Bell laboratories were first to fabricate a silicon-based semiconductor device called thyristor. Semiconductor devices having characteristics similar to thyristor are TRIAC, DIAC, silicon controlled switch, GTO, RCT, etc. This whole family of semiconductor devices is given the name thyristor. Silicon-controlled rectifier (SCR) is the oldest and most widely used member of the thyristor family. At present, the use of SCR is so extensive that the word thyristor has become synonymous with SCR.
Construction of Thyristor
A thyristor consists of four layers of alternate p-type and n-type silicon semiconductors forming three junctions J1, J2, and J3. The threaded portion is used for tightening the thyristor to the frame or heat sink using nuts. We mount thyristor on the heat sink to have better cooling. The rating of the thyristor is up to 10 kV, 3000 A, and 30 MW.
V-I Characteristics of Thyristor
The figure below shows the V-I characteristics of the thyristor. In fact, it is obvious from the V-I characteristics that it operates in three modes of operation. The figure below describes the three modes of operations.
Reverse blocking mode (OFF State):
Here anode is made negative and a cathode is made positive (VAK<0). The J-1 is reverse bias, J-2 is forward bias and J-3 is reverse bias. Here N+ region is heavily doped so the width of the depletion region is less in it and it has less voltage withstand capability. So, when we increase voltage, then J-3 breaks easily and the whole voltage appears across J-1. The J-1 has a lightly doped N– region which makes the voltage withstand capability of J-1 high. In this mode, a current of few mill amperes flows only called reverse leakage current. If we increase the reverse voltage beyond reverse breakdown voltage VBR, an avalanche occurs at J-1 and J-3 and reverses current increases rapidly which may damage SCR.
Forward Blocking mode:
Here anode is made positive and cathode is made negative (VAK> 0) keeping IG = 0. The J-1 is forward bias, J-2 is reverse bias and J-3 is forward bias. The reversed bias J-2 will not allow SCR to conduct. Only small a small forward leakage current will flow and the thyristor act as an open switch.
But if we increase VAK beyond VBO (break overvoltage) and keeping IG = 0 then it will initiate the conduction due to avalanche breakdown of J-2. To prevent the damaging of SCR, we have to prevent this.
Forward conducting mode:
Here anode is made positive and cathode is made negative (VAK> 0) with IG>0. As in forwarding blocking mode, the J-1 is forward bias, J-2 is reverse bias and J-3 is forward bias. When we apply gate current to J-2 then it will neutralize the charge present in depletion region of junction J-2 and SCR start conducting at voltage less than VBO.
As the gate current increase, we will be requiring lesser VAK to turn ON SCR decrease.
Once SCR starts conducting then it will continue to conduct even if the gate current is removed or it will not turn OFF.
Important Points to Remember
- Latching current (IL): Minimum amount of anode current required to turn ON SCR or Ia > IL to turn ON SCR.
- Holding Current (IH): Maximum amount of anode current below which SCR get turn OFF or Ia < IH to turn OFF SCR.
- Latching current is 2 to 3 times of holding current. In industrial applications holding current (typically 10 mA) is almost taken as zero.
- In Thyristor we can control only ON state but not of the state. Therefore it is called a semi-controlled switch.
- A thyristor can be termed as dc switch.
- It is not made of germanium because it has high leakage current.