Shockley Diode Characteristics, Thyristors, and SCRs

March 13, 2020 Cadence PCB Solutions

Shockley diode example

 

Not everyone is a fan of William Shockley. His erratic management style and extreme views made him a pariah later in life. Nevertheless, recognition should be given where it is due, and we have William Shockley to thank for his work on developing transistors. There is another important electronic component which bears his name: the shockley diode.

The important shockley diode characteristics make this diode useful in certain applications compared to the other common types of diodes. When we examine the structure of a Shockley diode, one can see how this diode functions like a pair of transistors with a unique configuration thanks to the way in which the p-type and n-type layers are stacked in the diode. Although these diodes are not commercially available, they form the basis of thyristors. Here’s how Shockley diode characteristics are related to thyristors and how to work with these devices in simulations.

Important Shockley Diode Characteristics

A Shockley diode is sometimes called a ‘pnpn’ diode due to its structure. Just like other diodes, it is a 2-electrode device, but it contains three p-n junctions, with two front-to-back p-n layers in series. The image below (left) shows the basic structure of a Shockley diode. As the p-type and n-type regions are placed in series, this structure can be rewritten as PNP and NPN transistors connected together. The collector of the PNP side is connected to the base of the NPN side, and the collector of the NPN side is connected to the base of the PNP side.

Shockley diode characteristics and structure

Basic structure and equivalent circuit for a Shockley diode

 

Some important Shockley diode characteristics can be seen by comparing the ‘pnpn’ structure and the equivalent circuit diagram. In the configuration shown above, the input voltage at the anode needs to rise to a sufficiently high level such that the collector current from the PNP side turns on the NPN side. At this point, a strong feedback loop is created that drives the PNP side to turn on. This then drives the output current through the cathode to increase to a high value once the voltage applied to the anode increases to a sufficiently high value.

In terms of p-n junctions, junctions J1 and J3 are driven in forward bias, while J2 is in reverse bias when a positive voltage is applied to the anode. If the applied voltage is large enough, junction J2 will enter breakdown, and the device will suddenly start conducting a much larger current. In this way, a Shockley diode acts like a 2-terminal switch, and the dual transistor equivalent circuit starts to appear intuitive. The device can be turned on simply by applying a sufficiently high forward voltage, which will latch the equivalent transistors into the on state. Similarly, the device can be turned off by applying a sufficiently large reverse voltage.

From Shockley Diodes to Thyristors and SCRs

The primary application for Shockley diodes is to control a silicon-controlled rectifier (SCR), which is a bistable switch. An SCR is just a Shockley diode with a gate connection to the p-type layer in the NPN stack of a Shockley diode. This is shown in the image below. The addition of a third terminal provides a way to externally modulate the gate in the second equivalent transistor with a second voltage source. If this gate connection is left floating, then an SCR will just behave as a regular Shockley diode.

 

Shockley diode characteristics compared to an SCR

Basic structure and equivalent circuit for an SCR

 

The I-V curves for both types of devices are quite different. A Shockley diode will have a hysteresis loop in the I-V curve, while the SCR will exhibit a highly nonlinear rectifying behavior. The DC characteristics can be determined in a simulation by sweeping the DC voltage between high and low values, followed by measuring the current at cathode. I-V curves for both devices are shown below.

 

Shockley diode characteristics compared to an SCR

SCR vs. Shockley diode characteristics and I-V behavior

 

Latching With a Switching Digital Signal

Rather than applying a steadily increasing DC voltage, one way to force a Shockley diode to latch is to use a fast-rising digital signal. When the rise time of the signal exceeds some threshold, it is possible to force the diode to latch due to the junction capacitance between each p-n junction. This can force the Shockley diode to turn on, even though the digital signal level may not be at the level required to switch the device into the ON state. Manufacturers will list the critical voltage rise to cause latching in a datasheet.

This form of latching is typically undesirable as you want to have the device operate as a controlled switch. This unintended latching can be eliminated by connecting a series RL circuit in series with the diode, and by connecting a series RC circuit in parallel with the diode (see the circuit diagram below). The RL portion slows down the rise time by creating a back EMF, and the RC portion slows down the voltage rise across the diode.

 

Shockley diode characteristics and a snubber circuit

Snubber circuit for a Shockley diode

 

When you need to investigate the Shockley diode characteristics of components and circuits, you’ll need to use the best PCB design and analysis software and a set of verified component models for simulations. The simulation and analysis tools in PSpice Simulator for OrCAD and the full suite of analysis tools from Cadence are ideal for evaluating the switching behavior of these and other components in a larger system. The manufacturing preparation tools also help ensure your components will be sourceable at scale.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

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