Electrical Engineering ⇒ Topic : Diode as Switch
DIODE AS SWITCH
The P-N diode can function very well as a switch with ON-OFF characteristics as it conducts on forward bias and almost does not conduct on reverse bias. As a switch in computer applications, the speed of operation from OFF to ON and ON to OFF is important.
The OFF-ON time or ON time is designated TON when the diode is changed from reverse to forward bias. It is referred to as forward recovery time tfr. It is the time required for the diode to reach 90% of the final value of voltage from 10%.
The ON-OFF time is designated ToFF. This is the time required to switch the diode from forward to reverse bias. ToFF time is more important than TON time and is the main cause of delay in switching operation.
As we observe from junction law on forward biasing, the holes on the p-side diffuse across the junction decay exponentially while the holes on the N-side which are minority carriers multiplying exponentially on the N-side. When reverse biased, the minority carriers are virtually zero near the junction. So the excess minority carriers on the inside of N-side should be swept back to the P-side to bring the P-N diode to equilibrium condition. While the excess minority carriers traverse back on reverse biasing, it causes a current flow in the reverse direction. After a time t1, the charge density decreases exponentially until it reaches the saturation current IS. The time from tO to t1 of current flow in the reverse direction is called storage time TS. The time for reverse current to decay exponentially to reverse saturation current IS is called transition time Tt. So, the time interval for the current to fall from ID to IS (see Figure (a) is the reverse recovery time Trr. The recovery time is stipulated as a specified diode current or resistance. Trr ranges from less than a nanosecond (ns) to as high as 1 microsecond (μS). Computer diodes have a typical time of 4 ns.
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