ECE 4339: Physical Principles of Solid State Devices

1. ECE 4339: Physical Principles of Solid State Devices Len Trombetta Summer 2007 Chapter 9: Optoelectronic Devices ECE 4339 L. Trombetta

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4. Photodiodes Incident light The number of photons decreases exponentially into the material. Also, the absorption rate increases with energy (see Fig. 3.20, next slide) so less light reaches the junction at higher energies. ECE 4339 L. Trombetta

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6. The region over which photo-generated electrons and holes are likely to find their way to the depletion region is (LN + W + LP). Hence where GL is the number of electron/hole pairs created per unit volume per second. The total current is then the sum of IL and whatever current is present with no light (the “dark” current)”: Frequency Response The frequency response of the photodiode is modest: a few 10’s of MHz. If the light intensity is changing faster than that, the photodiode will not respond. This is largely because the diffusion of carriers to the depletion region is slow. ECE 4339 L. Trombetta

7. E Eg The Spectral Response is measured by the current generated as a function of wavelength of incident light. In generating the data, the incident power was the same at all wavelengths, so the number of photons decreases with decreasing wavelength (increasing energy). ECE 4339 L. Trombetta

8. P-I-N Photodiode Intrinsic region can be “tuned” to the absorption depth of the wavelength of interest. Response is faster since most electron/hole pairs are generated in the depletion region and do not have to diffuse to it. ECE 4339 L. Trombetta

9. Si Materials issues: some III-V semiconductors have smaller band gaps than Si and can be used at lower wavelengths (for communications applications). But often these need to be deposited on substrates of different materials, so lattice matching becomes an issue. An important materials system is InGaAs – InP. ECE 4339 L. Trombetta

10. InGaAs – InP p-i-n photodiode In0.53Ga0.47As is the “active” region. This particular composition is lattice-matched to InP, so defects are minimized. InP is transparent at 1.3 mm and 1.55 mm, which are important wavelengths for communications applications. ECE 4339 L. Trombetta

11. Avalanche Photodiode This diode is operated near avalanche breakdown, so that a small photo-generated current induces avalanche multiplication. This improves the signal/noise ratio. Fig 6.12 ECE 4339 L. Trombetta

12. Vm Voc max power rectangle Im Isc Solar Cell For the curve GL = 4Go, the open-circuit voltage and short-circuit current are indicated. The maximum power the cell can deliver is Pmax = VmIm. We define the “fill factor”: Power conversion efficiency: ECE 4339 L. Trombetta

13. Multijunction solar cells use several materials to capture different parts of the solar spectrum. ECE 4339 L. Trombetta

14. “NREL's record-breaking triple-junction cell uses GaInP (1.8 electron-volt [eV] band gap) to absorb ultraviolet and visible-light wavelengths. The GaAs (1.4-eV band gap) layer absorbs near-infrared light, and the Ge layer absorbs the remaining lower-energy infrared light that still exceeds its 0.7-eV band gap.” Naval Research Labs (NRL) research center: http://www.nrel.gov/ncpv/higheff.html ECE 4339 L. Trombetta

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16. LED ECE 4339 L. Trombetta

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18. Streetman, “Solid State Electronic Devices”, 4ed., Prentice Hall ECE 4339 L. Trombetta