Definition of forward biasing: When a battery or a potential difference is used to the p-region of p-n junction for making p-region positive then this supplying method is called forward bias. The positive terminal of the battery is connected with the p-region and the negative terminal of the battery is connected with the n-region of the diode. When the positive charges from the battery go to the p-region, it repels the holes. On the other hand the negative charge from the battery go to the n-region and repels electrons. So, the barrier of p-n junction decreases and the resultant field decreases. Since the voltage of the potential barrier is very small about 0.1V to 0.3V, hence application of a small forward voltage decreases the barrier.
The electrons and the holes drift toward the junction and enter into depletion region. As a result the depletion layer becomes very thin and the height of the potential barrier decreases appreciably. At the same time the negative terminal of the battery supplies fresh supply of electrons in the n-type semiconductor and the positive terminal of the battery by attracting electrons from the p-type semiconductor creates new holes. As a result continuous flow of charge i.e. flow of current takes place.
This current is called forward current. This potential difference.
The basic features of forward biasing:
(i)Current is due to the electrons only in external circuit but in p-n junction, it is for majority carriers.
(ii)The current increases with the increases of applied potential difference. Current versus potential difference graph shows a nonlinear behavior.
(iii) There is no current for some applied voltage. It is known knee voltage or cut in voltage. The value of knee voltage is about 0.7V for Si diode and 0.2V for Ge diode.
(iv)After cut in voltage, barrier potential almost reduces to zero and current increases rapidly.
When a p-n junction p-region is connected negative terminal of a battery or a potential difference and n-region is connected with positive terminal of the battery this method of connection is called reverse bias. A circuit diagram is shown in figure.
As the negative terminal of the battery is connected with p-region, the holes of p-region come towards the negative terminal of the battery. On other side such thing happens as positive terminal of the battery is connected with the n-region the electrons come across to the positive charges for attraction. For this reason potential barrier increases and this widens the depletion layer. The junction appears to often a high resistance to pass of current and the circuit current is very low.
In p-region minority carriers come to the depletion region and in n-region minority carriers come to the depletion region and crossing the junction a small current in µA is produced in this circuit. In the reverse biasing majority carriers cannot cross the junction. For the flowing of small current reverse voltage is increased and the corresponding current is recorded. If V versus I graph is represented it will give those information below.
(i) The junction gives very small current (a few µA).
(ii) Current increases slightly with voltage and afterward it becomes constant. That is called reverse saturation current.
(iii) When voltage rises in reverse largely, current suddenly rises. It is called breakdown voltage.