Working of PN junction diode under forward biased condition with help of following circuit diagram and graph
Region A to B: In this region A to B of the forward characteristics shown in the fig, the forward voltage is small and less than the cut in voltage. Therefore the forward current flowing through the diode is small. With further increase in the forward voltage, it reaches the level of the cut in voltage and the width of depletion region grows on decreasing.
Region B to C: As soon as the forward voltage equals the cut in voltage, current through the diode increases suddenly. The nature of this current is exponential. The large forward current in the region B-C of the forward characteristics is limited by connecting a resistor ‘R’ in series with the diode. Forward current is of the order of a few mA. The forward current is a conventional current that flows from anode to cathode.
Therefore it is considered to be positive current, and the forward characteristics appears in the first quadrant as shown in the fig.
Cut in voltage (Knee Voltage): The voltage at which the forward diode current starts increasing rapidly is known as the cutin voltage of a diode. As shown in fig above, the cut in voltage is very close to the barrier potential. Cut-in voltage is denoted by . Cut-in voltage is also called as Knee voltage. Generally a diode is forward biased above the cut-in voltage. The cut-in voltage for a silicon diode is 0.6V and that for germanium diode is 0.3V.
If the p-region (anode) is connected to the positive terminal of the external DC source and nside (cathode) is connected to the negative terminal of the DC source then the biasing is said to be “forward biasing”.
Due to the negative terminal of external source connected to the n-region, free electrons from n-side are pushed towards the p-side. Similarly the positive end of the supply will push holes from p-side towards the n-side. With increase in the external supply voltage V, more and more number of holes (p-side) and electrons (n-side) start travelling towards the junction as shown in figure. The holes will start converting the negative ions into neutral atoms and the electrons will convert the positive ions into neutral atoms. As a result of this, the width of depletion region will reduce. Due to reduction in the depletion region width, the barrier potential will also reduce. Eventually at a particular value of V the depletion region will collapse. Now there is absolutely no opposition to the flow of electrons and holes. Hence a large number of electrons and holes (majority carriers) can cross the junction under the influence of externally connected DC voltage. The large number of majority carriers crossing the junction constitutes a current called as the forward current.