Varying Ability To Conduct Electrical Current Biology Essay

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A diode is the simplest sort of semiconductor device. a semiconductor is a material with a varying ability to conduct electrical current. Most semiconductors are made of a poor conductor that has had impurities. The process of adding impurities is called doping.

A semiconductor with extra electrons is called N-type material. Since it has extra negatively-charged particles, it is called N-type semi conductor. In N-type material, free electrons move from a negatively-charged area to a positively charged area.

A semiconductor with extra holes is called P-type material. Since it effectively has extra positively-charged particles, it is called P-type semiconductor. Electrons can jump from hole to hole, moving from a negatively-charged area to a positively-charged area. As a result, the holes themselves appear to move from a positively-charged area to a negatively-charged area.

A diode comprises a section of N-type material bonded to a section of P-type material, with electrodes on each end. This arrangement conducts electricity in only one direction. When no voltage is applied to the diode, electrons from the N-type material fill holes from the P-type material along the junction between the layers, forming a depletion zone. In a depletion zone, the semiconductor material is returned to its original insulating state; all of the holes are filled, so there are no free electrons or empty spaces for electrons, and charge can't flow.

In semiconductor the depletion region, also called depletion zone is an insulating region within a conductive, doped semiconductor substance where the mobile charge carriers have diffused away, or have been forced away by an electric field. The only elements left in the depletion region are ionized donor or acceptor impurities. The 'depletion region' is formed from a conducting region by removal of all free charge carriers, leaving none to carry a current.

Definition:-

The Depletion Zone (DZ) is a non conductive zone within a conductive, doped semiconductor material where the charge carriers have been swept away whereas P and N doped semiconductors are conductors and DZ is an insulator. The existence and shape of DZ is easily controlled by voltage applied to the electrodes containing semiconductor.

Let us take a crystal of a pure semiconductor to be doped so that one half of it is p-type and the other half is n-type. The surface at which both the surfaces meet is known as the p-n junction. Such a diode can be constructed by doping one half region of a single crystal of Germanium or Silicon with acceptor impurity and the other half with donor impurity. On either side of the p-n junction where there are hardly any mobile charges, the region is called the depletion region

Depletion Region Details:-

In the p-type region there are holes from the acceptor impurities and in the n-type region there are extra electrons.

When a p-n junction is formed, some of the electrons from the n-region which have reached the conduction band are free to diffuse across the junction and combine with holes.

Filling a hole makes a negative ion and leaves behind a positive ion on the n-side. A space charge builds up, creating a depletion region which inhibits any further electron transfer unless it is helped by putting a forward bias on the junction.

A ZD is formed instantaneously across P-N Junction. The electrons and holes diffuse together into zone with lower concentration of electrons and holes. The best example of this can be provided by ink drop which easily and uniformly diffuses in cup of water. In N-Type semiconductor there are excess of free electrons while as in P -Type semiconductor there are excess of free holes. When P-N doped semiconductors are placed together to form P-N Junction. Excess of free electrons of N -Type migrate to P-Type and in the same way excess of free holes of P -Type migrate to N-Type.in this way the free electrons moves from N type thus leaving behind a positive Donor and movement of free hole from P -Type leaves behind a Negative Acceptor. The injected free electrons come into contact with equal free holes on P- side and thus equal number of electrons and holes are eliminated. Opposite of this occurs on the N side leaving behind charged ions adjacent to the interface in the region with no mobile carriers called ZD .The utilized electrons and holes combine and thus an electric field is created. While uncompensated ions are positive in N side and negative in P-side. This also creates an electric field which provides a force opposing the continued exchange of charged carriers. When the electric field is sufficient to arrest the further transfer of electrons and holes the ZD reaches its equilibrium dimension.

To get rid of the depletion zone, you have to get electrons moving from the N-type area to the P-type area and holes moving in the reverse direction. To do this, you connect the N-type side of the diode to the negative end of a circuit and the P-type side to the positive end. The free electrons in the N-type material are repelled by the negative electrode and drawn to the positive electrode. The holes in the P-type material move the other way. When the voltage difference between the electrodes is high enough, the electrons in the depletion zone are boosted out of their holes and begin moving freely again. The depletion zone disappears, and charge moves across the diode.

If you try to run current the other way, with the P-type side connected to the negative end of the circuit and the N-type side connected to the positive end, current will not flow. The negative electrons in the N-type material are attracted to the positive electrode. The positive holes in the P-type material are attracted to the negative electrode. No current flows across the junction because the holes and the electrons are each moving in the wrong direction. The depletion zone increases. 

Physical aspects of p-n junction:-

1. Doped atoms near the metallurgical junction lose their free carriers by diffusion.

2. As these fixed atoms lose their free carriers, they build up an electric field which opposes the diffusion mechanism.

3. Equilibrium conditions are reached when:

Current due to diffusion = Current due to electric field

In the working of a P-N type semiconductor there are two conditions as:-

Forward bias

Reverse bias

Described as below……

Forward bias condition:-

 An external voltage applied to a PN junction is called BIAS. If, for example, a battery is used to supply bias to a PN junction and is connected so that its voltage opposes the junction field, it will reduce the junction barrier and, therefore the will current flow through the junction. This type of bias is known as forward bias. It causes the junction to offer only minimum resistance to the flow of current.

The positive terminal of the bias battery is connected to the P-type material and the negative terminal of the battery is connected to the N-type material. The positive potential repels holes toward the junction where they neutralize some of the negative ions. At the same time the negative potential repels electrons toward the junction where they neutralize some of the positive ions. Since ions on both sides of the barrier are being neutralized, the width of the barrier decreases. Thus, the effect of the battery voltage in the forward-bias direction is to reduce the barrier potential across the junction and to allow majority carriers to cross the junction. Current flow in the forward-biased PN junction is relatively simple. An electron leaves the negative terminal of the battery and moves to the terminal of the N-type material. It enters the N material, where it is the majority carrier and moves to the edge of the junction barrier. Because of forward bias, the barrier offers less opposition to the electron and it will pass through the depletion region into the P-type material. The electron loses energy in overcoming the opposition of the junction barrier, and upon entering the P material, combines with a hole. The hole was produced when an electron was extracted from the P material by the positive potential of the battery. The created hole moves through the P material toward the junction where it combines with an electron.

In the forward biased condition, conduction is by MAJORITY current carriers (holes in the P-type material and electrons in the N-type material). Increasing the battery voltage will increase the number of majority carriers arriving at the junction and will therefore increase the current flow. If the battery voltage is increased to the point where the barrier is greatly reduced, a heavy current will flow and the junction may be damaged from the resulting heat

Reverse bias:-

If the battery is connected across the junction so that its voltage will increase the junction barrier and thereby offers a high resistance to the flow of current through the junction. This type of bias is known as reverse bias.

To reverse bias a junction diode, the negative battery terminal is connected to the P-type material and the positive battery terminal to the N-type material. The negative potential attracts the holes away from the edge of the junction barrier on the P side, while the positive potential attracts the electrons away from the edge of the barrier on the N side. This action increases the barrier width because there are more negative ions on the P side of the junction, and more positive ions on the N side of the junction. This increase in the number of ions prevents current flow across the junction by majority carriers. However, the current flow across the barrier is not quite zero because of the minority carriers crossing the junction. When the crystal is subjected to an external source of energy, electron-hole pairs are generated. The electron-hole pairs produce minority current carriers. There are minority current carriers in both regions i.e. holes in the N material and electrons in the P material. With reverse bias, the electrons in the P-type material are repelled toward the junction by the negative terminal of the battery. As the electron moves across the junction, it will neutralize a positive ion in the N-type material. Similarly, the holes in the N-type material will be repelled by the positive terminal of the battery toward the junction. As the hole crosses the junction, it will neutralize a negative ion in the P-type material. This movement of minority carriers is called minority current flow, because the holes and electrons involved come from the electron-hole pairs that are generated in the crystal lattice structure, and not from the addition of impurity atoms.

 When a PN junction is reverse biased, there will be no current flow because of majority carriers but a very small amount of current because of minority carriers crossing the junction. However, at normal operating temperatures, this small current may be neglected.

The size of the depletion region in a diode is directly related to the bias. Forward biasing makes the region smaller by repelling the current carriers toward the PN junction. If the applied voltage is large enough, the negative particles will cross the junction and join with the positive particles. This forward biasing causes the depletion region to decrease, producing a low resistance at the PN junction and a large current flow across it. This is the condition for a forward-biased diode. On the other hand, if reverse-bias voltage is applied to the PN junction, the size of its depletion region increases as the charged particles on both sides move away from the junction. This condition produces a high resistance between the terminals and allows little current flow (only in the microampere range). This is the operating condition for the diode, which is nothing more than a special PN junction.

The insulation gap formed by reverse biasing of the diode is comparable to the layer of dielectric material between the plates of a common capacitor. Furthermore, the formula used to calculate capacitance

`

Where

A= plate area 

K = a constant value 

d = distance between plates

Application:-

DZ figures largely in explanation of:-

On/off of diodes

In control of

Emitter Junction Barrier in Bipolar Junction Transistors

Width / length of Conductive Channels in Field Effect Transistors

Width of Dielectric layer in Variable Capacitance Diodes

Depletion width:-

It is the region in which the depletion takes place. principle of charge neutrality in this case relates the depletion width wP in the p-region with acceptor doping NA to the depletion width wN in the n-region with donor doping ND:

The total depletion width in this case is the sum w = wN + wP. . This derivation is based on solving the Poisson equation in one dimension. The electric field is zero outside of the depletion width and therefore Gauss's law implies that the charge density in each region balance. Treating each region separately and substituting the charge density for each region into the Poisson equation eventually leads to a result for the depletion width. The width of the depletion region is as under

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