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Shockley Diode
The Shockley diode is a four-layer semiconductor device invented by William Shockley. It consists of three PN junctions (J1, J2, J3) constructed by connecting two transistors. In the forward bias state, J1 and J3 are forward biased while J2 is reverse biased, allowing current to flow. In reverse bias, J1 and J3 are reverse biased while J2 is forward biased, blocking current flow. The diode acts as a closed switch when conducting but turns off when current drops below the holding level. Key applications include use as a relaxation oscillator and triggering SCR devices.
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Shockley Diode
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- 3. Introduction The term Shockleydiode or PNPN diode is named from the inventor “William Shockley”. It is a four layer semiconductor device and it was the first semiconductor devices. It is similar to a thyristor with detached gate. This type of diode is not available commercially and not particularly useful. But, this diode is more useful in making other kinds of thyristors such as DIAC,TRIAC and SCR.
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- 6. Construction The construction ofthis diode is done by connecting the four layers to form PNPN junction.The basic circuit of this diode using two transistors. In the circuit, the collector terminal of the T1 transistor is connected to the base terminal of theT2 transistor. The J1 junction is designed at the EB junction ofT1 transistor, J2 is at common connected BC junction betweenT1 &T2 transistors, J3 is at BE junction ofT2 transistor. So, as the BE junctions, J1& J3 must be forward biased & as a CB junction, J2 must be reverse biased for linear operation.
- 7. Working The shockley diodefunctions like a normal thyristor without any trigger inputs . When the diode is in the forward, the current flows through the diode, whereas in reverse bias, there is no flow of current. The Shockley diode comprises of three junctions namely J1, J2 and J3. Whenever the voltage is applied to the diode then the anode terminal is made positive with respect to the cathode, J1 and J3 junctions are forward biased where J2 is reverse biased.
- 8. Forward Biasing When thevoltage is applied to the shockley diode in such a way that anode is made positive with respect to cathode, junctions J1 and J3 are forward biased and J2 is reverse biased. Until the voltage across the diode is less than the break over voltage, as an open switch shockley diode exhibits a very high resistance and allows no current to flow through it. Once the break over voltage is reached (as the forward voltage is increased), it exhibits a very low resistance due to the breakdown of junction J2. Therefore, it acts like a short circuit and allows the current to flow until the current reaches to the holding current level of the diode. ON-state current flows only when the voltage is more than the break over voltageVBO.
- 9. Reverse Biasing When theanode is made negative with respect to the cathode, junctions J1 and J3 are reverse biased and junction J2 is forward biased. If the reverse bias voltage is increased (beyond the breakdown voltage of the shockley diode), J1 and J3 are reverse biased, then the reverse current will flow through the diode. This reverse current produces the heat, further this could ruin the entire diode. Therefore, shockley diode should never be operated in reverse biased condition with a voltage equal to the reverse breakdown voltage.
- 10. To Turn OffDiode .. Once the shockley diode is ON it acts like a closed switch offer a very low resistance to the current flow.To turn OFF the diode, the applied voltage must be reduced to a value such that current flowing through the diode is less than the holding current IH of the diode. In this state, junction J2 comes from reverse breakdown state and restores its high resistance value.
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- 12. Applications 1. Shockley Diodeused as Relaxation Oscillator: When the battery voltage is applied to the circuit, the capacitor charges through a resistance R. When the applied voltage or voltage across the capacitor is more than the break over voltage of shockley diode, it becomes turned ON and acts as a Switch. This causes rapidly to discharge the capacitor through the diode. When the current through the diode is less than the holding current of the diode, the diode becomes turned OFF and capacitor charges again. The voltage across the capacitor isVBO where the voltage at the reference is more than zero volts because the capacitor will not discharge completely.
- 13. 2. Shockley Diodeused asTrigger Switch: Most common applications of shockley diode are switching circuits , to turn ON the SCR. SCR is triggered by the shockley diode.The Resistive and capacitor RC network is fed with a DC supply which drives the shockley diode. When theVDC is applied, shockley is forward biased and also capacitor starts charging through the resistance. When the charging voltage of the capacitor reaches to the break over voltage of the diode, diode starts conducting and the capacitor starts discharging through the diode.This conduction of shockley diode drives the SCR into turn ON state, then the buzzer gives an alarm. Once the SCR is turned ON it will remain in the latching or ON state until the power is removed.The triggering times of SCR is controlled by selecting proper values of capacitor and resistors.
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