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 vA(t) x xn t For t > 0: EE130/230A Fall 2013 Lecture 14, Slide 2

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

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

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

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

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: EE130/230A Fall 2013 Lecture 14, Slide 9

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

Open Circuit Voltage, VOC EE130/230A Fall 2013 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 R.F. Pierret, Semiconductor Fundamentals, Figure 9.5 EE130/230A Fall 2013 Lecture 14, Slide 14

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

Answer: To increase energy conversion efficiency 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/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: 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