Both diode and SCR (Silicon Controlled Rectifier) are semiconductor devices with P type and N type semiconductor layers. They are used in many electronic switching applications. Both devices have terminals called ‘anode’ and ‘cathode’ but SCR has an additional terminal called ‘gate’. Both these devices have application dependant advantages.
Diode
Diode is the simplest semiconductor device and it consists of two semiconductor layers (one P-type and one N-type) connected to each other. Therefore diode is a PN junction. Diode has two terminals known as the anode (the P-type layer) and cathode (the N-type layer).
Diode allows current flows through it only in one direction that is anode to cathode. This direction of current is marked on its symbol as an arrow head. Since diode restricts the current to only one direction, it can be used as a rectifier. The full bridge rectifier circuit which is made of four diodes can rectify an alternative current (AC) to a direct current (DC).
The diode starts acting as a conductor when a small voltage is applied in the direction of anode to cathode. This voltage drop (known as the forward voltage drop) is always there when a current flow happens. This voltage is usually about 0.7V for normal silicon diodes.
Silicon Controlled Rectifier (SCR)
SCR is a type of thyristor and widely used in current rectification applications. SCR is made of four alternating semiconductor layers (in the form of P-N-P-N) and therefore consists of three PN junctions. In analysis, this is considered as a tightly coupled pair of BJTs (one PNP and other in NPN configuration). The outermost P and N type semiconductor layers are called anode and cathode respectively. Electrode connected to inner P type semiconductor layer is known as the ‘gate’.
In operation, SCR acts conducting when a pulse is provided to the gate. It operates at in either ‘on’ or ‘off’ state. Once the gate is triggered with the pulse, SCR goes to the ‘on’ state and keep conducting until the forward current become less than a threshold known as ‘holding current’.
SCR is a power device and most of the times it is used in applications where high currents and voltages are involved. The most used SCR application is controlling (rectifying) alternating currents.
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What is the difference between BJT and SCR? 1. Diode has only two layers of semiconductor, whereas SCR has four layers of them. 2. Two terminals of diode are known as anode and cathode, whereas SCR has three terminals known as anode, cathode and gate 3. SCR can be considered as a pulse controlled diode in analysis. 4. SCR can operate at higher voltages and currents than diodes. 5. Power handling is better for SCRs than diodes. 6. Symbol of the SCR is derived by adding a gate terminal to the symbol of diode.
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