Understanding the Diode Transient

The pn junction is typically used as an electrical switch. In forward bias, referred to as the on state, a relatively large current can be produced by a small applied voltage; in reverse bias, referred to as the off state, only a very small current will exist. Our primary interest in circuit applications is the speed of the pn junction diode in switching states. We will qualitatively discuss the transients that occur and the charge storage effects. We will simply state the equations that describe the switching times without any mathematical derivations.

 

The Turn-off Transient:- Suppose we want to switch a diode from the forward bias on state to the reverse-bias off state. There is excess minority carrier charge stored in both the p and n regions of the diode. The excess minority carrier concentrations at the space charge edges are supported by the forward-bias junction voltage. When the voltage is switched from the forward- to the reverse-bias state, the excess minority carrier concentrations at the space charge edges can no longer be supported and they start to decrease, as shown in Figure. 

 But the junction capacitances do not allow the junction voltage to change instantaneously. After storage time, the voltage across the junction will begin to change.

When a pn junction is switched from forward bias to reverse bias, the stored excess minority carrier charge must be removed from the junction. The time required to remove this charge is called the storage time and is a limiting factor in the switching speed of a diode. If the minority carrier are removed at fast then the diode turns of rapidly. So to switch the diode quickly, we need to be able to produce a large reverse current as well as have a small minority carrier lifetime. In the design of diode circuit then, the designer must provide a path for the transient reverse-bias current pulse in order to be able to switch the diode quickly.

The Turn-on Transient:- The turn-on transient occurs when the diode is switched from its "off" state into the forward-bias "on" stale. The turn-on can be accomplished by applying a forward-bias current pulse. The process is completed in two stages.

The first stage of turn-on occurs very quickly and is the length of time required to narrow the space charge width from the reverse-bias value to its thermal-equilibrium value. During this time, ionized donors and acceptors are neutralized as the space charge width narrows.

The second stage of the turn-on process is the time required to establish the minority-carrier distributions. During this time the voltage across the junction is increasing toward its steady-state value.

A small turn-on time is achieved if the minority carrier lifetime is small and if the forward-bias current is small.