SCR, Diac, Triac, UJT, Transistor: Similarities and Differences

This page compares SCR, Diac, Triac, UJT (Unijunction Transistor), and a standard junction transistor. It outlines their similarities and differences to help understand their individual applications.

SCR (Silicon Controlled Rectifier)

Figure-1 depicts structure and symbol of SCR.

• Full Form: Silicon Controlled Rectifier
• Terminals: Three
• Layers: Four semiconductor layers
• Switching: Unidirectional. It conducts current only in one direction. Therefore, it can control DC power or the forward-biased half cycle of an AC input to the load.
• Control: SCRs can only control either the positive or negative half cycle of an AC input.

Figure-2 depicts characteristics of SCR.

TRIAC

Figure-3 depicts structure and symbol of TRIAC.

• Name Origin: Combination of “TRI” (three) and “AC” (alternating current).
• Terminals: Three
• Layers: Five semiconductor layers
• Control: Controls both positive and negative half cycles of an AC signal input.
• Switching: Bidirectional.
• Characteristics: The forward and reverse characteristics of a TRIAC are similar to the forward characteristics of an SCR.
• Construction: Equivalent to two separate SCR devices connected in inverse parallel.
• Operation: Once the TRIAC is triggered into conduction, the gate loses control. To turn it OFF, reduce the current below the holding value.
• Demerits: TRIACs typically have lower current capabilities compared to SCRs. Most are available in ratings less than 40 Amps and voltages up to 600 Volts.

Figure-4 depicts V-I characteristics of TRIAC.

The following can be derived from TRIAC characteristics:

• V-I characteristics in the first and third quadrants are the same, except for the direction of voltage and current flow. This is identical to the SCR characteristic in the first quadrant.
• TRIACs can function with either positive (+ve) or negative (-ve) gate control voltage. In normal operation, gate voltage is +ve in the 1st quadrant and -ve in the 3rd quadrant.

DIAC

Figure-5 depicts structure and symbol of DIAC.

• Terminals: Two
• Layers: Three, bidirectional
• Switching: Can be switched from its OFF state to ON state for either polarity of applied voltage.
• Construction: Can be made in either PNP or NPN structure form. The figure depicts a DIAC in PNP form, which has two p-regions of silicon separated by an n-region.

Let’s compare DIAC vs TRANSISTOR and understand the similarities and differences between them:

• Structure: DIAC structure is similar to a transistor’s structure.
• Base Terminal: There’s no terminal attached to the base layer in a DIAC, unlike a transistor.
• Region Size: All three regions in a DIAC are identical in size, unlike a transistor.
• Doping Concentration: The doping concentrations are identical in all three regions of a DIAC, unlike a bipolar transistor. This gives the DIAC symmetrical properties.

Figure-6 depicts characteristics of DIAC.

UJT (Unijunction Transistor)

Figure-7 depicts structure and symbol of UJT.

As shown, it’s an n-type silicon bar with connections on both ends. The leads are referred to as “B1” and “B2”. Along the bar between the two bases, a PN junction is constructed between the P-type Emitter and the N-type Bar. This lead is referred to as “Emitter Lead-E”.

• Full Form: Unijunction Transistor
• Terminals: Three, switching device made of semiconductor materials.
• Operation: When a UJT is triggered, $I_E$ (emitter current) increases re-generatively until it’s limited by $V_E$ (emitter power supply). Due to this feature, UJTs are used in applications such as sawtooth generators, pulse generators, switching circuits, etc.
• Junctions: Has only one PN junction, hence the term “UNI.”
• Also Known As: “Double Based Diode” due to having only one PN junction. The two base terminals are derived from one single section of diode (or semiconductor material).
• Doping: The emitter part is heavily doped, and the n region is lightly doped. Hence, the resistance between two base terminals is quite high when the emitter terminal is left open (about 5 to 10 KOhm).

UJT structure is similar to an N-channel FET device. However, the difference is that the P-Type Gate material surrounds the N-type material in the UJT.

Figure-8 depicts characteristics of UJT.

Junction Transistor (BJT - Bipolar Junction Transistor)

Figure-9 depicts structure and symbol of Bipolar junction Transistor.

Figure-10 depicts output characteristics of transistor in common base and common emitter configurations.

Comparison Table

In summary, these electronic components serve different functions and have distinct operating principles. SCR is used for one-way current control, Diac and Triac are employed for AC triggering and power control, UJT functions as a trigger pulse generator, and BJT serves as a versatile amplifier and switching device in electronic circuits.