PN Junction


When a P-type semiconductor is joined to a N-type semiconductor such that the crystal structure remains continuous at the boundary, a PN junction is formed. A PN junction forms a very useful device and can be called a semiconductor diode, PN junction diode, or simply a crystal diode. Simply joining two pieces together cannot form a PN junction, because it would produce a discontinuous crystal structure. Special techniques are required to assemble a PN junction. Normally P-type carriers are to the left of the junction and N-type carriers to the right.

PN junction PN JunctionP-Type combining with N-Type

 Since the junction diode is a two terminal device, the application of voltage across its terminals leaves three possibilities:

  1. No bias
  2. Forward bias
  3.  Reverse bias

No bias – This occurs when there is no external voltage applied. The holes from the P-region diffuse into the N-region. They then combine with the free electrons in the N-region. The free electrons from the N-region diffuse into the P-region. These electrons combine with the holes.

Forward bias – When a battery is connected to the PN junction diode such that the positive terminal is connected to the P-side and negative terminal to the N-side, forward bias is created. When the PN junction is forward biased, the holes are repelled from the positive terminal and are forced to move towards the junction.  Similarly, the electrons are repelled from the negative terminal of the battery and drift towards the junction. The current within the PN junction is the sum of electron current and hole current.

Reverse bias – A voltage source is connected with the positive terminal attached to the N-region and negative to the P-region. The holes in the P-region are attracted towards the negative terminal of the applied voltage, and the electrons in the N-region are attracted to the positive terminal. Thus the majority carriers are drawn away from the junction. This increases the barrier potential, which makes it more difficult for the carriers to diffuse across the junction.


Resistance in a PN junction

  1. Static Resistance – is calculated when the diode is connected in a d.c circuit. This resistance is also known as d.c resistance or static resistance.  It is the ratio of d.c voltage across the diode to d.c current flowing through it.
  2. Dynamic Resistance – is equal to the slope of V-I of the diode


Diode Capacitance

There are usually two types of diode capacitances:

  1. Depletion layer capacitance or transition capacitance
  2. Diffusion or storage capacitance

Depletion Layer Capacitance – Exists in reverse biased. This capacitance arises due to immobile charges at the junction varying with the applied voltage.

Diffusion Capacitance – Exists in a forward biased junction. This capacitance occurs due to the arrangement of the minority carrier density. The value of this capacitance is much larger then that of depletion capacitance.


Different PN Junction Ratings

  1. Maximum Power Rating – Is defined as the max power that a PN junction can dissipate without damaging it
  2. Peak Inverse Voltage Rating – Can be defined as the max value of reverse voltage that a PN junction can withstand without taking damage.
  3. Maximum Forward Current Rating – Is described as the max value of forward current that a PN junction diode can carry without damaging itself.

For more information on PN junctions, watch the video

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