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If we increase the voltage above 0.7v; there will be enough electrons supplied to the N-
type to reduce the barrier to nothing; a large number of electrons can then cross the bar-
rier (because both sides can conduct electricity), but energy is still lost across the barrier
causing a 0.7V drop in voltage.
The overall effect here is to drop the voltage across the diode. FIGnition uses two signal
diodes in series in this way to drop the 5v supply to 3.6v for the memory chips to operate
(see section
9.2).
If we pump electrons to the P-type Silicon it has the opposite effect. The P-type silicon ac-
cepts the electrons and this in turn repels electrons near the barrier of the N-type silicon;
increasing the barrier. Electrons aren't able to cross the barrier and so current does not
flow (except for a very small number of electrons).
Here, the diode is acting as a valve, preventing current from flowing if a negative voltage
appears at the P-type silicon. This behaviour is used in FIGniton's video circuit to protect
outputs when Sync is high while Video out is low and vice versa
(see section
3.2.2).
If enough voltage is applied to the P-type silicon, they will eventually have enough energy
to overcome the barrier causing a break down in the diode and this is likely to damage it
permanently
B.2 Zener Diodes
Zener diodes are types of diodes designed to 'break down' in a controlled way without
damaging the diode. We can think of this as the Zener diode having a limited barrier.
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