Zener Diode - this device is designed to operate in the breakdown region under large reverse bias. Breakdown occurs due to the overlapping of the bands in the p and n-type materials. This device is used in voltage stabilisers since fluctuations in current produce very little change in voltage in this region.
Tunnel Diode - these devices are heavily doped with a narrow depletion region. A large current will flow under forward bias due to tunneling. Such devices are useful as oscillators and amplifiers.
Light Emitting Diodes (LEDs) - when a pn junction is forward biased the barrier is lowered from V0 to V0-V. This facilitates the passage of majority carriers (electrons) from the n-type material into the p-type material where they produce a population inversion ( ie excess electrons in the conduction band of the p-type material compared with the thermal electrons). In this situation the recombination of electrons and holes will exceed thermal excitation and so light will be emitted by this process. Different colours are possible depending on the material used. Crystalline silicon is not used for LEDs because its band gap is in the infrared. The most popular materials are amorphous silicon and GaP.
Solar Cells - These operate on the reverse process to LEDs. If a pn junction is illuminated with light of sufficient energy, electron-hole pairs will be formed. The electric field in the junction region will separate these pairs by sweeping the electrons into the p-layer and the holes into the n-layer . This separation of the electron-hole pairs by the built in field of the pn junction is called the photovoltaic effect. The electrons and holes may recombine by passing the electrons through an external circuit where they do work before recombining with the excess holes in the n-layer. It consists of a number of layers:
an outer glass coating to protect it from the weather
an anti-reflection coating to reduce light reflection
front contacts or fingers of metal connected to a bus bar
a p-layer
an n-layer
a back contact of metal
a substrate to support the cell
The individual solar cells are interconnected in series or in parallel to produce a solar panel. Silicon solar cells generally provide about 0.5V and 40mA cm-2 under normal sunlight giving an efficiency of conversion of sunlight to electricity of about 15 % .
The efficiency ? of a solar cell is given by
where ff is the fill factor for the cell and 100mW cm- 2 is the approximate intensity of sunlight on the earth's surface.
For a solar cell the current is the photocurrent Iph caused by illumination minus the thermionic diffusion current for the diode. Thus
For a real solar cell we must include a shunt resistance and a series resistance in the analysis. This yields
