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
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
Outline
1 HeterostructuresIntroductionHomojunctionHeterojunction
2 Heterojunction Bipolar TransistorStructureApplication
3 Thyristorspnpn JunctionDiac
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
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
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
IntroductionHomojunctionHeterojunction
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
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
IntroductionHomojunctionHeterojunction
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
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
IntroductionHomojunctionHeterojunction
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
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
IntroductionHomojunctionHeterojunction
Isolated n-type and p-type
Have same vacuum ref
Fermi Levels Differ
Both materials Neutral
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
IntroductionHomojunctionHeterojunction
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)
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
IntroductionHomojunctionHeterojunction
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
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
IntroductionHomojunctionHeterojunction
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
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
IntroductionHomojunctionHeterojunction
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)
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
IntroductionHomojunctionHeterojunction
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)
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
IntroductionHomojunctionHeterojunction
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)
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
IntroductionHomojunctionHeterojunction
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)
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
IntroductionHomojunctionHeterojunction
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)
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
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
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
StructureApplication
What made the difference?
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
Thyristors
StructureApplication
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
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
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
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
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
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
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
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
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
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
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
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
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
Shuvan Prashant Heterostructures, HBTs and Thyristors
HeterostructuresHeterojunction Bipolar Transistor
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
Shuvan Prashant Heterostructures, HBTs and Thyristors