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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.



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.

SCR characteristics

Figure-2 depicts characteristics of SCR.


TRIAC Structure and Symbol

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.

Triac characteristics

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 construction and symbol

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.

Diac characteristics

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


UJT Structure and Symbol

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, IE increases re-generatively until it is limited by VE . Here IE is emitter current and VE 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.

UJT characteristics

Figure-8 depicts characteristics of UJT.

Junction Transistor


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

transistor characteristics

Figure-10 depicts output characteristics of transistor in common base and common emitter configurations.
Refer BJT vs FET for more information.


Specifications SCR Diac Triac UJT BJT
Function Controlled rectification Bidirectional trigger Bidirectional switch Pulse generation Amplification and switch
Current control One way Bidirectional Bidirectional N/A (Not a switch) N/A (Variable control)
Triggering Requires gate current Voltage threshold Gate current or voltage External voltage pulse Base current
Latching behavior Latches on until anode current drops below a certain threshold N/A (Only triggers) Latches on until anode current drops below a threshold N/A (No Latching) N/A (No Latching)
Applications Power control, rectification Phase control, triggering circuits AC power switching, dimming, motor control, heating control Pulse generators, timing circuits Amplifiers, signal switching, amplification

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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