Optoelectronic Devices Chapter 8 Optoelectronic Devices Optoelectronic Devices are semiconductors devices in which the basic particle of light, the photon, plays a major role. Photodetectors Solar cells Light-emitting diodes (LEDs) Semiconductor lasers Photodiodes Optoelectronic Devices

8.1 Photodiodes Photodiodes Two-terminal devices designed to respond to photon absorption. Since modern electronic devices often involves optical as well as electrical signals, photodiodes serve very important functions as electronic devices. Although a bulk semiconductor sample can respond to light signal (photoconductor), most photodiodes are junction devices p-n junction under light illumination

Recall for a p-n junction in dark, the reverse saturation current I0 due to the drift of minority carrier across a junction is a generation current. Carriers generated within the depletion region W are separated by the electric field, electrons being collected in the n-region and holes in the p-region. Minority carriers generated thermally within a diffusion length (Lp or Ln) of each side of the junction diffuse to the depletion region and are swept to the other side by the electric field.

If the junction is illuminated uniformly by photons with h > Eg Added generation rate gop (EHP/cm3.s) participates in this current. The additional current due to illumination is Iop=qAgop(Lp + Ln + W) where: AgopLp is the number of holes created within a diffusion length of the transition region on the n-side AgopLn is the number of electrons created within a diffusion length of the transition region on the p-side AgopW is the number of carriers created within the transition region (3) This current Iop always flows from n to p.

Diode equation for an illuminated p-n junction Analysis: The I-V curve is lowered by an amount proportional to the generation rate (Fig. 8-1c) When the device is short circuited (V=0), there is a net current flowing across the junction from n to p. - This is called the short-circuit current Isc=-Iop When the device is open circuited (I=0), there is a voltage appearing across the junction. This voltage is called the open-circuit voltage Voc, Voc will increase with gop, but cannot exceed the equilibrium contact potential Vo Voc Isc Photovoltaic Effect

8.1.2 Solar Cells - a solar cell is a semiconductor device that converts solar energy directly to electricity by the photovoltaic effect. Some design issues: To utilize a maximum amount of available optical energy, the solar cell must have a large area junction. The surface is coated to reduce reflection. The junction depth must be less than Lp in the n material to allow holes generated near the surface to diffuse to the junction before they recombine. Proper match between Ln, the thickness of p region and the mean optical penetration depth 1/ The series resistance of the device must be small Large V0 means higher doping, but a doping too high will reduce the lifetime of the carriers.

Important Performance Parameters of a Solar Cell Short-circuit current ISC  10 to 100 mA/cm2 Open-circuit voltage Voc  less than 1 Volt Fill factor F.F.=ImVm/IscVoc close to 80% by choosing a proper load Conversion efficiency =F.F. IscVoc /optical power around 18% for a well-made Si cell. I-V characteristic of an illuminated solar cell. The maximum power rectangle is shaded. Operation of a solar cell

The Solar Energy 6000 K The radiative energy output from the sun derives from a nuclear fusion reactive (same as H-bomb). In every second, about 6*1011 kg of hydrogen is converted to helium, with a net mass loss of about 4*103kg, which is converted to 4*1020 J through the Einstein equation (E=mc2) This energy is emitted primarily as electromagnetic radiation in the ultraviolet to infrared region (0.2 to 3 m)

a) Single Crystal solar cells in panel b) Polycrystalline solar panel c) a-Si solar panel

Solar Cell Applications