DIODE

Diode:

A semiconductor PN junction diode is a two terminal electronic device that allows current in only one direction. The diode is firmed by doping a semiconductor (like silicon or germanium) with trivalent impurity (e.g. Boron or Aluminium) from one end to form p-type region and with pentavalent impurity like Phosphorous from the other end to form n-type region on the other end. The metal contacts taken out from p-region and n-region are called anode and cathode respectively. Please refer the applet to know the fabrication process of the diode

pn Junction:

As the pn junction is formed, holes from p-region diffuse across the junction and recombine with the electrons in the n-region near the junction resulting in formation of positive ions in the n-region near the junction. Similarly, electrons from n-region diffuse across the junction and recombine with the holes in p-region near the junction to form negative ions. Thus there exists a narrow region extended in both the regions, that does not have any mobile charge carriers (neither holes nor electrons) is called depletion region, the region depleted of charge carriers. This region has negative ions on one side and positive ions on the other, resulting in an electric field across the junction. The resulting electric field opposes further diffusion of hole and electrons across the junction as the voltage drop across the junction acts as a barrier for any charge carrier to cross the junction. The more the barrier, fewer charge carriers will be able to overcome it and result in lower magnitude of diffusion current ID.  This electric field also causes the minority carriers from respective junctions to drift across the junction and constitute drift current IS which flows in the direction opposite to that of diffusion current. This component of current is due to minority carriers which are thermally generated and hence this current strongly depends upon temperature.

At equilibrium, these two currents are equal and opposite i. e. ID = IS. This condition is maintained by barrier potential. The built-in potential across the junction is given by,

                       

  , called thermal voltage,   , ND  are doping concentrations of p side and n side. Typically the value of V0 at room temperature ranges between 0.6 to 0.8V

Forward biasing a pn junction:

The diode is said to be forward biased when the positive terminal of the battery is connected to anode and negative terminal to cathode. Under these conditions, the holes in p-region are repelled by the positive terminal of the battery and electrons are repelled by the negative terminal. These charge carriers are then pushed across the junction. The applied voltage supplies majority charge carriers to both the regions. These charge carriers neutralise some of the uncovered bound charges causing the space charge region to store less charge reducing the depletion region barrier voltage. This helps the electrons from n-region and holes from p-region to cross the junction easily. As the applied voltage increased, the diode current increases exponentially once the potential barrier is overcome and is given by,

where, Is is the saturation current which is given by,

 Is is directly proportional to junction area A and     which is a strong function of temperature. η is called ideality factor 1 for Ge and 2 for Si. Fig 3 shows the I-V characteristics od pn junction diode. The characeristic in first quadrant (positive  voltage and current) shows the exponential relationship between diode current and diode voltage. 

Hole current:

The positive terminal of the battery supplies large number of holes to the p-type semiconductor and attracts or accepts large number of free electrons from the n-type semiconductor. In other words, the large number of holes begins their journey at the positive terminal whereas the large number of free electrons finishes their journey at the positive terminal. The holes, which begin their journey from the positive terminal, produce a large positive electric field at p-type semiconductor. The direction this positive electric field is opposite to the direction of negative electric field of depletion region (negative ions) near the p-n junction.  Due to the large number of positive charge carriers (holes) at p-type semiconductor, they get repelled from each other and try to move from higher concentration region (p-type semiconductor) to a lower concentration region (n-type semiconductor). However, before crossing the depletion region, some of the holes finds the negative ions and replaces the electrons position with holes. Thus, the holes are disappeared.The negative ions, which lose the electrons, become neutral atoms. Thus, the depletion region or negative ions (negative electric field) at p-type semiconductor near the p-n junction decreases until it disappears.

 The remaining holes will cross the depletion region and attracted to the negative terminal of battery or terminate at the negative terminal of battery. Thus, the positive charge carriers (holes) that are crossing the depletion region carry the electric current from one point to another point in the p-n junction diode.

Reverse biasing a PN junction:

When the polarity of the battery is reversed, the cathode is connected to positive end of the battery and anode to negative. This causes electrons from n-region to get attracted to positive terminal of the battery and holes by the negative terminal. This causes uncovered positive bound charge to increase in n-region and negative bound charge to increase in p-region resulting in wider depletion region and higher barrier potential reducing the diffusion current ID. The small reverse current flows through the diode which is due to thermally degenerated charge carriers (minority carriers) in respective regions. The reverse current is nearly equal to IS and is of the order of a few nanoamperes. This current is independent of the barrier voltage but strongly depends on the temperature

type of diode:

1. Backward diode:   This type of diode is sometimes also called the back diode. Although not widely used, it is a form of PN junction diode that is very similar to the tunnel diode in its operation. It finds a few specialist applications where its particular properties can be used.

1. BARITT diode:   This form of diode gains its name from the words Barrier Injection Transit Time diode. It is used in microwave applications and bears many similarities to the more widely used IMPATT diode.

1. Gunn Diode:   Although not a diode in the form of a PN junction, this type of diode is a semiconductor device that has two terminals. It is generally used for generating microwave signals. 

1. Laser diode:   This type of diode is not the same as the ordinary light emitting diode because it produces coherent light. Laser diodes are widely used in many applications from DVD and CD drives to laser light pointers for presentations. Although laser diodes are much cheaper than other forms of laser generator, they are considerably more expensive than LEDs. They also have a limited life. 

1. Light emitting diodes:   The light emitting diode or LED is one of the most popular types of diode. When forward biased with current flowing through the junction, light is produced. The diodes use component semiconductors, and can produce a variety of colours, although the original colour was red. There are also very many new LED developments that are changing the way displays can be used and manufactured. High output LEDs and OLEDs are two examples. 

1. Photodiode:   The photo-diode is used for detecting light. It is found that when light strikes a PN junction it can create electrons and holes. Typically photo-diodes are operated under reverse bias conditions where even small amounts of current flow resulting from the light can be easily detected. Photo-diodes can also be used to generate electricity. For some applications, PIN diodes work very well as photodetectors. 

1. PIN diode:   This type of diode is typified by its construction. It has the standard P type and N-type areas, but between them there is an area of Intrinsic semiconductor which has no doping. The area of the intrinsic semiconductor has the effect of increasing the area of the depletion region which can be useful for switching applications as well as for use in photodiodes, etc. 

1. PN Junction:   The standard PN junction may be thought of as the normal or standard type of diode in use today. These diodes can come as small signal types for use in radio frequency, or other low current applications which may be termed as signal diodes. Other types may be intended for high current and high voltage applications and are normally termed rectifier diodes.

1. Schottky diodes:   This type of diode has a lower forward voltage drop than ordinary silicon PN junction diodes. At low currents the drop may be somewhere between 0.15 and 0.4 volts as opposed to 0.6 volts for a silicon diode. To achieve this performance they are constructed in a different way to normal diodes having a metal to semiconductor contact. They are widely used as clamping diodes, in RF applications, and also for rectifier applications. 

1. Step recovery diode:   A form of microwave diode used for generating and shaping pulses at very high frequencies. These diodes rely on a very fast turn off characteristic of the diode for their operation. 

1. Tunnel diode:   Although not widely used today, the tunnel diode was used for microwave applications where its performance exceeded that of other devices of the day. 

1. Varactor diode or varicap diode:   This type of diode is used in many radio frequency (RF) applications. The diode has a reverse bias placed upon it and this varies the width of the depletion layer according to the voltage placed across the diode. In this configuration the varactor or varicap diode acts like a capacitor with the depletion region being the insulating dielectric and the capacitor plates formed by the extent of the conduction regions. The capacitance can be varied by changing the bias on the diode as this will vary the width of the depletion region which will accordingly change the capacitance. 

1. Zener diode:   The Zener diode is a very useful type of diode as it provides a stable reference voltage. As a result it is used in vast quantities. It is run under reverse bias conditions and it is found that when a certain voltage is reached it breaks down. If the current is limited through a resistor, it enables a stable voltage to be produced. This type of diode is therefore widely used to provide a reference voltage in power supplies. Two types of reverse breakdown are apparent in these diodes: Zener breakdown and Impact Ionisation. However the name Zener diode is used for the reference diodes regardless of the form of breakdown that is employed.

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