SCR , Silicon controlled ractifier

SCR is also known as SILICON CONTROLLED DEVICE . It is widely use in many electronic circuit. SCR is made up by P-type semiconductor and N-type semiconductor.the main use of SCR is CURRENT CONTROLLED DEVICE.

In SCR has three terminals ...
-ANODE , CATHODE and GATE

The cathode is the most heavily doped , the gate and -ANODE are the next heavily doped the lowest doping level is within the central N type layer.Anode is connected on P-type semiconductor and cathode is connected on N-type semiconductor , and the GATE is On P-type semiconductor.in SCR has three junction.
J1 , J2 AND J3 , junction J1 is between P-type and n N-type material and junction J2 is between N-type and P-type material , similarly tje junction J3 is between p P-type and N-type material according showing in given figure.

SCR is showing in bellow figure..

Before study of working of SCR , we will do study about following terms...

●Avalanche brackdown
●Triggering of SCR

●Avalanche brackdown :-

Avalanche brackdown is accurs is pn - junction diode , in avalanche brackdown , when we apply high reverse biasing voltage on Diode and , so as we increase the applied reverse voltage the electric field across junction will keep increasing . The generated electric field exerts a force on the electrons at junction and it frees them from covalent bonds. These free electrons will gain acceleration and it will start moving across the junction with high velocity. This results in collision with other neighboring atoms. These collisions in high velocity will generate further free electrons. These electrons will start drifting and electron-hole pair recombination occurs across the junction. This results in net current that rapidly increases. it's condition is known as AVALANCHE BRACKDOWN.

●Triggring of SCR:-

Triggring means ON the devices from its OFF stage. SCR is normally start on forward brack over voltage , when we apply voltage on GATE terminal of SCR which is called triggring when SCR is turn ON. If we remove triggring voltage , the SCR will remain ON , this voltage is called FIRING VOLTAGE and this process in triggring of SCR

●Working process of SCR:-

Generally the SCR is work in three mode

1. Forward Blocking mode

SCR is work when it forward biasing , in forward bias ANODE is on positive voltage and CATHODE is on negative voltage. Forward voltage triggring occurs when the -ANODE CATHODE Forward-voltage is increases , when GATE is open circuit then junction J2 reach it hight value and Brackdown , this condition in AVALANCHE BREAKDOWN , in this process the junction J2 will BREAKDOWN it means junction J2 in reverse bias . And junction J1 and J3 is in forward bias. this mode is known as Forward Blocking mode.

2. Reverse Blocking mode

When a negative voltage is applied to the ANODE and a positive voltage to the CATHODE , the SCR is in reverse blocking mode, making J1 and J3 reverse biased and J2 forward biased. The device behaves as two reverse-biased diodes connected in series. A small leakage current flows. the device offers a high impedance in this mode until the voltage applied is less than the reverse breakdown voltage This is the reverse blocking mode.

3. Forward conduction mode

In this mode SCR come conduction through Blocking mode and the conduction mode can be comes through two ways , first apply hight positive voltage on Anode ...

And second way , apply triggring voltage on GATE terminal .Once any one of these methods is applied, the avalanche breakdown occurs at junction J2. Therefore the SCR turns into conduction mode and acts as a closed switch thereby current starts flowing through it.if gate current is increasing, the voltage required to turn ON the SCR is less if gate biasing is preferred. The current at which the SCR turns into conduction mode from blocking mode is called as latching current .

And also when the forward current reaches to level at which the SCR returns to blocking state is called as holding current (IH). At this holding current level, depletion region starts to develop around junction J2. Hence the holding current is slightly less than the latching current