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.
