Electrical Engineering ⇒ Topic : Action of P-N Junction Diode

Action of P-N Junction Diode

In N-type semiconductor, the majority carriers are electrons and in P-type, the majority carriers are holes. When a battery is connected to a P-N junction with its positive terminal connected to P-type and negative terminal to N-type as shown in Figure 1 (a), the holes of P-type will move towards the junction between P and N and the electrons of N-type will move towards the junction between P and N. So the electrons and holes rush towards each other and recombine. Due to electron-hole recombination at the junction, the covalent bond in P-region near the positive end of the battery breaks down liberating an electron. The liberated electron enters the positive terminal of the battery and a new hole is created, which moves towards the junction. Thus, a continuous movement of electrons and holes takes place at the junction and lasts as long as the external battery remains in the circuit. As the flow of electrons and holes is very large, there is a large current generated in the circuit even for the small potential difference applied. Such a circuit is said to be forward biased.

                                          figure (1) Biasing of semiconductor diodes

In Figure 1 (b), the negative terminal of the battery is connected to P-type and positive terminal of battery is connected to N-type. The free electrons in N-type moves towards the positive terminal of battery. So a few electrons and holes are left for recombination. This leads to increase the resistance of the circuit and very little current flows across the junctions. This current is known as leakage current. The circuit is said to be reverse biased

   Figure (2) shows the forward and reverse bias characteristics of a semiconductor diode. Since it is a plot of potential difference across the P-N junction and the current through the junction, it is also called the volt-ampere characteristic for the P-N junction diode

                                          Figure (2) Forward and reverse bias characteristics.

For a voltage of 2 V during forward bias, the current is very high. During reverse bias, the leakage current is very small and it is very small even up to a voltage less than V_Z There is a rise of current in the reverse direction for V = V_z due to breakdown of covalent bonds in a very large number of covalent bonds. This voltage is known as Zener voltage. It is defined as the reverse voltage at which breakdown occurs. It is also called the breakdown voltage.