SCR, Diac, Triac, UJT, Transistor | Similarities and difference between SCR, Diac, Triac, UJT(Unijunction Transistor)

This page compares SCR vs Diac vs Triac vs UJT vs Transistor and mentions similarities and difference between SCR, Diac, Triac, UJT (Unijunction Transistor) and normal junction transistor.

SCR

Figure-1 depicts structure and symbol of SCR.
• The full form of SCR is Silicon Controlled Rectifier.
• It is a three terminal device.
• It has 4 layers of semiconductor.
• It is a unidirectional switch. It conducts current only in one direction. Hence it can control DC power only OR it can control forward biased half cycle of AC input in the load.
• Basically SCR can only control either positive or negative half cycle of AC input.

Figure-2 depicts characteristics of SCR.

TRIAC

Figure-3 depicts structure and symbol of TRIAC.
• The name "TRIAC" is derived from combination of "TRI" means three and "AC" or alternating current.
• It is a three terminal semiconductor device.
• It has 5 layers of semiconductor.
• It can control both positive and negative half cycles of AC signal input.
• It is a bidirectional switch.
• The forward and reverse characteristics of TRIAC is similar to forward characteristics of SCR device.
• Contruction of TRIAC is equivalent to 2 separate SCR devices connected in inverse parallel as shown in the figure.
• Similar to the SCR, once the triac is fired into conduction, the gate will lose all the control. At this stage, the TRIAC can be turned OFF by reducing current in the circuit below the holding value of current.
• The main demerit of TRIAC over SCR is that TRIAC has lower current capabilities. Typically most of the TRIACs are available in ratings less than 40 Amp and at voltages upto 600 Volt.

Figure-4 depicts V-I characteristics of TRIAC. Following can be derived from TRIAC characteristics.
• VI characteristics in first and third quadrants are same except direction of voltage and current flow. This characteristic in the 1st and 3rd quadrant is identical to SCR characteristic in the 1st quadrant.
• TRIAC can function with either positive(+ve) or negative(-ve) gate control voltage. In normal operation, gate voltage is +ve in 1st quadrant and -ve in 3rd quadrant.

DIAC

Figure-5 depicts structure and symbol of DIAC.
• It is a two terminal device.
• It is 3 layer bidirectional device.
• Diac can be switched from its off state to ON state for either polarity of applied voltage.
• The DIAC can be made either in PNP or NPN structure form. The figure depicts DIAC in PNP form which has two p-regions of silicon separated by n-region.

Let us compare DIAC vs TRANSISTOR and understand similarities and difference between DIAC and TRANSISTOR.
• Structure of DIAC is similar to the structure of transistor.
• There is no terminal attached with base layer in DIAC unlike transistor.
• All the three regions in DIAC are identical in size unlike transistor.
• The doping concentrations are identical in these three regions in DIAC unlike bipolar transistor. This will give DIAC device symmetrical properties.

Figure-6 depicts characteristics of DIAC.
Refer Diac vs Triac for more information.

UJT

Figure-7 depicts structure and symbol of UJT.
As shown it is n-type silicon bar with connections on both ends. The leads are referred as "B1" and "B2". Along the bar between the two bases , PN junction is constructed between P-type Emitter and N-type Bar. This lead is referred as "Emitter Lead-E".

• It is the short form of Unijunction Transistor.
• It is 3 terminal switching device made of semiconductor materials.
• When UJT is triggered, I_E increases re-generatively until it is limited by V_E . Here I_E is emitter current and V_E is emitter power supply. Due to this feature, UJT is used in wide variety of applications such as sawtooth generator, pulse generator, switching etc.
• Device has only one PN junction and hence the term "UNI" in Unijunction Transistor (UJT).
• The UJT is also known as "Double Based Diode". This is due to the fact that it has only one PN junction. The two base terminals are derived from one single section of diode(or semiconductor material).
• In UJT, emitter part is heavily doped and n region is lightly doped. Hence resistance between two base terminals is quite high when emitter terminal is left open. The value of resistance is about 5 to 10 KOhm.

UJT structure is similar to N-channel FET device. But the difference between UJT and FET is that P-Type Gate material is surrounded around N-type material.

Figure-8 depicts characteristics of UJT.

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.
Refer BJT vs FET for more information.

Comparison

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.

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