Question

Why does current flow in the forward-bias direction but not flow in the reverse-bias direction in a PN junction?

Answer 1

Hint: In the forward biased mode, the P site of the P-N junction diode is connected to the positive terminal of the battery and the N side is connected to the negative terminal. In reverse biased mode, the P side is connected to the negative terminal of the battery and therefore the N side is connected to the positive terminal.

Complete answer:
A PN junction diode is forward biased when the P side and the N side are connected to the positive and negative terminals of a battery. In reverse bias, they're connected to the other terminals. When a contact diode has forward biased the electrons from the negative end of the battery travel into the N side of the diode and cross over to the P side and combine with the holes Electrons are also emitted from the P side into the positive terminal of the battery producing more free holes that can travel over to the N side and then it will combine with the electrons. Therefore the depletion layer along with the barrier potential of the junction decreases and will decrease the overall resistance of the diode.

When the diode is reverse biased, the battery attracts and pulls out the bulk charge carriers from all sides of the junction diode. The holes from the P side and electrons from the N side and the holes from the P side enter the negative terminal of the battery and the electrons go into the positive terminal of the battery. This widens the depletion region at the junction and increases the barrier potential. Hence, it becomes more difficult for the current to pass through the diode and the resistance of the diode increases very much. The diode almost acts sort a resistor of infinite resistance and it acts like a circuit.

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Note: This behavior of a P-N junction diode under different kinds of bias is what makes it so unique and useful. Hence, due to the above behavior, P-N junction diodes find a wide range of practical applications ranging from acting as rectifiers to photovoltaic cells