Diodes are fundamental building blocks in electronics, playing a vital role in controlling current flow, protecting circuits and enabling various functionalities. Because of their one-direction current flow, they shield circuits from voltage spikes or reverse current flow that could damage sensitive components. This article will discuss a Shockley diode, named after physicist William Shockley, which is a unique type of diode with some distinct characteristics.
What is a Shockley diode?
Shockley diodes are a particular type of diode with a four-layer PNPN structure that acts as a voltage-triggered switch. Though less common now, they were historically important in the development of semiconductor devices. Similar to other devices called thyristors, a Shockley diode acts like a switch.
Operating principles
This diode operates based on the interaction between two interconnected bipolar transistors (one PNP and one NPN) formed by its PNPN structure. It stays in a high resistance "off" state until a specific voltage threshold is reached. This is because both the embedded transistors are also "off" due to the lack of current flow through their base-emitter junctions.
Once the voltage exceeds this trigger voltage, the resistance falls sharply, and the diode switches "on," allowing significant current flow. At this voltage, a phenomenon called avalanche breakdown occurs within one of the transistor junctions. This breakdown signifies a sudden increase in current flow through that junction. The current surge caused by the breakdown in one transistor acts as the base current for the other transistor, turning it on as well and creating a regenerative effect. The newly turned-on transistor injects even more current into the first transistor, further amplifying the current flow. This positive feedback loop rapidly drives both transistors into saturation, resulting in a very low resistance state for the entire Shockley diode. Now, current can flow freely through the device.
Turn it off
Once triggered and turned on, a Shockley diode exhibits a crucial property called latching. This means it will stay in the low resistance "on" state even if the applied voltage drops below the threshold voltage. This is because the current flowing through the device itself is sufficient to maintain the saturation of both transistors.
To turn the Shockley diode off, the current flowing through it needs to be reduced below a certain level called the holding current. This can be achieved by reducing the applied voltage or by introducing a temporary current path that bypasses the diode. Once the current falls below the holding current, both transistors come out of saturation, and the diode returns to its high resistance "off" state.
Differences from other diodes and thyristors
Shockley diodes are a specific type of thyristor, but with a simpler structure and no gate for control. It relies solely on the voltage applied between its anode and cathode for triggering. Most thyristors, on the other hand, have a dedicated gate terminal that allows for more precise control in turning the device on and off. Applying a current pulse to the gate triggers the switch on, and reducing the current below a holding value can turn it off.
In comparison to regular diodes that typically have just one p-n junction and allow current flow in one direction only, Shockley diodes have a more complex structure and function more like a switch that turns on or off based on voltage. They are simpler and more widely used for rectification applications, while Shockley diodes and thyristors are used in situations where switching functionality is needed.
What are its applications?
· One key use of Shockley diodes is as a trigger for silicon-controlled rectifiers (SCRs), which are more advanced thyristors with a gate for control. By applying a voltage across the Shockley diode, it can reach its breakover voltage and turn on. This surge of current can then be used to trigger the gate of the SCR, precisely controlling the switching of much higher power currents.
· They can also be used in relaxation oscillator circuits. These circuits generate a sawtooth waveform, which is a voltage that gradually increases and then rapidly decreases. The Shockley diode's ability to switch states based on voltage makes it suitable for creating this type of oscillating signal.
Why are they not preferred today?
It's important to note that Shockley diodes are not as widely used today compared to other types of diodes and thyristors because:
- The absence of a dedicated gate terminal for control makes Shockley diodes less versatile than SCRs, which offer more precise switching capabilities.
- Other types of circuits and devices can often achieve similar functionalities to Shockley diodes, potentially with better efficiency or control.
Conclusion
Shockley diodes function like a voltage-controlled switch. By exceeding a specific voltage threshold, you can flip it from a high-resistance blocking state to a low-resistance conducting state. They occupy a middle ground between regular diodes and thyristors. They offer switching behavior like thyristors but lack the precise control of a gate terminal. This makes them less versatile but can be suitable for specific applications where simpler control based on voltage is sufficient.
