PHOTODIODES




A photodiode is a p-n junction  structure. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a free electron and a positively charged electron hole. This mechanism is also known as the photoelectric effect. If the absorption occurs in the junction's depletion region, these carriers are swept from the junction by the built-in field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. This photocurrent is the sum of both the dark current (without light) and the light current, so the dark current must be minimised to enhance the efficiency of the device in light.
Following  are various modes of operations of these photodiodes.

Photovoltaic mode
When used in forward  bias  or photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up. This mode accomplish the photovoltaic effect, which is the basis for solar cells – a  solar cell is just a large area photodiode.

Photoconductive mode

In this mode the diode is often reverse biased , reducing the response time at the expense of increased noise. This increases the width of the depletion layer, which decreases the junction's capacitance resulting in faster response times. The reverse bias induces only a small amount of current (known as saturation or back current) along its direction while the photocurrent remains virtually the same. For a given spectral distribution, the photocurrent is linearly proportional to the illuminance. Although this mode is faster, the photoconductive mode tends to exhibit more electronic  noise i.e.  avalanche noise.
                                                                                                                                        
Avalanche breakdown mode
Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. Materials conduct electricity if they contain mobile charge carriers. There are two types of charge carrier in a semiconductor: free electrons and electron holes. A fixed electron in a reverse-biased diode may break free due to its thermal energy, creating an electron-hole pair. If there is a voltage gradient in the semiconductor, the electron will move towards the positive voltage while the hole will "move" towards the negative voltage. Most of the time, the electron and hole will just move to opposite ends of the crystal and stop. Under some circumstances i.e. when the voltage is high enough, the free electron may move fast enough to knock other electrons free, creating more free-electron-hole pairs (ie. more charge carriers), increasing the current. Fast-"moving" holes may also result in more electron-hole pairs being formed. In a fraction of a nanosecond, the whole crystal begins to conduct. Avalanche breakdown usually destroys regular diodes, but avalanche diodes are designed to break down this way at low voltages and can survive the reverse current. The voltage at which the breakdown occurs is called the breakdown voltage. 

Bipolar junction transistor mode
A bipolar junction transistor (BJT) is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes. Charge flow in a BJT is due to bidirectional diffusion of charge carriers across a junction between two regions of different charge concentrations. This mode of operation is contrasted with unipolar transistors, such as field-effect transistors, in which only one carrier type is involved in charge flow due to drift. By design, most of the BJT collector current is due to the flow of charges injected from a high-concentration emitter into the base where they are minority carriers that diffuse toward the collector, and so BJTs are classified as minority-carrier devices.