Heterostructures, HBTs and Thyristors : Exploring the "different"
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The document discusses heterostructures, heterojunction bipolar transistors (HBTs), and thyristors. It begins by explaining homojunctions and heterojuctions, how they differ in material composition and resulting energy band structures. It then describes HBTs, noting they can achieve higher speeds than bipolar junction transistors (BJTs) due to reduced injection of minority carriers into the emitter. Finally, it discusses thyristors, four-layer pnpn semiconductor devices that can operate in either conducting or blocking states, and diacs, bidirectional thyristor variants used in alternating current switching applications.
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Heterostructures, HBTs and Thyristors : Exploring the "different"
- 1. Heterostructures Heterojunction Bipolar Transistor Thyristors Heterostructures, HBTs and Thyristors Exploring the “Different” Shuvan Prashant June 16, 2014 as part of PHY 1001 Physical Electronics Coursework. Shuvan Prashant Heterostructures, HBTs and Thyristors
- 2. Heterostructures Heterojunction Bipolar Transistor Thyristors Outline 1 Heterostructures Introduction Homojunction Heterojunction 2 Heterojunction Bipolar Transistor Structure Application 3 Thyristors pnpn Junction Diac Shuvan Prashant Heterostructures, HBTs and Thyristors
- 3. Heterostructures Heterojunction Bipolar Transistor Thyristors Introduction Homojunction Heterojunction 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
- 4. Heterostructures Heterojunction Bipolar Transistor Thyristors Introduction Homojunction Heterojunction Energy Band Diagram Construction Assumption There are negligible number of traps or generation-recombination centers at the interface of two dissimilar SCs Validity: SCs have matched Lattice Constants Requirements The Fermi Level must be same on both sides of the interface The vacuum Level must be continuous and parallel to the band edges Discontinuity in band edges is unaffected because of doping Materials Used III-V Compound Semiconductors GaAs Eg = 1.42eV Lattice Constant = 5.6533 Ao Alx Ga1−x As where x can vary from 0 to 1 Eg = 2.17eV Lattice Constant = 5.6605 Ao Shuvan Prashant Heterostructures, HBTs and Thyristors
- 5. Heterostructures Heterojunction Bipolar Transistor Thyristors Introduction Homojunction Heterojunction Different Possibilities Band Engineering The three possibilities are Straddling Staggered Broken Gap Types of Junction Where dopant changes at junction– Anisotype e.g nP, Pn Where dopant doesn’t change – Isotype e.g nN, pP Shuvan Prashant Heterostructures, HBTs and Thyristors
- 6. Heterostructures Heterojunction Bipolar Transistor Thyristors Introduction Homojunction Heterojunction Energy band Edge Picture Band Edge energies The band edge energies relative to the Vacuum Ref are the property of SC Electron Affinity,χ, CB end to Vacuum Ref Energy Gap Eg , Valence Band Edge to Conduction Band Edge Fermi Level Depends on doping level Work Function,Φ: Fermi Level to Vacuum ref Shuvan Prashant Heterostructures, HBTs and Thyristors
- 7. Heterostructures Heterojunction Bipolar Transistor Thyristors Introduction Homojunction Heterojunction Isolated n-type and p-type Have same vacuum ref Fermi Levels Differ Both materials Neutral Shuvan Prashant Heterostructures, HBTs and Thyristors
- 8. Heterostructures Heterojunction Bipolar Transistor Thyristors Introduction Homojunction Heterojunction Electrically Connected n-type and p-type Charge shifts between sides Fermi Levels Shift until equal Vacuum Ref is -qφ Depletion Approximation is good for estimating ρ(x) Shuvan Prashant Heterostructures, HBTs and Thyristors
- 9. Heterostructures Heterojunction Bipolar Transistor Thyristors Introduction Homojunction Heterojunction Isolated N type and p type Similar to homojunction except that the two materials have different electron affinities, energy gaps, dielectric constants and effective masses. Electron affinity of the wide bandgap material is less than that of narro band gap material Shuvan Prashant Heterostructures, HBTs and Thyristors
- 10. Heterostructures Heterojunction Bipolar Transistor Thyristors Introduction Homojunction Heterojunction 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 Electron Affinity Rule Shuvan Prashant Heterostructures, HBTs and Thyristors
- 11. Heterostructures Heterojunction Bipolar Transistor Thyristors Introduction Homojunction Heterojunction Electrically Connected N-type and p-type Charge shifts between sides Fermi Levels Shift until equal Vacuum ref. is now -qφ(x) where φ(x) = (q/ )ρ(x) dx dx Ec(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
- 12. Heterostructures Heterojunction Bipolar Transistor Thyristors Introduction Homojunction Heterojunction 2D electron gas Electrons from wide gap AlGaAs flow into GaAs Form an accumulation layer of electrons in Potential well 2d Electron Gas – electrons have quantized energy levels in one spatial direction but are free in other two So What? Electron Mobility increases in the low impurity doping region (abrupt heterojunction) Shuvan Prashant Heterostructures, HBTs and Thyristors
- 13. Heterostructures Heterojunction Bipolar Transistor Thyristors Introduction Homojunction Heterojunction 2D electron gas Electrons from wide gap AlGaAs flow into GaAs Form an accumulation layer of electrons in Potential well 2d Electron Gas – electrons have quantized energy levels in one spatial direction but are free in other two So What? Electron Mobility increases in the low impurity doping region (abrupt heterojunction) Shuvan Prashant Heterostructures, HBTs and Thyristors
- 14. Heterostructures Heterojunction Bipolar Transistor Thyristors Introduction Homojunction Heterojunction 2D electron gas Electrons from wide gap AlGaAs flow into GaAs Form an accumulation layer of electrons in Potential well 2d Electron Gas – electrons have quantized energy levels in one spatial direction but are free in other two So What? Electron Mobility increases in the low impurity doping region (abrupt heterojunction) Shuvan Prashant Heterostructures, HBTs and Thyristors
- 15. Heterostructures Heterojunction Bipolar Transistor Thyristors Introduction Homojunction Heterojunction 2D electron gas Electrons from wide gap AlGaAs flow into GaAs Form an accumulation layer of electrons in Potential well 2d Electron Gas – electrons have quantized energy levels in one spatial direction but are free in other two So What? Electron Mobility increases in the low impurity doping region (abrupt heterojunction) Shuvan Prashant Heterostructures, HBTs and Thyristors
- 16. Heterostructures Heterojunction Bipolar Transistor Thyristors Structure Application Limitations of BJT 1 Limit on Current Gain- Emitter Injection Efficiency γ 2 γ - accounts for minority carrier hole diffusion current in the emitter 3 Doesn’t contribute to transistor action 4 Increase in emitter doping - bandgap narrowing offsets the improvement 5 Solution: Use wideband gap material to minimise carrier injection from base to emitter n*GaAs n*GaAs nGaAlAs Emitter nGaAs nGaAlAs pGaAs Collector Base nGaAlAs Emitter and p GaAs Base Junc Energy Band Diagram Shuvan Prashant Heterostructures, HBTs and Thyristors
- 17. Heterostructures Heterojunction Bipolar Transistor Thyristors Structure Application What made the difference? Shuvan Prashant Heterostructures, HBTs and Thyristors
- 18. Heterostructures Heterojunction Bipolar Transistor Thyristors Structure Application 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 doping needn’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⇒ Shorter Base transit Time Cutoff Frequencies of the order of 40 GHz Shuvan Prashant Heterostructures, HBTs and Thyristors
- 19. Heterostructures Heterojunction Bipolar Transistor Thyristors pnpn Junction Diac Thyristors Three pn junctions in series - pnpn diode With a gate terminal – Semiconductor Controlled Rectifier or Thyristor Switching from an OFF or blocking state to an ON or conducting state Wider range of current and voltage handling capabilities than transistors Shuvan Prashant Heterostructures, HBTs and Thyristors
- 20. Heterostructures Heterojunction Bipolar Transistor Thyristors pnpn Junction Diac Basic Characteristics Figure: Thyristor in forward region Regions 1 Forward Blocking - OFF State with high impedance Forward Breakover (switching) dV/dI = 0 V= VBF I= Is 2 Negative Resistance Region 3 Forward Conducting - ON State with low impedance dV/dI = 0 V= Vh I= Ih 4 Reverse Blocking State 5 Reverse Breakdown region Shuvan Prashant Heterostructures, HBTs and Thyristors
- 21. Heterostructures Heterojunction Bipolar Transistor Thyristors pnpn Junction Diac Basic Characteristics Figure: Thyristor in forward region Regions 1 Forward Blocking - OFF State with high impedance Forward Breakover (switching) dV/dI = 0 V= VBF I= Is 2 Negative Resistance Region 3 Forward Conducting - ON State with low impedance dV/dI = 0 V= Vh I= Ih 4 Reverse Blocking State 5 Reverse Breakdown region Shuvan Prashant Heterostructures, HBTs and Thyristors
- 22. Heterostructures Heterojunction Bipolar Transistor Thyristors pnpn Junction Diac Basic Characteristics Figure: Thyristor in forward region Regions 1 Forward Blocking - OFF State with high impedance Forward Breakover (switching) dV/dI = 0 V= VBF I= Is 2 Negative Resistance Region 3 Forward Conducting - ON State with low impedance dV/dI = 0 V= Vh I= Ih 4 Reverse Blocking State 5 Reverse Breakdown region Shuvan Prashant Heterostructures, HBTs and Thyristors
- 23. Heterostructures Heterojunction Bipolar Transistor Thyristors pnpn Junction Diac Understanding pnpn as coupled transistors Bistable device pnpn diode in forward region is Bistable device high impedance low current OFF low impedance high current ON Shuvan Prashant Heterostructures, HBTs and Thyristors
- 24. Heterostructures Heterojunction Bipolar Transistor Thyristors pnpn Junction Diac Bidirectional Thyristor Diac Diac Diode for Alternating current Diac diode as an ac switch ON OFF States for positive and negative anode voltages Shuvan Prashant Heterostructures, HBTs and Thyristors
- 25. Heterostructures Heterojunction Bipolar Transistor Thyristors pnpn Junction Diac Bibliography Thank You References Semiconductor Devices S M Sze I edition Semiconductor Devices D A Neamann Third Edition MIT Lectures OCW 6.772 Compound Semiconductor Devices As taught in: Spring 2003 by Clifton Fonstad Jr. Pictures for thyristors from www.wikipedia.com Shuvan Prashant Heterostructures, HBTs and Thyristors