Difference between Diode and Silicon Controlled Rectifier(SCR)

Both diode and SCR are types of semiconductor devices that act as a static switch to control the flow of electric current. Both diode and SCR are unidirectional switches used in a number of electronic circuits. One major application of diode and SCR is that both are used for rectification purpose. However, there are many differences between diode and SCR based on their basic definition, construction, working, applications, etc.

In this article, we will explain all the major differences between diode and SCR along with their short description.

What is a Diode?

A diode is a two-terminal PN junction semiconductor electronic device which allows the electric current to pass in only one direction. A diode is formed by combining a p-type semiconductor material with an n-type semiconductor material.

The two terminals of the diode are − anode and cathode. When the positive terminal of the power supply is connected to the anode terminal and negative terminal of supply to the cathode terminal of the diode, then the diode is said to be forward biased and it allows the electric current to flow through it. On the other hand, when positive terminal is connected to cathode and negative terminal to the anode, then the diode becomes reverse biased and it block the passes of current through it. Therefore, the diode being a unidirectional device, is used for rectification purpose (for converting AC in DC).

The diode is extensively used in many electronic circuits such as rectifiers, static switch, clipper circuit, clamper circuit, etc.

What is an SCR?

SCR stands for Silicon Controlled Rectifier. An SCR is a four-layer, three-terminal and three-junction semiconductor power electronic device. The SCR is a member of the thyristor family in power electronics. Therefore, it is usually known as a Thyristor.

It consists of three terminals viz. anode, cathode and gate. Where, the anode and cathode are the main terminals for the current conduction while the gate is the terminal provided for controlling or triggering of the SCR. The SCR is constructed by four alternating layers of P-type and N-type semiconductor materials forming a PNPN structure as shown in the figure of SCR. Therefore, three PN junctions are in its structure. The anode of SCR is taken from the external P-region whereas the cathode is taken from the external N-region. The gate is taken out from the middle P-region of the SCR.

The most common application of SCR is in controlled rectification, i.e. for the conversion of AC into DC with controlled switching.

Difference between Diode and SCR

Since both diode and SCR are widely used in switching applications in many electronic circuits. However, there are several differences between them which are given in the following table −

Basis of Difference	Diode	SCR
Definition	A diode is a two-terminal, unijunction semiconductor electronic device.	An SCR or Silicon Controlled Rectifier is a three-terminal, four-layer and three-junction semiconductor device used for switching in electronic circuits.
Circuit symbol
Terminals	Diode has two terminals viz. Anode and Cathode.	SCR has three terminals viz. Anode, Cathode and Gate.
Number of semiconductor layers	Diode has two layers: one is of p-type semiconductor and another is of n-type semiconductor.	SCR has four semiconductor layers which are arranged in alternating manner to form a PNPN structure.
Number of PN junctions	Diode has only one PN junction.	SCR has three PN junctions.
Starting of conduction	Diode starts conducting when the supply voltage exceeds the cut in voltage, which is 0.4 for germanium and 0.7 for silicon.	SCR starts conducting when the Gate signal is provided.
Size	Diode is relatively smaller in size.	SCR is larger in size when compared to a diode.
Cost	The cost of diode is less.	SCR is comparatively expensive than diode.
Switching	Diode provides uncontrolled switching in the circuit.	SCR provides controlled switching of the current flow.
Power rating	The power rating of the diode is comparatively lower.	SCR has high power rating.
Forward blocking mode	Diode does not have forward blocking mode, i.e. it can never block current in forward bias.	SCR has forward blocking mode, where there is no gate pulse is provided or the applied voltage is less than the breakdown voltage.
Controlling of output power	Diode does not provide controlling of output power.	In case of SCR, the output power can be controlled by varying the firing angle of it.
Voltage rating	Diode is used for low voltage applications.	SCR is a power electronic device, hence it has high voltage ratings.
Applications	A diode is used in several electronic circuits such as rectifier circuit, clipper circuit, clamper circuit, etc.	SCR is mainly used as controlled rectifier and power electronic switch.

Conclusion

We have enlisted all the noticeable differences between diode and SCR in the above table. The most significant difference is that a diode provides uncontrolled switching operation, whereas an SCR provides controlled switching operation.