A thyristor (, from a combination of Greek language θύρα, meaning "door" or "valve", and transistor) is a solid-state semiconductor device which can be thought of as a highly robust and switchable diode, allowing the passage of current in one direction but not the other, often under control of a gate electrode, that is used in high-power applications like inverters and radar generators. It usually consists of four layers of alternating P- and N-type materials. It acts as a bistable switch (or a latch). There are two designs, differing in what triggers the conducting state. In a three-lead thyristor, a small current on its gate lead controls the larger current of the anode-to-cathode path. In a two-lead thyristor, conduction begins when the potential difference between the anode and cathode themselves is sufficiently large (breakdown voltage). The thyristor continues conducting until the voltage across the device is reverse-biased or the voltage is removed (by some other means), or through the control-gate signal on newer types.
Some sources define "silicon-controlled rectifier" (SCR) and "thyristor" as synonymous.Christiansen, Donald; Alexander, Charles K. (2005); Standard Handbook of Electrical Engineering (5th ed.). McGraw-Hill, Other sources define thyristors as more complex devices that incorporate at least four layers of alternating N-type and P-type substrate.
The first thyristor devices were released commercially in 1956. Because thyristors can control a relatively large amount of power and voltage with a small device, they find wide application in control of electric power, ranging from light dimmers and electric motor speed control to high-voltage direct-current power transmission. Thyristors may be used in power-switching circuits, relay-replacement circuits, inverter circuits, oscillator circuits, level-detector circuits, chopper circuits, light-dimming circuits, low-cost timer circuits, logic circuits, speed-control circuits, phase-control circuits, etc. Originally, thyristors relied only on current reversal to turn them off, making them difficult to apply to direct current; newer device types can be turned on and off through the control-gate signal. The latter is known as a gate turn-off thyristor, or GTO thyristor.
Unlike transistors, thyristors have a two-valued switching characteristic, meaning that a thyristor can only be fully on or off, while a transistor can lie in between on and off states. This makes a thyristor unsuitable as an analog amplifier, but useful as a switch.
History
The silicon controlled rectifier (SCR) or thyristor proposed by William Shockley in 1950 and championed by Moll and others at Bell Labs was developed in 1956 by power engineers at General Electric (GE), led by Gordon Hall and commercialized by GE's Frank W. "Bill" Gutzwiller. The Institute of Electrical and Electronics Engineers recognized the invention by placing a plaque at the invention site in Clyde, New York, and declaring it an IEEE Historic Milestone.
In 1960, Transitron Electronic Corporation marketed an PNPN tetrode with a high current gain at the time referred to as Binistor. The device was predicted to be applied both in switching and data-storage circuits. One research paper using Transitron's tetrode demonstrated an "unusual" (at the time) switched-mode power-supply circuit.
An earlier gas-filled tube device called a thyratron provided a similar electronic switching capability, where a small control voltage could switch a large current. It is from a combination of "thyratron" and "transistor" that the term "thyristor" is derived.
In recent years, some manufacturersExample: Silicon Carbide Inverter Demonstrates Higher Power Output in Power Electronics Technology (2006-02-01) have developed thyristors using silicon carbide (SiC) as the semiconductor material. These have applications in high-temperature environments, being capable of operating at temperatures up to .
Design
The thyristor is a four-layered, three-terminal semiconductor device, with each layer consisting of alternating N-type or 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 transistors, 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
The thyristor has three p-n junctions (serially named J1, J2, J3 from the anode).
When the anode is at a positive potential VAK with respect to the cathode with no voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place (Off state). Now if VAK is increased beyond the breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and the thyristor starts conducting (On state).
If a positive potential VG is applied at the gate terminal with respect to the cathode, the breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value of VG, 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 the potential VAK is removed or the current through the device (anode−cathode) becomes less than the holding current specified by the manufacturer. Hence VG 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 (VGT) and gate trigger current (IGT). Gate trigger current varies inversely with gate pulse width in such a way that it is evident that there is a minimum gate charge required to trigger the thyristor.
Switching 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 (IL). As long as the anode remains positively biased, it cannot be switched off unless the current drops below the holding current (IH). In normal working conditions, the latching current is always greater than holding current. In the above figure, IL has to come above the IH on y-axis since IL>IH.
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 (tQ). Attempting to positively bias the anode within this time causes the thyristor to be self-triggered by the remaining charge carriers (holes and electrons) that have not yet recombined.
Frequency Thyristors are designed for low-frequency applications; that is, those below . Frequencies higher than the domestic AC mains supply (e.g. or ), require thyristors with lower values of tQ. Such fast thyristors can be made by diffusing heavy metal ions 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.
Types
ACS: AC switch: a high-commutation and overvoltage-protected TRIAC with gate buffer ACST: AC switch: a high-commutation and overvoltage-protected TRIAC AGT: anode gate thyristor: a thyristor with gate on n-type layer near to the anode ASCR: asymmetrical SCR BCT: bidirectional control thyristor: a bidirectional switching device containing two thyristor structures with separate gate contacts BOD: breakover diode: a gateless thyristor triggered by avalanche current DIAC: bidirectional trigger device Dynistor: unidirectional switching device Shockley diode: unidirectional trigger and switching device SIDAC: bidirectional switching device Trisil, SIDACtor: bidirectional protection devices BRT: base resistance controlled thyristor ETO: emitter turn-off thyristorRashid, Muhammad H.(2011); Power Electronics (3rd ed.). Pearson, GTO: gate turn-off thyristor DB-GTO: distributed-buffer gate turn-off thyristor MA-GTO: modified-anode gate turn-off thyristor IGCT: integrated gate-commutated thyristor Ignitor: spark generators for fire-lighter circuits LASCR: light-activated SCR, or LTT: light-triggered thyristor LASS: light-activated semiconducting switch MCT: MOSFET controlled thyristor: It contains two additional FET structures for on/off control CSMT or MCS: MOS composite static induction thyristor PUT or PUJT: programmable unijunction transistor: a thyristor with gate on n-type layer near to the anode used as a functional replacement for unijunction transistor Quadrac: special type of thyristor which combines a DIAC and a TRIAC into a single package RCT: reverse conducting thyristor SCS: silicon-controlled switch or thyristor tetrode: A thyristor with both cathode and anode gates ‒ effectively a combination of SCR and PUT in one device SCR: silicon-controlled rectifier SITh: static induction thyristor, or FCTh: field-controlled thyristor: containing a gate structure that can shut down anode current flow TRIAC: triode for alternating current: a bidirectional switching device containing two thyristor structures with common gate contact
Reverse-conducting thyristor A reverse-conducting thyristor (RCT) has an integrated reverse diode, so is not capable of reverse blocking. These devices are advantageous where a reverse or freewheel diode must be used. Because the SCR and diode never conduct at the same time, they do not produce heat simultaneously and can easily be integrated and cooled together. Reverse-conducting thyristors are often used in frequency changers and inverters.
Photothyristors
Photothyristors are activated by light. The advantage of photothyristors is their insensitivity to electrical signals, which can cause faulty operation in electrically noisy environments. A light-triggered thyristor (LTT) has an optically sensitive region in its gate, into which electromagnetic radiation (usually infrared) is coupled by an optical fiber. Since no electronic boards need to be provided at the potential of the thyristor in order to trigger it, light-triggered thyristors can be an advantage in high-voltage applications such as HVDC. Light-triggered thyristors are available with in-built over-voltage (VBO) protection, which triggers the thyristor when the forward voltage across it becomes too high; they have also been made with in-built forward recovery protection, but not commercially. Despite the simplification they can bring to the electronics of an HVDC valve, light-triggered thyristors may still require some simple monitoring electronics and are only available from a few manufacturers.