Eg , ValenceBand Edge to ConductionBand Edge

Fermi LevelDepends on doping levelWork Function,Φ: Fermi Level to Vacuum ref



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Isolated n-type and p-type

Have same vacuum ref

Fermi Levels Differ

Both materials Neutral



Thyristors


Electrically Connected n-type and p-type

Charge shifts between sidesFermi Levels Shift until equalVacuum Ref is -qφDepletion Approximation is good for estimating ρ(x)



Thyristors


Isolated N type and p type

Similar to homojunction except that the two materials havedifferent electron affinities, energy gaps, dielectric constants andeffective masses.Electron affinity of the wide bandgap material is less than that ofnarro band gap material



Thyristors


Isolated N type and p type

In ideal abrupt heterojunction using nondegenerately doped SCs,the vacuum level is ‖ to CBs and VBs.If vacuum level is continuous , then same ∆Ec and ∆Ev

discontinuities will exist at heterojunction interface ElectronAffinity Rule



Thyristors


Electrically Connected N-type and p-type

Charge shifts between sidesFermi Levels Shift until equalVacuum ref. is now -qφ(x) where φ(x) = (q/ε)ρ(x) dx dxEc(x) is -qφ(x) - χ(x) and Ev(x) = -qφ(x) - [χ(x) +Eg(x)]Depletion Approximation is good for estimating ρ(x)



Thyristors


2D electron gas

F Electrons from wide gapAlGaAs flow into GaAs

F Form an accumulation layerof electrons in Potential well

F 2d Electron Gas – electronshave quantized energy levelsin one spatial direction

F but are free in other two SoWhat?

F Electron Mobility increasesin the low impurity dopingregion (abruptheterojunction)



Thyristors

StructureApplication

Limitations of BJT

1 Limit on Current Gain-Emitter Injection Efficiencyγ

2 γ - accounts for minoritycarrier hole diffusion currentin the emitter

3 Doesn’t contribute totransistor action

4 Increase in emitter doping -bandgap narrowing offsetsthe improvement

5 Solution: Use widebandgap material to minimisecarrier injection from baseto emitter

n*GaAs

n*GaAsnGaAlAs

EmitternGaAsnGaAlAspGaAs

Collector

Base

nGaAlAs Emitter and p GaAs Base JuncEnergy Band Diagram



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What made the difference?



Thyristors


What made the difference?

The holes and electrons see a different barrier

The holes are not allowed to go back into emitter

Drastic reduction in Hole Injection – high emitter dopingneedn’t be done

Reduction in band narrowing effect too..

So What’s the use ?

High Frequency Device

Lower emitter Doping⇒ Smaller junction Capacitance⇒Higher Speed

Electron Mobility for npn GaAs is 5 times that of Si⇒ ShorterBase transit Time

Cutoff Frequencies of the order of 40 GHz



Thyristors

pnpn JunctionDiac

Thyristors

Three pn junctions in series - pnpn diode

With a gate terminal – Semiconductor Controlled Rectifier orThyristorSwitching from an OFF or blocking state to an ON or conductingstateWider range of current and voltage handling capabilities thantransistors



Thyristors

pnpn JunctionDiac

Basic Characteristics

Figure: Thyristor in forward region

Regions

1 Forward Blocking - OFFState with high impedanceForward Breakover(switching) dV/dI = 0 V=VBF I= Is

2 Negative Resistance Region

3 Forward Conducting - ONState with low impedancedV/dI = 0 V= Vh I= Ih

4 Reverse Blocking State

5 Reverse Breakdown region



Thyristors

pnpn JunctionDiac

Understanding pnpn as coupled transistors

Bistable device

pnpn diode in forward region is Bistable devicehigh impedance low current OFFlow impedance high current ON



Thyristors

pnpn JunctionDiac

Bidirectional Thyristor Diac

Diac Diode for Alternating current

Diac diode as an ac switchON OFF States for positive and negative anode voltages



Thyristors

pnpn JunctionDiac

Bibliography

Thank You

References

F Semiconductor Devices S M Sze I edition

F Semiconductor Devices D A Neamann Third Edition

F MIT Lectures OCW 6.772 Compound Semiconductor DevicesAs taught in: Spring 2003 by Clifton Fonstad Jr.

F Pictures for thyristors from www.wikipedia.com


heterostructures, hbts and thyristors : exploring the "different"

Science

ideal abrupt heterojunction

dierent shuvan prashant

vacuum level

vacuum ref energy gap

valence band edge

band edges discontinuity

energy levels

d electron gas electrons