The P and N-type semiconducting materials, when kept touching together, it gives a PN Junction or a semiconductor diode. Popularly, it is known as "Rectifier diode", as it is used for rectification purposes. The other semiconductor diodes used for other than rectification are Power diodes, Zener diodes and Varactor diodes.
PN JUNCTION
The P and N type materials taken separately are of no practical use, but PN Junction has vital importance in the field of electronics, as all semiconductors have one or more PN Junctions.
| When a piece of P-type material is kept with a piece of N-type material their contact surface is called a P-N junction. It is important that the crystal structure should remain continuous at the boundary, i.e., it should have a simple crystal structure. The P-N junctions are the control elements of all solid semi- conductor devices. |
The P-N junction is commercially known as a 'semiconductor diode' and has replaced the vacuum diode in many applications.
Definition: The P-N junction is known as semiconductor diode.
It is also popularly known as 'crystal diode' as it is grown out of a crystal. It is also called junction or rectifier diode'.
P-N JUNCTION THEORY
Suppose a P-type material has been kept just touching an N-type material the so formed and junction has not been connected to any external voltage, the following phenomena occur.
(a) Diffusion : Immediately after the junction is formed, electrons from N region are 'diffused' into the P region crossing the junction (contact surface AB) and recombine with holes. Similarly, some holes from P diffuse into region the N region and recombine with electrons. This establishes a net positive charge in the N region and a net in the P negative charge region.
(b) Formation of Potential Barrier: The process of recombination goes on for sometime. Each recombination 'eliminates' a hole and a free electron. In this process a layer of positive ions near the junctions AB on the N side and a layer of negative ions near the junction on the P side are established. At this stage, further electrons and holes crossing the junction are repelled back by these positive and negative ions respectively, and diffusion is stopped. These positive and negative ions in other words, establish a potential difference (0.3 V in case of Ge and 0.7 V in case of Si) near junction AB. This is called Junction barrier voltage' and can be represented by a cell of 0.3 V (or 0.7 V) which prevents further diffusion.
Definition : The potential difference built up across the junction, which prevents further diffusion is called Junction barrier.
(c) Depletion Layer/Space Charge Region: The above layer of immobile positive and negative ions on both sides of junction is called 'depletion layer' (or space charge region) because all electrons and holes are depleted (emptied) from this region. As the region has immobile (fixed) ions having a particular charge, this is also called the 'space charge region'. The distance of the depletion layer is called its 'width' and the potential difference between the two sides of the layer is called its height. As already mentioned its value is 0.3 V for Ge and 0.7V for Si.
MECHANISM OF CURRENT FLOW IN A PN JUNCTION DIODE
We have seen that a semiconductor diode (P-N junction), when not biased (i.e., not connected with an emf), does not conduct any current due to the formation of a depletion layer. Now we will see what happens when a diode is biased.
A semiconductor diode can be applied bias in two ways :
1. Forward biasing
2. Reverse biasing
1. Forward Biasing (FB)
When an external bias (emf) is applied to a P-N junction diode such that the P side of the junction is connected with positive terminal and N side with negative terminal, such biasing is known as 'Forward biasing'.
In this biasing, the electric field produced by the external battery works opposite to the electric field produced by junction barrier. As a result, the height of junction barrier goes on reducing and at 0.3 V (for Ge) and 0.7 V (for Si), the barrier totally vanishes, i.e., depletion layer disappears. This creates a conducting path and holes from P side and electrons from N side start moving through the junction. This establishes a current; in other words, the diode conducts current which depends upon the amount of applied forward bias.
2. Reverse biasing (RB)
When an external field (bias or emf) is applied to a P-N junction diode, such that the P side of the junction is connected with the negative terminal and N side with the positive terminal, such a biasing is known as "Reverse biasing'.
In this biasing, the electric field produced by the external battery acts in the same direction as the junction barrier, as a result the height of the barrier goes on increasing further. This strengthens the depletion layer still higher and not a single electron or hole can cross through the junction. In other words, no current conduction occurs in the diode.
Conclusion: A semiconductor diode conducts only when it is forward biased. It does not conduct, when it is reverse biased. The diode should, therefore, be always connected in a circuit in forward bias mode.
ZENER AND AVALANCHE BREAKDOWN
(a) Zener Breakdown: This breakdown is different from avalanche breakdown as it takes place in the semiconductor devices with thin depletion layer. In this breakdown the electric field at the depletion layer becomes very large with a very small reverse bias. A small number of electrons jump across the barrier and "Zener breakdown' occurs. In this breakdown, the junction is not damaged permanently. If the reverse bias is removed, the device can regain its original position. This phenomenon occurs in zener diode.
(b) Avalanche Breakdown: However, if number of electrons jumping across the junction becomes too large and the current (due to these electrons) exceeds the "burn value' for the device, 'avalanche breakdown' occurs that may destroy the device permanently.
Thus, the avalanche breakdown is the stage next to the zener breakdown. This type of breakdown takes place in devices with thicker junction like in ordinary semiconductor diodes. In this breakdown, electric field of the depletion layer becomes very high such that at high reverse voltage, electrons acquire very high velocities that they dislodge valence electrons from the semiconductor atom. This is a 'cumulative' process and the field at the depletion layer attains such a high value that the process leads to the flow of an infinite large current which breaks down the junction permanently. The device cannot regain its original position and is burnt off.
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