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ECSE-2210 Microelectronics Technology
Class Activity 17 – Solution
1. What is the root cause of the delay in switching from the on-state to the off state?
There are stored minority carriers in the quasi-neutral region immediately adjacent to
the depletion region when the device is on state. When moving from the on state to
the off sate removal of these charges does not happen instantaneously. This is the
cause of delay in switching from the on to the off state.
2. Name the two mechanisms that act to remove the stored excess charge during the
turn-off transient.
i) Recombination
ii) Reverse Current Flow
Carriers can be eliminated by recombination, but this would require several minority
carrier lifetimes to move from the on state to the off state. On the other hand, when a
reverse bias is applied the carriers drift back across the depletion region leading to a
reverse current flow. But the number of carriers removed by this method is limited
by the switching circuitry. In either case, neither method is considered rapid enough
to be thought as instantaneous.
3. An ideal p+-n diode carrying a forward current IF is suddenly reverse biased such that
a current IR flows through it at t = 0+. The storage time measured was ts. Determine
what happens to ts (increase or decrease) if:
a. Only IF is increased. ts will increase
As IF increases there are more stored charges in the quasi neutral region adjacent
to the depletion region. Hence it takes longer for the charges to be removed.
b. Only |IR| is increased. ts will decrease
As IR increases, stored charge carriers can be removed by the reverse current
flow. Hence ts will decrease as less time will be required to remove the stored
charges.
c. The hole lifetime,
τ
p, is increased. ts will increase
As the hole lifetime is increased, the carriers can exist in the quasi neutral region
for a longer time before they can recombine. As a result the storage delay time
increases.
4. a. An ideal p+-n diode carrying a forward current IF is suddenly open circuited at
t = 0. Sketch the expected variation of pn(x, t) versus x at progressively increasing
times after open circuiting the diode.
