Diode

The material made by the combination of a P-type semiconductor and an N-Type semiconductor is known as a diode. The N-type portion of a diode has electrons and the P-type has electron holes. There is a neutral region between these two semiconductors known as a junction barrier. The P-type portion of a diode is known as the anode and the N-type portion of it is known as the cathode. Normally there is a flow of electrons in the diode. An external potential is needed in prefect direction and quantity in order to make flow of current.

Current flow in a diode
A diode can be connected to a battery in two ways, either forward or reversed- biased. First is reverse-bias, the anode of the diode to negative terminal of the battery and the cathode to the positive terminal of the battery. Second method is forward bias, to connect the anode of the diode to the positive terminal of the battery and the cathode to the negative terminal of the battery. In order to make current flow of diode more clear, the effects of connecting the diode with the battery by the first and second method, respectively, are explained with the help of figure given below. It is cleared from this method that on connecting anode to the negative terminal and cathode to the positive terminal of the battery there will be no current flow through the diode. This method of connecting battery is known as reverse biasing of the diode.
 * First method :On connecting negative on anode and positive end on cathode. On seeing this figure, it is clear that the negative terminal of the battery will attract the holes of P-type and electrons of N-type will be attracted by the positive terminal of the battery.As a result of this process, holes of P-type get collected on the end of the P-type and electrons will get collected on the end of N-type, as it is shown in the figure. Now if the figure is seen clearly, then it is clear that junction barrier near the junction will widen and, as a result, there will no be current between the junction. The N type semiconductor of the diode.

When there is reduction in the pressure from a definite pressure than there will be no current flow through the diode because of junction resistance.Connection of diode with battery in this position in which positive terminal of the battery connected to anode and negative terminal to cathode is known as forward biasing of diode. In forward biasing junction resistance because very low.
 * Second Method :On connecting the positive terminal to the anode and negative terminal of the battery to the cathode of the diode. On seeing the above figures it is clear that electrons of the N-type are repelled by the negative terminal of the battery towards the junction and, in the same way, holes of the P-type are repelled towards the junction by the positive terminal of the battery. When the positive and negative voltages of the battery on the P and N type semiconductors are increased to such a level that holes of P-type start filling with the electrons of N-type means junction resistance of N-type gets eliminated than there is current flow through diode.

What is rectification?
Ans. Mostly, electrical energy is used in the form of AC, but at some places it is used in the form of DC. the process of making DC from AC is known as rectification.

Why is a diode known as a rectifier?
Ans. After knowing the characteristics of a diode, it is clear that only during forward bias current flows through the diode. No current flows through the diode during reverse bias. In the one cycle of AC, the first half of the cycle is positive and second half cycle is negative.

Types of Diode

 * Rectifier diode
 * Signal diode
 * Zener diode
 * Varactor diode
 * Schottky diode
 * Tunnel diode
 * Light emmiting diode
 * Photo sensitive diode

Radio demodulation
The first use for the diode was the demodulation of amplitude modulated (AM) radio broadcasts. The history of this discovery is treated in depth in the radio article. In summary, an AM signal consists of alternating positive and negative peaks of voltage, whose amplitude or 'envelope' is proportional to the original audio signal, but whose average value is zero. The diode (originally a crystal diode) rectifies the AM signal, leaving a signal whose average amplitude is the desired audio signal. The average value is extracted using a simple filter and fed into an audio transducer, which generates sound.

Power conversion
Rectifiers are constructed from diodes, where they are used to convert alternating current (AC) electricity into direct current (DC). Similarly, diodes are also used in Cockcroft-Walton voltage multipliers to convert AC into very high DC voltages.

Over-voltage protection
Diodes are frequently used to conduct damaging high voltages away from sensitive electronic devices. They are usually reverse-biased (non-conducting) under normal circumstances, and become forward-biased (conducting) when the voltage rises above its normal value. For example, diodes are used in stepper motor and relay circuits to de-energize coils rapidly without the damaging voltage spikes that would otherwise occur. Many integrated circuits also incorporate diodes on the connection pins to prevent external voltages from damaging their sensitive transistors. Specialized diodes are used to protect from over-voltages at higher power (see Diode types above).

Logic gates
Diodes can be combined with other components to construct AND and OR logic gates. This is referred to as diode logic.

Ionising radiation detectors
In addition to light, mentioned above, semiconductor diodes are sensitive to more energetic radiation. In electronics, cosmic rays and other sources of ionising radiation cause noise pulses and single and multiple bit errors. This effect is sometimes exploited by particle detectors to detect radiation. A single particle of radiation, with thousands or millions of electron volts of energy, generates many charge carrier pairs, as its energy is deposited in the semiconductor material. If the depletion layer is large enough to catch the whole shower or to stop a heavy particle, a fairly accurate measurement of the particle's energy can be made, simply by measuring the charge conducted and without the complexity of a magnetic spectrometer or etc. These semiconductor radiation detectors need efficient and uniform charge collection and low leakage current. They are often cooled by liquid nitrogen. For longer range (about a centimetre) particles they need a very large depletion depth and large area. For short range particles, they need any contact or un-depleted semiconductor on at least one surface to be very thin. The back-bias voltages are near breakdown (around a thousand volts per centimetre). Germanium and silicon are common materials. Some of these detectors sense position as well as energy. They have a finite life, especially when detecting heavy particles, because of radiation damage. Silicon and germanium are quite different in their ability to convert gamma rays to electron showers.

Semiconductor detectors for high energy particles are used in large numbers. Because of energy loss fluctuations, accurate measurement of the energy deposited is of less use.

Temperature measuring
A diode can be used as a temperature measuring device, since the forward voltage drop across the diode depends on temperature. This temperature dependence follows from the Shockley ideal diode equation given above and is typically around -2.2 mV per degree Celsius.

Charge coupled devices
Digital cameras and similar units use arrays of photo diodes, integrated with readout circuitry.