What is Shockley diode:
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| Basic structure and symbol of Shockley diode |
Shockley diode or PNPN junction is a four layer diode, it has more than one PN junction. This diode is named after its inventor William Shockley.
This diode is not commercially available, but it is more useful in making other types of thyristors like DIAC, TRIAC and SCR.
The symbol of Shockley diode looks like the symbol of PN junction diode but only half of the triangle is included in the symbol. It has two terminals, anode and cathode.
Construction of Shockley diode:
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| Construction of Shockley diode |
The above picture is two transistor model and its equivalent circuit. The construction of Shockley diode consists of two interconnected transistors, one PNP and the other NPN.
Working of Shockley diode:
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| Two transistor equivalent circuit of Shockley diode |
Junction 1 is formed between emitter base junction of T1. The common connected with base and collector junction of T1 and T2 forms the junction 2. The junction of base emitter of T2 forms the junction 3.
When the diode is forward biased, the anode is made positive and cathode negative, junction 1 and junction 3 are forward biased but the junction 2 is reverse biased.
The diode exhibits high resistance so that current does not flow till the applied voltage is near the breakdown voltage. When the voltage is increased further junction 2 breaks and it becomes short circuit and allows current to flow.
When the diode is reverse biased Junction 2 is forward biased and the other two junctions, junction 1 and junction 3 are reverse biased.
When the applied external voltage exceeds the breakdown voltage excessive heat is produced and the device gets damaged. So Shockley diode never operates in the reverse biased condition.
V-I Characteristics of Shockley diode:
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| V-I characteristics of Shockley diode |
When the applied external
voltage is increased and till it reaches
the break over voltage the device will be in OFF state. After it crosses the
breakover voltage the device will be in the ON state.
The function of Shockley diode is greater when the voltage applied is higher than the knee voltage and when the current is higher than the holding current. When the current drops below the holding current, the device returns to the OFF state which is the dotted line.
Advantages:
- It has high switching speed
- Since two transistors are connected, it is more than a transistor.
Disadvantages:
- The manufacturing and the construction process is complex, since it involves four layers of semiconductors.
Applications:
- Used as trigger switch
- Used as relaxation oscillator
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