Ample pioneering work on theory and fabrication of the power-switching device, which later came to be known as a thyristor (because its characteristics are similar to those of the gas-tube thyratron), was done at the Bell Laboratories in the U.S.A. The first prototype was introduced by the General Electric Company (USA) in 1957. Since then, many improvements have been made, both in the technique of its fabrication and in adapting it to numerous industrial applications. With the development of a number of other devices of similar type and characteristic, the whole family of such power-switching devices has come to be known as "thyristors". Since, the basic semiconductor material used for the device is silicon, it is also designated ao a silicon-controlled rectifier (SCR). The term SCR is often used for the oldest member of the thyristor family which is the most widely used power-switching device.
         The rating of SCR has been very much improved since its introduction and now SRCs of voltage rating 10 kV and current rating 500 A are available, corresponding to a power-handling capacity of about 5 MW. This device can be switched by a low-voltage supply of about 1 A and Jo W, which shows the tremendous control capability of the device.
 Because SCR is compact and has high reliability and low losses, it has more or less replaced the thyratron and the magnetic amplifier as a switching device in many applications.

Advantages of a Thyristor Over Thyratron 
In comparison with the thyratron, thyristor possesses the following advantages: 
  1.  It is more robust and smaller in size.
  2.  It has a longer working life. 
  3.  It has no filament.
  4.  The voltage drop in the forward direction is only about 1 to 2 volts, compared 10 to 15 volts for the thyratron
  5.  The triggering and recovery periods are much shorter, so that it is more suitable for high-frequency switching operations.
  6. The arc ionizing and deionizing ties for a thyratron are comparatively large and so the device applications are limited to a frequency of 1 kHz. A thyristor can operate over a much greater range of frequency.

 Construction, Operation and Characteristics of a Thyristor Construction: 

thyristor types
  • The cross-sectional view of a typical SCR is shown in Fig.C Basically, the SCR consists of a four-layer pallet of P- and N-type semiconductor materials. Silicon is used as the intrinsic semiconductor to which the proper impurities are added. The junctions are either diffused or alloyed.  
  • The planer construction shown in Fig. (a) is used for low-power SCRs. The technique is useful for making a number of units for a single silicon wafer. Here, all the junctions are diffused.
  • The other technique the mesa construction is shown in Fig. (b). This is used for high power SCRs. Here, the inner junction J2 is obtained by diffusion and then the outer two layers are alloyed to it. Because the PNPN pallet is required to handle large currents, it is properly braced with tungsten or molybdenum plates to provide greater mechanical strength. One of these plates is hand-soldered to a copper or an aluminium stud, which is threaded for attachment to a heat sink. This provides an efficient thermal path for conducing the internal losses to the surrounding medium. The use of hand solder between the pallet and backup plates minimises thermal fatigue when the SCRs are subjected to temperature-induced stresses. For medium and low-power SGRs, the pallet is mounted directly on the copper stud or casing, using a soft-solder which absorbs the thermal stresses set up by differential expansion and provides a good thermal path for heat transfer. When a larger cooling the press-pack or hockey pack con. arrangement is required for high-power SCRs, the press- pack or hockey pack construction in used, which provides for double- sided air or water cooling.
symbol of thyristor
  • fig.C shows the symbolic diagram and terminal configuration of an SCR.
  • fig. D shows a thyristor. It has four layer alternately of P and N  with outer layers heavely doped, The control current generally applied via middle p layer and N emitter.


junction J1 and J3 are forward biased while junction J2 is reverse biased. If the voltage gradient between anode and cathode is too high then due to increased inherent current the thyristor may be switched on.
 thyristor characteristics
  • fig. (E) shows the forward and reverse characteristics of a thyristor.

When the anode is positive compared to the cathode, junctions J1 and J3 are forward biased and J2 is reverse biased. Under such circumstances a very small current due to inherent conductivity will flow through the device which is in forward blocking state. If the voltage at anode is continuously increased the inherent current is increased and at one stage it switches on the device. The `switching-on of the device results because of the breakdown of reverse biased junction J2 due to high voltage gradient. The 'switching on' condition of thyristor is referred to as conducting state and during this state current through the device is only limited by external resistance. 
                             If the anode voltage is reduced, the device will continue to conduct till the forward 
current reduces below the holding current Ih Below this value of current the depletion layer 
will start appearing across junction J2 and the device will be 'blocked' 

Reverse blocking characteristics: 

if the cathode is positive as compared to anode, the junction J2 reverse biased and only a small current flows through the device and the characteristics are called reverse blocking characteristics. If the voltage is continuously increased at one stage it may result breaking of depletion layers at junction J1 and J3 and the current through the device suddenly increases to a very high value. This is called reverse breakdown and the voltage is called reverse  breakdown voltage VBR.
                    As the outer layers of thyristor are highly doped compared to inner layers. the thickness
in deplection layer at J2 during forward bias is much more as compared to the total thickness
of.depletion layer at junctions J1 and J3 during reverse bias. Hence. the forward breakdown voltage VBF is normally greater than reverse breakdown voltage VBR. 

Thyristor Family 

Thyristor family include the following 
  1. Triac 
  2.  Reverse conducting thyristor
  3. Gate assisted thyristor.
  4. Gate turn off thyristor.
  5. light activated thyristor.

Comparison between Transistors and Thyristors.

 Although both are from the same family of semiconductor, a thyristor claims some of the following advantages over transistor:
  1.  A thyristor is a four layer device whereas a transistor has three layer.
  2.  Due to difference in fabrication and operation it is possible to have thyristors with higher   voltage and current ratings.
  3.  While a transistor requires a continuous current to remain in conducting state, a thyristor only   requires a pulse to make it conducting and thereafter remain conducting.
  4. The rating of a thyristor is in kilowatts while that of a transistor is in watts.

  Applications of Thyristor

 The thyristors are used for the following purposes: 
  1.  To control excitation current of synchronous motors.
  2.  To get a continuously variable output frequency.
  3.  To get standby A.C. power supplies. 
  4.  To control fluorescent lighting banks.
  5.  Other applications include: (i) Radar modulators; (ii) Servo systems; (111) latching relays; (iv)  D.C to A.C inverters. 


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