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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 Turn-On Transient Consider a p+n diode (Qp >> Qn): i(t) Dpn(x) t x vA(t) xn dpn For t > 0: dx EE130/230A Fall 2013 x xn i 0 qAD p Lecture 14, Slide 2 t dQ p dt i Qp τp IF Qp τp for t 0 • By separation of variables and integration, we have Qp (t ) I F τ p 1 e t / τ p • If we assume that the build-up of stored charge occurs quasi-statically so that Qp (t ) I diffusionτ p I 0 eqvA / kT 1 τ p kT I F t / τ p then v A (t ) ln 1 1 e q I0 EE130/230A Fall 2013 Lecture 14, Slide 3 • If tp is large, then the time required to turn on the diode is approximately DQ/IF where DQ DQ p DQ j EE130/230A Fall 2013 Lecture 14, Slide 4 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 quasineutral regions. EE130/230A Fall 2013 Lecture 14, Slide 5 • The ideal diode equation stipulates the relationship between JN(-xp) and JP(xn): Dn L p N D ni 2 p side 2 J P ( xn ) D p Ln N A ni n side J N ( x p ) 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 • 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 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 Varactor Diode • Voltage-controlled capacitance – Used in oscillators and detectors (e.g. FM demodulation circuits in your radios) – Response changes by tailoring doping profile: C j Vr n for n 1 m 2 Vr Vbi EE130/230A Fall 2013 Lecture 14, Slide 9 Optoelectronic Diodes I I 0 (e qVA kT 1) I L I L qA( LP W LN )GL EE130/230A Fall 2013 Lecture 14, Slide 10 R.F. Pierret, Semiconductor Fundamentals, Figure 9.2 Open Circuit Voltage, VOC Voc VA EE130/230A Fall 2013 I 0 L W L kTq ln L p p L n GL 1 t p pn n t n n p Lecture 14, Slide 11 C. C. Hu, Modern Semiconductor Devices for ICs, Figure 4-25(b) Solar Cell Structure Cyferz at en.wikipedia EE130/230A Fall 2013 Lecture 14, Slide 12 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 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 EE130/230A Fall 2013 Lecture 14, Slide 14 R.F. Pierret, Semiconductor Fundamentals, Figure 9.5 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 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 i) The absorption depth (average distance a photon travels before transferring its energy to an electron) for longwavelength 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. ii)There is more recombination in heavily doped contacts than at a good Si/SiO2 interface; most of the back surface should be covered by SiO2 so that generated carriers have a high probability of diffusing to the depletion region before they recombine. EE130/230A Fall 2013 Lecture 14, Slide 16 Question 2 (re: Slide 15): What limits the lifetime of an LED? Answer: i) 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.) ii) 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: 1.Degradation of epoxy package causing partial absorption of light 2.Mechanical stress weakening the wire bond (electrical connection) 3.Formation/growth of crystalline defects, or diffusion of metal into the semiconductor, resulting in increased recombination via midgap states EE130/230A Fall 2013 Lecture 14, Slide 17