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ECE 874: Physical Electronics

ECE 874: Physical Electronics. Prof. Virginia Ayres Electrical & Computer Engineering Michigan State University ayresv@msu.edu. Lecture 24, 20 Nov 12 Chp. 05: Recombination-Generation Processes.

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ECE 874: Physical Electronics

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  1. ECE 874:Physical Electronics Prof. Virginia Ayres Electrical & Computer Engineering Michigan State University ayresv@msu.edu

  2. Lecture 24, 20 Nov 12Chp. 05: Recombination-Generation Processes VM Ayres, ECE874, F12

  3. Recombination-Generation ProcessesThese two mechanisms are important at 300K and higher temperatures: Absorption and Spontaneous emission Direct bandgap materials like GaAs: important Recombination-generation (R-G) of electrons and/or holes via a trap (a local defect). Indirect bandgap materials like Si: very important. Will show that this process is most efficient for traps near the mid-gap Chp. 05 concentrates on (b) VM Ayres, ECE874, F12

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  6. Recombination-Generation ProcessesThese two mechanisms are important at low temperatures: Donor or acceptor sites act as local impurity traps but are not near the mid-gap. Inefficient version of R-G mechanism. Exciton formation creates a non-dopant type of bandgap state typically close to Ec or Ev. When excitons form, they alter the n, p headcount. When they annihilate they can produce photons with close to the bandgap energy/wavelength. This adds extra photons but also a spread to emitted wavelengths. Important for direct bandgap optoelectonic materials like GaAs at low temps. VM Ayres, ECE874, F12

  7. Recombination-Generation Processesthis mechanism is important at high n or p concentrations: Auger process: band-to-band recombination or trap recombination is going on when a collision with an outside n or p also occurs. The orginal n or p gets and subsequently loses a lot of extra energy. This is important for direct bandgap materials like GaAs when what you want is recombination that gives you bandgap energy/wavelength photon emission and what you get instead is a lot of thermal energy waste. VM Ayres, ECE874, F12

  8. Recombination-generation (R-G) via a trap (a local defect): why this is important: Rate for this steady state happening is proportional to the trap density NT = dn/dt or dp/dt J = R width VM Ayres, ECE874, F12

  9. Recombination-generation (R-G) via a trap (a local defect): why this is important: pn junction in Si at equilibrium ( no bias) - - - - + + + + p = p+ = 1019 cm-3 n = 1015 cm-3 Si W VM Ayres, ECE874, F12

  10. Same pn junction in Si in reverse bias: - 5V - Vrev + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - p+ = 1019 cm-3 n = 1015 cm-3 Si W Reverse bias goal: turn the device OFF: no current flowing. VM Ayres, ECE874, F12

  11. Diode equation Given: tp = stay-alive time for holes on the n-side = 10-6 sec VM Ayres, ECE874, F12

  12. This seems to be a good solid OFF. VM Ayres, ECE874, F12

  13. Find the depletion width WD too: VM Ayres, ECE874, F12

  14. VM Ayres, ECE874, F12

  15. In the Depletion region; n, p and np are small: J = R width time - Given: tg = generation time for holes on the n-side = same = 10-6 sec VM Ayres, ECE874, F12

  16. You didn’t turn your device OFF as well as you thought you did by four orders of magnitude. VM Ayres, ECE874, F12

  17. What happened: trap-mediated recombination-generation (R-G) processes act to restore what ever the previous steady state was. In this example: in the old steady state, the n-side was largely a neutral region with: Now the same place is a depletion region WD with: Result: Jgen: traps released carriers in WD: new steady state VM Ayres, ECE874, F12

  18. Example problem conditions: steady state VM Ayres, ECE874, F12

  19. General info: VM Ayres, ECE874, F12

  20. General info: Processes the change the e- headcount Processes the change the hole headcount VM Ayres, ECE874, F12

  21. Each one of these processes happens with better or worse efficiencies: Hole capture Hole emission VM Ayres, ECE874, F12

  22. General info: Processes the change the e- headcount Processes the change the hole headcount VM Ayres, ECE874, F12

  23. Equilibrium: 0 = 0 = Under equilibrium conditions you can solve for the emission coefficients in terms of the capture coefficients (p. 145). Then, assuming that even away from equilibrium, the capture coefficient values don’t change too much: OK: cn, cp, n, p, nT, pT Need: n1, p1 (p. 145) VM Ayres, ECE874, F12

  24. 4.68: in the skipped Chp. 04 section on ionization of dopants as a function of temperature, and also traps as a function of temperature. VM Ayres, ECE874, F12

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  26. Example problem: calculate n1 for O in Si at 300K for the closest to mid-gap trap. VM Ayres, ECE874, F12

  27. Oxygen traps: .16 eV below EC .38 eV below EC .51 eV below EC Oxygen traps: .41 eV above EV VM Ayres, ECE874, F12

  28. Oxygen trap nearest mid-gap is: .51 eV below EC EC – ET’ = .51 eV Where is it relative to Ei? EC – Ei = .56 eV - .0073 eV = 0.5527 EV VM Ayres, ECE874, F12

  29. ET versus E T’. What’s the difference? ET’ includes the temperature dependence of the trap.Equation 4.69: in the skipped Chp. 04 section on ionization of traps as a function of temperature: 1 or 2 is typical ET’ is what you experimentally measure so the .51 eV below EC level on the graph is ET’ in our problem. VM Ayres, ECE874, F12

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