P-N junction diode and its working
This article explains the construction and operation of a P-N junction diode. A P-N junction diode is created by combining P-type and N-type materials. A semiconductor diode is a unidirectional electronic device, that is current flow only in one direction.
figure shows the symbol of a P-N junction diode. The terminal connected to the P material is called Anode and the terminal connected to the N-type material is called Cathode.
Intrinsic semiconductor: Semiconductor in its purest form is called Intrinsic semiconductor.
Doping: The process of adding other element's material(impurity)to the pure semiconductor material is called Doping.
Extrinsic semiconductor: Adding other elements to pure semiconductor by the process of doping and gets an impure semiconductor(Doped semiconductor)called Extrinsic semiconductor.
P-type Semiconductor material
P-type material is created by adding trivalent impurity(That is the impurity atom have, three valance electrons).
So the P-type material have, majority of holes and a few electrons. Therefor holes are called majority carriers and thermally generated electrons are called minority carriers.
We can see from the figure P-type material have, large amount of acceptor ions and corresponding number of holes.
Impurity atoms which are commonly used for making P-type materials are Arsenic, Antimony and Phosphorus.
N-type Semiconductor material
N-type material is created by adding pentavalent impurity(That is the impurity atom have, five valance electrons).
So the N-type material have, majority of electrons and a few holes. Therefor electrons are called majority carriers and holes are called minority carriers. We can see from the figure N-type material have large amount of donor ions and corresponding number of electrons. Impurity atoms which are commonly used for making N-type materials are Indium, Gallium and Boron.
Formation of a P-N junction
Already discuss in this article, p-type material have large number of holes and n-type material have large number of free electrons. When the p-type and n-type semiconductors are joined , there will be a flow of electrons from n-type material to the p-type material. This process is due to the diffusion process of electrons and holes.Then the carrier flow will stop due to the immobile ions near the junction. N-type material have positively charged immobile ions near the junction and P-type material have negatively charged immobile ions near the junction.This region near the junction acts as a barrier which blocks movement of carriers.This region which blocks the movement of charge carriers, is called depletion region.
P-N junction diode with external bias voltage
We can apply external voltage to the semiconductor diode. So we can connect a semiconductor diode terminals(Anode and cathode)to the external source terminals in two ways.
(1) positive terminal to the anode and negative terminal to the cathode.
(2) positive terminal to the cathode and negative terminal to the anode.
The first one is called, forward biasing and the second is called reverse biasing.
P-N junction diode with forward bias voltage
As already mentioned forward biasing of a P-N junction diode means anode of P-N junction diode is connected to the positive terminal of power source and the cathode is connected to the negative terminal of the power source. If the applied voltage is more than that of the barrier voltage(potential barrier) the device will conduct. We can see from the figure the width of the depletion layer decreases and now the external voltage overcome the barrier potential. So the carriers(electrons and holes) can cross the depletion layer. By this process, a current flow occurs in the device.
P-N junction diode with Reverse bias voltage
In reverse bias condition anode is connected to the negative terminal of the power source and the cathode is connected to the positive terminal of the power source. We can see the figure, the width of the depletion layer increases with reverse bias voltage. And the device will not conduct at this stage. But there is a very small current flow in reverse direction due to minority carriers and it is negligible. This small current is called leakage current or reverse saturation current. If the reverse voltage is above some certain level kinetic energy of minority carriers will increase and breaks the covalent bonds in the structure.So more and more electrons will escape from the covalent bound and a very large current flow occurs which is known as break down of the device.By this large current flow the device may damage. This particular voltage is called break down voltage.