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Lecture 14: pn Junction Diodes and Transient Response

This lecture continues the discussion on pn junction diodes, focusing on the transient response during turn-on. It also summarizes important concepts, explores diode applications such as varactor and optoelectronic diodes, and discusses their unique characteristics. Additional reading materials are recommended for further understanding.

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Lecture 14: pn Junction Diodes and Transient Response

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  1. Lecture 14 OUTLINE • pn Junction Diodes (cont’d) • Transient response: turn-on • Summary of important concepts • Diode applications • Varactor diodes • Tunnel diodes • Optoelectronic diodes Reading: Pierret 9; Hu 4.12-4.15

  2. Turn-On Transient Consider a p+n diode (Qp >> Qn): i(t) Dpn(x) t vA(t) x xn t For t > 0: EE130/230A Fall 2013 Lecture 14, Slide 2

  3. By separation of variables and integration, we have If we assume that the build-up of stored charge occurs quasi-statically so that then EE130/230A Fall 2013 Lecture 14, Slide 3

  4. If tp is large, then the time required to turn on the diode is approximately DQ/IF EE130/230A Fall 2013 Lecture 14, Slide 4

  5. Summary of Important Concepts • Under forward bias, minority carriers are injected into the quasi-neutral regions of the diode. • The current flowing across the junction is comprised of hole and electron components. • If the junction is asymmetrically doped (i.e. it is “one-sided”) then one of these components will be dominant. • In a long-base diode, the injected minority carriers recombine with majority carriers within the quasi-neutral regions. EE130/230A Fall 2013 Lecture 14, Slide 5

  6. The ideal diode equation stipulates the relationship between JN(-xp) and JP(xn): For example, if holes are forced to flow across a forward-biased junction, then electrons must also be injected across the junction. EE130/230A Fall 2013 Lecture 14, Slide 6

  7. Under reverse bias, minority carriers are collected into the quasi-neutral regions of the diode. Minority carriers generated within a diffusion length of the depletion region diffuse into the depletion region and then are swept across the junction by the electric field. The negative current flowing in a reverse-biased diode depends on the rate at which minority carriers are supplied from the quasi-neutral regions. Electron-hole pair generation within the depletion region also contributes negative diode current. EE130/230A Fall 2013 Lecture 14, Slide 7

  8. pn Junction as a Temperature Sensor C. C. Hu, Modern Semiconductor Devices for ICs, Figure 4-21 EE130/230A Fall 2013 Lecture 14, Slide 8

  9. Varactor Diode • Voltage-controlled capacitance • Used in oscillators and detectors (e.g. FM demodulation circuits in your radios) • Response changes by tailoring doping profile: EE130/230A Fall 2013 Lecture 14, Slide 9

  10. Optoelectronic Diodes EE130/230A Fall 2013 Lecture 14, Slide 10 R.F. Pierret, Semiconductor Fundamentals, Figure 9.2

  11. Open Circuit Voltage, VOC EE130/230A Fall 2013 Lecture 14, Slide 11 C. C. Hu, Modern Semiconductor Devices for ICs, Figure 4-25(b)

  12. Solar Cell StructureCyferz at en.wikipedia EE130/230A Fall 2013 Lecture 14, Slide 12

  13. Textured Si surface for reduced reflectance • Achieved by anisotropic wet etching (e.g. in KOH) M. A. Green et al., IEEE Trans. Electron Devices, Vol. 37, pp. 331-336, 1990 P. Papet et al., Solar Energy Materials and Solar Cells, Vol. 90, p. 2319, 2006 EE130/230A Fall 2013 Lecture 14, Slide 13

  14. p-i-n Photodiodes • W  Wi-region, so most carriers are generated in the depletion region faster response time (~10 GHz operation) • Operate near avalanche to amplify signal R.F. Pierret, Semiconductor Fundamentals, Figure 9.5 EE130/230A Fall 2013 Lecture 14, Slide 14

  15. Light Emitting Diodes (LEDs) • LEDs are made with compound semiconductors (direct bandgap) R.F. Pierret, Semiconductor Fundamentals, Figure 9.13 R.F. Pierret, Semiconductor Fundamentals, Figure 9.15 EE130/230A Fall 2013 Lecture 14, Slide 15

  16. Question 1 (re: Slide 12): Why are the contacts to the back (non-illuminated) side of a solar cell made only at certain points (rather than across the entire back surface)? Answer: To increase energy conversion efficiency • The absorption depth (average distance a photon travels before transferring its energy to an electron) for long-wavelength photons is greater than the Si thickness.  The bottom surface oxide and metal layer effectively form a mirror that reflects light back into the silicon. • There is more recombination in heavily doped contacts than at a good Si/SiO2interface; most of the back surface should be covered by SiO2so that generated carriers have a high probability of diffusing to the depletion region before they recombine. EE130/230A Fall 2013 Lecture 14, Slide 16

  17. Question 2 (re: Slide 15): What limits the lifetime of an LED? Answer: • LED lifetime is defined to be the duration of operation after which the light output falls to only 70% of original. (Even afterwards, the LED will continue to function.) • The power density of an LED can be high (up to 10 W/cm2, comparable to an electric stove top), causing significant heating which can degrade the light output through various mechanisms: • Degradation of epoxy package causing partial absorption of light • Mechanical stress weakening the wire bond (electrical connection) • Formation/growth of crystalline defects, or diffusion of metal into the semiconductor, resulting in increased recombination via mid-gap states EE130/230A Fall 2013 Lecture 14, Slide 17

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