P-N junction semiconductor diode

P-N junction semiconductor diode

What is a P-N junction semiconductor diode?

A P-N junction diode is a two-terminal or two-electrode semiconductor device, which allows the electric current in only one direction while blocking the electric current in the opposite or reverse direction. If the diode is forward-biased, it allows the electric current to flow. On the other hand, if the diode is reverse-biased, it blocks the electric current flow. A P-N junction semiconductor diode is also called a p-n junction semiconductor device.

In n-type semiconductors, free electrons are the majority charge carriers whereas in p-type semiconductors, holes are the majority charge carriers. When the n-type semiconductor is joined with the p-type semiconductor, a P-N junction is formed. The P-N junction, which is formed when the p-type and n-type semiconductors are joined, is called a p-n junction diode.

The p-n junction diode is made from the semiconductor materials such as silicon, germanium, and gallium arsenide. For designing the diodes, silicon is preferred over germanium. The p-n junction diodes made from silicon semiconductors work at higher temperatures when compared with the p-n junction diodes made from germanium semiconductors.

The basic symbol of P-N junction diode under forward bias and reverse bias is shown in the below figure

In the above figure, the arrowhead of a diode indicates the conventional direction of electric current when the diode is forward-biased (from a positive terminal to the negative terminal). The holes which move from the positive terminal (anode) to the negative terminal (cathode) are the conventional direction of the current.

The free electrons moving from the negative terminal (cathode) to the positive terminal (anode) actually carry the electric current. However, due to the convention we have to assume that the current direction is from a positive terminal to a negative terminal.

  • Biasing of p-n junction semiconductor diode

The process of applying the external voltage to a P-N junction semiconductor diode is called biasing. External voltage to the P-N junction diode is applied in any of the two methods: forward biasing or reverse biasing.

If the P-N junction diode is forward-biased, it allows the electric current to flow. Under the forward-biased conditions, the p-type semiconductor is connected to the positive terminal of the battery whereas; the n-type semiconductor is connected to the negative terminal of the battery.

If the P-N junction diode is reverse-biased, it blocks the electric current flow. Under the reverse-biased condition, the p-type semiconductor is connected to the negative terminal of the battery whereas; the n-type semiconductor is connected to the positive terminal of the battery.

  • Terminals of P-N junction diode

Generally, terminal refers to a point or place at which any object begins or ends. For example, a bus terminal or terminus is a place at which all the buses begin or end. Similarly, in a  p-n junction diode, the terminal refers to a point at which charge carriers begin or end.

The P-N junction diode consists of two terminals: positive and negative. At the positive terminal, all the free electrons will end and all the holes will begins whereas at the negative terminal all the free electrons will begin and all the holes will ends.

  • Terminals of the diode under forward bias

In forward biased P-N junction diode (p-type connected to the positive terminal and n-type connected to the negative terminal), the anode terminal is a positive terminal whereas the cathode terminal is a negative terminal.

An anode terminal is a positively charged electrode or conductor, which supplies holes to the p-n junction. In other words, the anode or anode terminal or positive terminal is the source of positive charge carriers (holes), the positive charge carriers (holes) begin their journey at the anode terminal and travel through the diode and end at the cathode terminal.

 

The cathode is the negatively charged electrode or conductor, which supplies free electrons to the p-n junction. In other words, the cathode terminal or negative terminal is the source of free electrons, the negative charge carriers (free electrons) begin their journey at the cathode terminal and travel through the diode and ends at the anode terminal.

The free electrons are attracted towards the anode terminal or positive terminal whereas the holes are attracted towards the cathode terminal or negative terminal.

  • Terminals of the diode under reverse bias

If the diode is reverse biased (p-type connected to the negative terminal and n-type connected to the positive terminal), the anode terminal becomes a negative terminal whereas the cathode terminal becomes a positive terminal.

The anode terminal or negative terminal supplies free electrons to the p-n junction. In other words, the anode terminal is the source of free electrons, the free electrons begin their journey at the negative or anode terminal and fills a large number of holes in the p-type semiconductor. The holes in the p-type semiconductor get attracted towards the negative terminal. The free electrons from the negative terminal cannot move towards the positive terminal because the wide depletion region at the p-n junction resists or opposes the flow of free electrons.

Cathode terminal or positive terminal supplies holes to the p-n junction. In other words, the cathode terminal is the source of holes, the holes begin their journey at the positive or cathode terminal and occupies the position of the electron in the n-type semiconductor. The free electron in the n-type semiconductor gets attracted toward the positive terminal. The holes from the positive terminal cannot move towards the negative terminal because the wide depletion region at the p-n junction opposes the flow of holes.

Silicon and germanium semiconductor diodes

  • For designing the diodes, silicon is preferred over germanium. 
  • The p-n junction diodes made from silicon semiconductors work at high temperatures than the germanium semiconductor diodes.
  • The forward bias voltage for a silicon semiconductor diode is approximately 0.7 volts whereas for a germanium semiconductor diode is approximately 0.3 volts.
  • Silicon semiconductor diodes do not allow the electric current flow, if the voltage applied on the silicon diode is less than 0.7 volts.
  • Silicon semiconductor diodes start allowing the current flow, if the voltage applied on the diode reaches 0.7 volts. 
  • Germanium semiconductor diodes do not allow the electric current flow, if the voltage applied on the germanium diode is less than 0.3 volts.
  • Germanium semiconductor diodes start allowing the current flow, if the voltage applied on the germanium diode reaches 0.3 volts.
  • The cost of silicon semiconductors is low when compared with germanium semiconductors.

Advantages of p-n junction diode

The P-N junction diode is the simplest form of all semiconductor devices. However, diodes play a major role in many electronic devices.

  • A P-N junction diode can be used to convert the alternating current (AC) to the direct current (DC). These diodes are used in power supply devices.
  • If the diode is forward-biased, it allows the current flow. On the other hand, if it is reverse-biased, it blocks the current flow. In other words, the p-n junction diode becomes on when it is forward biased whereas the p-n junction diode becomes off when it is reversed biased (I.e. it acts as a switch). Thus, the p-n junction diode is used as an electronic switch in digital logic circuits.

Types of Diodes

The various types of diodes are as follows:

  1. Zener diode
  2. Avalanche diode
  3. Photodiode
  4. Light Emitting Diode
  5. Laser diode
  6. Tunnel diode
  7. Schottky diode
  8. Varactor diode
  9. P-N junction diode

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