Silicon Controlled Rectifier (SCR) is a four layer p-n-p-n device where p and n layers are alternately arranged. The outer layers are heavily doped. There are three p-n junctions called J1, J2 and J. The outer p layer is called anode while outer n layer is called cathode. Middle p layer is called gate. The three terminals are taken out respectively from these three layers, as shown in the Fig. 6.1.
Anode must be positive with respect to cathode to forward bias the SCR. But this is not sufficient criterion to turn SCR ON. To make it ON, a current is to be passed through the gate terminal denoted as IGT. Thus, it is a current operated device.
Types of constructions which are used to manufacture SCR
, 1) Planar type
2) Mesa type
3) Press pack type
1. Planar type:
This construction is used for low current SCRs. In this type, all the p-n junctions come to the same surface on the cathode side. This is shown in the Fig. 6.2 All the junctions are diffused in this type of construction. The disadvantage of this type is more silicon per ampere current is required. The advantage is that the mass production is possible and large number of SCRs can be manufactured with uniform characteristics.
2. Mesa type :
In this construction, the junction J2 is diffused while the outer layers are alloyed to it. This is shown in the Fig. 6.3. To handle the large currents, the molybdenum or tungsten plates are braced to p-n-p-n silicon pellet. This provides the additional mechanical strength. When SCR is ON, the area around the gate starts the conduction first. But in this construction, area around the gate is small hence this construction is not suitable for high di/dt ratings.
3. Press Pack Type:
This construction is used for the centre gate SCRs. The entire circular area around the gate takes part in the initial conduction hence di/dt capability of such SCRs is large. This is shown in the Fig. 6.4. The construction is used for high power SCRs. A silicon wafer is used to make such a high power SCR. This type of construction provides double sided cooling arrangement which is necessary for high power SCRs.
The Fig. 6.4 (a) shows the symbol of SCR The IGT is the gate trigger current required to make the SCR ON. Basically it is a diode Cathode symbol, with a third terminal gate connected to it. Thus, the basic material used for the SCR fabrication is silicon whose reverse current is very small compared to the other important semiconductor material like Germanium.
Why SCR is called controlled rectifier?
The SCR conducts only in one direction and not only that the point at which it starts conduction can be controlled. Due to this, the device is called silicon controlled rectifier (SCR).
Operation of SCR
The operation of SCR is divided into two categories,
1. When gate is open :
Consider that the anode is positive with respect to the cathode and gate is open. The junctions Ji and Ja are forward biased and junction J2 is reverse biased. There is depletion region around J, and only leakage current flows which is negligibly small Practically the is to be OFF. This is called forward blocking state of SCR and voltage applied to anode and cathode with anode positive is called forward voltage. This is shown in the figure 6.5(a):
With the gate open, if the cathode is made positive with respect to anode, the junctions Jul become reverse biased and J, forward biased. Still the current flowing is leakage current, which can be neglected as it is very small. The voltage applied to make cathode positive is called reverse voltage and SCR is said to be in reverse blocking state. This is shown in the Fig. 6.5 (b).
In the forward blocking state, if the forward voltage is increased, the current remains almost zero up to a certain limit. At a particular value, the reverse biased junction J breaks down and SCR conducts heavily. This voltage is called forward break over voltage Video of SCR. In such condition, SCR is said to be ON or triggered
2. When the gate is closed :
Consider that the voltage is applied between gate and cathode when the SCR is in forward blocking state. The gate is made positive with respect to the cathode. The electrons from n-type cathode which are majority, cross the junction J; to reach to positive of battery.
While holes from p type move towards the negative of the battery, this constitutes the gate current. This current increases the anode current as some electrons cross junction Jz. As anode current increases, more electrons’ junction Jz and the anode current further increases. Due to regenerative action, within a short time, the junction J2 breaks and SCR conducts heavily. The connections are shown in the Fig. 6.6. The resistance R is required to limit the current. Once the SCR conducts, the gate loses its control.