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Posted on 27 March 2019

Structure and Functional Principle of Thyristors

 

 

 

 

 

 

 

A thyristor is a semiconductor component with a minimum of three pn-junctions which can be switched from off-state to on-state. Often, "thyristor" specifically designates the reverse-blocking triode thyristor which cannot be switched in reverse direction but blocks. In addition to the two terminals that a diode provides, a thyristor has a gate which serves to switch the thyristor to the on-state (Figure 1).

Current-voltage characteristic of thyristor

Figure 1. Current-voltage characteristic of a thyristor with voltage directions and operating states

A thyristor consists of four alternate n-doped and p-doped regions (Figure 2). Together with the adjacent p-doped regions, the middle n- region forms the high-blocking pn-junctions in forward and reverse direction.

Diagram of thyristor

Figure 2. Diagram of a thyristor (glassivation acts as protection from environmental impacts and stabilizes reverse currents)

Passiviation (here: glassivation) must be performed for both pn-junctions. In order to be able to understand how a thyristor works, you can first imagine the thyristor divided into an NPN transistor and a PNP transistor (Figure 3).

Diagram showing the splitting a thyristor into two coupled NPN and PNP transistors

Figure 3. Splitting a thyristor into two coupled NPN and PNP transistors

If the cathode is negatively polarized with respect to the anode, a current will flow from the gate to the cathode. As a result, the cathode, which is the emitter of the NPN transistor, will inject electrons. The gate current is amplified by the NPN transistor. Some of these electrons will reach the low-doped region which simultaneously acts as collector of the NPN transistor and base of PNP transistor. In the PNP transistor, this current is further amplified and conducted to the base of the NPN transistor. This coupling of the transistor parts is crucial for the functioning of the thyristor.

The current amplification in the transistor is current-dependent. As soon as the current in the gate region becomes so high that the sum of current gains (in basis circuit) results in αnpn + αpnp ≥ 1, the current is amplified beyond all measure and the thyristor is triggered, meaning it switches to an on-state. A short current pulse to the gate (e.g. lasting 10 μs) is sufficient to trigger the thyristor. If the main current has exceeded the latching current IL at the end of the trigger pulse, the thyristor will stay in on-state. Only if the main current falls below the holding current IH will the thyristor switch back to forward off-state (blocking state). All current flowing through the pn-junction from the gate to the cathode acts as trigger current if it is high enough:

  • External trigger current from gate to cathode
  • Off-state current by exceeding the maximum blocking voltage (breakover voltage) ("break-over triggering")
  • Current generated in the space charge region by light incidence (light triggering)
  • Excessive temperature (thermally generated off-state current)
  • Capacitive displacement current owing to steeply rising anode voltage (dv/dt)

 

For more information, please read:

Dynamic Properties of Thyristors

Thyristor Basics

Bipolar Junction Transistor

Reliable Thyristor Triggering

 

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