The term Shockley diode or PNPN diode is named from the inventor “William Bradford Shockley”. It is a four layer semiconductor device and it was the first semiconductor devices. It is similar to a thyristor with detached gate. Even though, this type of diode is not available commercially and not particularly useful. But, this diode is most useful in making other kinds of thyristors such as DIAC, TRIAC and SCR. Once we understand the basic concept of this diode, we will easily understand the other concepts covered in thyristors. Let us know the working of a Shockley diode and applications.
What is a Shockley Diode?
The Shockley diode is a two terminal and four layer semiconductor switching diode. Where four layers are P-N-P-N and two terminals are anode and cathode. The function of the Shockley diode is similar to a normal diode without any initiate i/ps. When the diode is in the forward, the current flows through the diode, whereas in reverse bias, there is no flow of current. The basic symbol, two transistor analogy and construction are shown below.
The construction of this diode is done by connecting the four layers to form PNPN junction. The basic circuit of this diode using two transistors is shown in above. In the above circuit, the collector terminal of the T1 transistor is connected to the base terminal of the T1 transistor.
The J1 junction is designed at the EB junction of T1 transistor, J2 is at common connected BC junction between T1 & T2 transistors, and the J3 is at BE junction of T2 transistor. So, as the BE junctions, J1& J3 must be forward biased & as a CB junction, J2 must be reverse biased for linear operation.
Circuit Diagram of Shockley Diode
Shockley diodes are mostly used as a switch in many circuits to turn on an SCR. The circuit diagram of the Shockley diode is shown below. The circuit below uses a Shockley diode to turn on the SCR, then it turns in the buzzer until the power is disconnected.
The first part of the circuit is RC circuit, where the RC network is fed DC voltage. Here, the capacitor will charge up through the resistor. When the DC power is detached from the circuit, the capacitor will discharge through the diode. This discharge will be the trigger voltage of the Shockley diode and it will turn the diode on. Once the diode is switched ON, the SCR will be turned ON and generates an alarm.
The negative terminal of the diode must be connected to the gate terminal of the SCR. Once the gate terminal of the SCR receives the ample voltage, it will conduct the current from anode to cathode until the power is detached. Detaching the gate current at this point has no effect and the current will still conduct from positive to negative. For this reason SCRs are very good for switching of devices like alarm circuits which continue ON until the power is deactivated. And another reason is, the switch is placed in the positive terminal of the SCR. Whenever you want to disable the buzzer, you should detach the power from the positive terminal the SCR. To do this, we kept a normally closed switch (SMPS) on this anode terminal.
Characteristics of Shockley Diode
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. The V-I characteristics of a Shockley diode are shown in the following figure. The diode consists of two operating modes such as conducting and non conducting modes, which operates on the lower line with small current and a voltage less than break over voltage
When the voltage tries to beat the break over voltage, the device breaks down & turns on along the dotted line to the on-state. This line specifies a temporary condition. The device can have voltage and current values on the dotted line as it turns on b/n the two stable operating states. In on-state or conducting state, the device functions on the greater line. As long as the current through the device is bigger than the holding current IH, then the voltage across it is somewhat greater than knee voltage, VK. When the current drops below the holding current’s level IH, the device turns back along with the dotted line to the off-state.
Application of the Shockley Diode
The application of the Shockley diode is a relaxation oscillator circuit that is shown above. This circuit uses a Shockley diode, that is connected across the power supply battery and capacitor. When the voltage is applied to the circuit using the battery, then the capacitor in the circuit charges through a resistor. When the applied voltage across the capacitor is greater than the break over voltage of the diode, then it activates and works as a switch.This causes quickly to discharge the capacitor through the Shockley diode. And when the current through the Shockley diode is below the holding current of the diode, then the diode becomes inactivated and again capacitor charges up.
This is all about what is a Shockley diode, Circuit Diagram, V-I Characteristics and its Application. We hope hat you have got a better understanding of this concept. Furthermore, any doubts regarding this diode or electronic project kits please give your feedback by commenting in the comment section below. Here is a question for you, what is the function of the Shockley diode?