Shockley Diode


Shockley Diode

 

Shockley Diode

 






Named after its inventor, a Shockley diode is a PNPN device having two terminals as
shown in Fig. 1 (i).





This *device acts as a switch and consists of four alternate P-type
and N-type layers in a single crystal. The various layers are label as P 1, N 1, P 2 and N 2
for identification.





Fig-1




Since a p-region adjacent to an N-region may be consider a junction
diode, the shockly diode is equivalent to three junction diodes connected in series as
shown in Fig. 1 (ii). The symbol of Shockley diode is show in Fig.1 (iii).
…………………………………………………………………………………………………………………………..





*Note that if we remove the gate terminal of as SCR, the resulting device is Shockley diode.





Working






(i)





When diode is forward biase (i.e., anode is positive w.r.t.
cathode), diodes D 1 and D 3 would be forward-biase while diode D 2 would
be reverse-biase. Since diode D 2 offers very high resistance (being
reverse biased) and the three diodes are in series, the diode
presents a very high resistance. As the *forward voltage increases, the
reverse bias across D 2 is also increase.





At some forward voltage (called
breakover voltage V BO ), reverse breakdown of D 2 occurs. Since this
breakdown results in reduced resistance, the diode presents a
very low resistance. From now onwards, the diode behaves as a
conventional forward-biased diode; the forward current being determine
by the applied voltage and external load resistance. This behavior of
diode is indicate on its V-I characteristic in Fig. 2.





Fig-2





(ii)





When Shockley diode is reverse biased (i.e., anode is negative w.r.t.
cathode), diode D 1 and D 3 would be reverse-biased while diode D 2 would
be forward-biased.





If reverse voltage is increase sufficiently, the reverse
voltage breakdown (point A in Fig. 2) of diode is reach. At
this point, diodes D 1 and D 3 would go into reverse-voltage breakdown, the
reverse current flowing





…………………………………………………………………………………………………………………………..
Since D 1 and D 3 offer very low resistance (being forward biased), the entire applied voltage
appears as reverse voltage across D 2.
through them would rise rapidly and the heat produced by this current flow could ruin
the entire device. For this reason, diode should never be operate with a
reverse voltage sufficient to reach the reverse-voltage breakdown point.
Conclusion. The above discussion reveals that Shockley diode behaves like a
switch. So long as the forward voltage is less than breakover voltage. Diode





offers very high resistance (i.e., switch is open) and practically conducts no current. At
voltages above the break-over value, Shockley diode presents a very low resistance
(i.e.,switch is close) and diode conducts heavily. It may be note that
Shockley diode is also known as PNPN or four layer diode or reverse-blocking
diode thyristor.





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Note: Once Shockley diode is turned ON (i.e.,it starts conducting), the only way
to turn it OFF is to reduce the applied voltage to such a value so that current flowing
through Shockley diode drops below its holding current (I H ) value.





Diode D 2 then comes
out of its reverse-breakdown state and its high-resistance value is restored. This, in
turn, causes the entire Shockley diode to revert to its hiht-resistance (switch open)
state.


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