Editing Thyristor (section)

== Design ==
[[File:thyristor.svg|thumb|Structure on the physical and electronic level, and the thyristor symbol]]

The thyristor is a four-layered, three-terminal semiconductor device, with each layer consisting of alternating [[N-type semiconductor|N-type]] or [[P-type semiconductor|P-type]] material, for example P-N-P-N. The main terminals, labelled anode and cathode, are across all four layers. The control terminal, called the gate, is attached to p-type material near the cathode. (A variant called an SCS—silicon controlled switch—brings all four layers out to terminals.) The operation of a thyristor can be understood in terms of a pair of tightly coupled [[bipolar junction transistor]]s, arranged to cause a self-latching action.

Thyristors have three states:

# Reverse blocking mode: Voltage is applied in the direction that would be blocked by a diode
# Forward blocking mode: Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not been triggered into conduction
# Forward conducting mode: The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the "holding current"

=== Gate terminal ===
[[File:Thyristor layers.svg|thumb|upright=0.5|Layer diagram of thyristor]]

The thyristor has three [[p-n junction]]s (serially named J<sub>1</sub>, J<sub>2</sub>, J<sub>3</sub> from the anode).

When the anode is at a positive potential V<sub>AK</sub> with respect to the cathode with no voltage applied at the gate, junctions J<sub>1</sub> and J<sub>3</sub> are forward biased, while junction J<sub>2</sub> is reverse biased. As J<sub>2</sub> is reverse biased, no conduction takes place (Off state). Now if ''V''<sub>AK</sub> is increased beyond the breakdown voltage ''V''<sub>BO</sub> of the thyristor, [[avalanche breakdown]] of J<sub>2</sub> takes place and the thyristor starts conducting (On state).

If a positive potential ''V''<sub>G</sub> is applied at the gate terminal with respect to the cathode, the breakdown of the junction J<sub>2</sub> occurs at a lower value of ''V''<sub>AK</sub>. By selecting an appropriate value of ''V''<sub>G</sub>, the thyristor can be switched into the on state quickly.

Once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until: (a) the potential ''V''<sub>AK</sub> is removed or (b) the current through the device (anode−cathode) becomes less than the holding current specified by the manufacturer. Hence ''V''<sub>G</sub> can be a voltage pulse, such as the voltage output from a [[UJT]] [[relaxation oscillator]].

The gate pulses are characterized in terms of gate trigger voltage (''V''<sub>GT</sub>) and gate trigger current (''I''<sub>GT</sub>). Gate trigger current varies inversely with gate pulse width in such a way that it is evident that there is a minimum gate [[electric charge|charge]] required to trigger the thyristor.

=== Switching characteristics ===
[[File:Thyristor I-V diagram.svg|thumb|''V''—''I'' characteristics]]

In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state (i.e. does not need a continuous supply of gate current to remain in the on state), providing the anode current has exceeded the latching current (''I''<sub>L</sub>). As long as the anode remains positively biased, it cannot be switched off unless the current drops below the holding current (''I''<sub>H</sub>). In normal working conditions the latching current is always greater than holding current. In the above figure ''I<sub>L</sub>''  has to come above the ''I<sub>H</sub>'' on y-axis since ''I<sub>L</sub>''>''I<sub>H</sub>''.

A thyristor can be switched off if the external circuit causes the anode to become negatively biased (a method known as natural, or line, commutation). In some applications this is done by switching a second thyristor to discharge a capacitor into the anode of the first thyristor. This method is called forced commutation.

Once the current through the thyristor drops below the holding current, there must be a delay before the anode can be positively biased ''and'' retain the thyristor in the off-state. This minimum delay is called the circuit commutated turn off time (''t''<sub>Q</sub>). Attempting to positively bias the anode within this time causes the thyristor to be self-triggered by the remaining charge carriers ([[Electron hole|holes]] and [[electron]]s) that have not yet [[Carrier generation and recombination|recombined]].

=== Frequency ===
Thyristors is designed for low frequency applications i.e. those below 1kHz.<ref>{{Cite |title=Basics on the thyristor (SCR) structure and its application, AN4607 |date=2018 |url=https://www.st.com/resource/en/application_note/an4607-basics-on-the-thyristor-scr-structure-and-its-application-stmicroelectronics.pdf |url-status=live |publisher=[[NXP Semiconductors]]}}</ref> Frequencies higher than the domestic AC mains supply (e.g. 50&nbsp;Hz or 60&nbsp;Hz), require thyristors with lower values of ''t''<sub>Q</sub>. Such fast thyristors can be made by diffusing [[heavy metal (chemistry)|heavy metal]] [[ion]]s such as [[gold]] or [[platinum]] which act as charge combination centers into the silicon. Today, fast thyristors are more usually made by [[electron]] or [[proton]] [[irradiation]] of the silicon, or by [[ion implantation]].  Irradiation is more versatile than heavy metal doping because it permits the dosage to be adjusted in fine steps, even at quite a late stage in the processing of the silicon.