SWITCHING CHARACTERISTICS OF DIODE
- When diode is switched from forward biased to the reverse biased state or viceversa, it takes finite time to attain a steady state.
- This time consists of a transient and an interval of time before the diode attains a steady state. The behavior of the diode during this time is called switching characteristics of the diode.
- In the forward-bias state, there are a large number of electrons from the n
- side diffusing into p side and a large number of holes diffusing into n side from p side. This diffusion process establishes a large number of minority carriers in each material.
- When forward biased, let n is concentration of electrons on p side at thermal equilibrium and p is concentration of holes on n side thermal equilibrium. This is concentration level far away from the junction.
- It increases towards the junction and becomes n and Pn on p and n side respectively in steady state. These minority charge carriers are supplied from other side of the junction, where those carriers are majority in number.
- When the diode is reverse biased, again far from the junction the minority charge concentration is n on p side and Pno on n side.
- In reverse biased condition, as they approach the junction, they quickly cross the junction. Hence minority carrier concentration decreases to zero at the junction in steady state. This is .shown in the Fig.
- Now when a forward biased diode is suddenly reverse biased, it takes finite time to change the minority charge carrier concentration and to attain new steady state value.
- The diode can not attain steady state till the minority charge carrier concentration changes from that corresponding to the forward biased to that corresponding to the reverse biased.
- Till the excess charge carrier concentration pn-pno and np-npo reduces to zero, the diode continues to conduct. This current is decided by the current limiting external resistance connected in the circuit.
- Hence in switching applications, the time required by the diode to attain new steady state, plays an important role.
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