Difference Between BJT and SCR


Both BJT (Bipolar Junction Transistor) and SCR (Silicon Controlled Rectifier) are semiconductor devices with alternating P type and N type semiconductor layers. They are used in many switching applications due to many reasons such as efficiency, low cost, and small size. Both of them are three terminal devices, and they provide a good control range of current with a small controlling current. Both these devices have application dependant advantages.

Bipolar Junction Transistor (BJT)

BJT is a type of transistor, and it consists of two PN junctions (a junction made by connecting a p type semiconductor and n type semiconductor). These two junctions are formed by connecting three semiconductor pieces in the order of P-N-P or N-P-N. There for two types of BJTs known as PNP and NPN.

Three electrodes are connected to these three semiconductor parts, and the middle lead is called ‘base’. Other two junctions are ‘emitter’ and ‘collector’.

In BJT, large collector emitter (Ic) current is controlled by the small base emitter current (IB) and this property is exploited to design amplifiers or switches. There for it can be considered as a current driven device. BJT is mostly used in amplifier circuits.

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 in either ‘on’ or ‘off’ state. Once the gate is triggered with a pulse, SCR goes to the ‘on’ state and keep conducting until the forward current become less than the threshold value 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.

In brief:

Difference between BJT and SCR

1. BJT has only three layer of semiconductor, whereas SCR has four layers of them.

2. Three terminals of BJT are known as emitter, collector, and base, whereas SCR has terminals known as anode, cathode, and gate

3. SCR is considered as tightly coupled pair of transistors in analysis.