2N2646 Uni Junction Transistor & Its Working

A semiconductor switching device like Unijunction Transistor or UJT includes three terminals with two layers. As the name suggests, it includes only a single junction and it is capable of limiting huge power through a small input signal. It is a low-cost device, so applicable in different electronic circuits like pulse generators, oscillators, triggers, etc. This article discusses an overview of a 2N2646 Uni Junction Transistor and its working.

What is 2N2646 Uni Junction Transistor?

2n2646 UJT transistor is a 3-terminal semiconductor device that contains only one junction & it behaves like an electrically controlled switch. This UJT or uni-junction transistor cannot be used as a linear amplifier however it is widely used in triggered oscillators, free-running oscillators, and pulse generation circuits of low frequencies.

This transistor is composed of n-type semiconductors wherever p-type material is fixed with its length to assist in fixing the intrinsic standoff ratio parameter (η). Typical 2n2646 UJT includes three terminals like emitter, base & collector.

2N2646 Uni Junction Transistor
2N2646 Uni Junction Transistor

In this transistor, the base terminal is composed of n-type silicon which is lightly doped whereas the emitter terminal is doped heavily & includes p-type material. This device mainly depends on the formation of a single junction among two materials like p-type & n-type. Once the emitter terminal behaves like an open-circuit, then the resistance between two base terminals (B1 & B2) is known as inter-base resistance.

The emitter (E) is closed to base terminal B2 than base terminal B1, then this device doesn’t include a symmetrical construction but some amount of current will be supplies through B1 to B2 whenever there is no potential dissimilarity between two terminals like E & B. But, if the huge voltage is given at the emitter (E) terminal then large current will supply from two base terminals like B1 to B2 so that it will generate huge B2 o/p current.

The UJT structure & N-Type JFET is quite similar, however, the surface of the gate terminal within N-Type JFET is larger as compared to the emitter junction within UJT. This transistor generally works once the emitter junction is connected in forwarding bias whereas the JFET functions normally whenever the gate terminal is connected in reversed bias.

Working of 2N2646 Uni Junction Transistor

Once this transistor is turned ON then it permits the emitter current to enhance until it is restricted through the power supply in the emitter terminal. This transistor can also be used as an oscillator because it exhibits negative resistance.


Once the positive supply is given to the two base terminals of this transistor then a potential drop will occur. The current will flow from the emitter terminal to the base, once the voltage of the emitter is increased one diode voltage over the voltage wherever P-type material diffusion occurs.

This extra current on the base region permits the resistance among emitter (E) & B2 which leads to reduction; therefore the conductivity of the UJT improves. So this resistance drop will make the emitter terminal connect in forwarding bias so that the current will be injected more. So, this resistance drop will make the UJT operate above negative resistance. So it is very useful for oscillator circuits.

Pin Configuration

The pin configuration of a 2N2646 Uni junction transistor is shown below. This transistor includes three terminals which are discussed below.

2N2646 UJT Symbol
2N2646 UJT Symbol
  • Pin1 (Base 1): This pin is connected normally to power
  • Pin2 (Emitter): This pin is used to turn ON/OFF the transistor
  • Pin3 (Base 2): This pin is connected normally to Load
  • Pin4 (Case): This pin is connected internally to the base2 pin

Features & Specifications

The features and specifications of the 2N2646 Uni Junction Transistor include the following.

  • The maximum two bases voltage (VB2B1) is 35V
  • Emitter reverse voltage Max (VB2E) is 30V
  • RMS emitter current Max (Ie) is 50mA
  • Peak emitter current max (Ie) is 2A
  • The range of operating temperature is -65ºC to +150ºC
  • Power dissipation max is 300mW
  • Accessible like high reliability (HR)
  • Accessible like Sn/Pb plating (non-RoHS)
  • The typical low emitter reverse current is 0.005µA (Typ)
  • The surface is passivated for uniformity & reliability

The similar 2N2646 UJTs are 2N2647, 2N4871, 2N4870, & 2N1671.

Once the power is turned ON, then the uni-junction transistor will activate to high resistance condition & whole voltage supply come into view across it. As a result, throughout the beginning period, the voltage drop of UJT will be equivalent to VCC.

How to use 2N2646 Uni Junction Transistor?

The circuit diagram of the LED flasher using the 2N2646 unijunction transistor is shown below. This circuit operates with 9V- 12V DC. The required components to build this circuit are a 9V to 12V battery, 2N2646 UJT, LED, 10KΩ & 390Ω resistors, breadboard, and connecting wires.

This circuit can be built with a 2N2646 transistor which is a PN UJT. This transistor includes three pins like bases-2 & emitter-1. This circuit works like an oscillator using a resistor -10K and a capacitor – 100uF.

LED Flasher with 2N2646 UJT
LED Flasher with 2N2646 UJT

In this circuit, a resistor with 390 ohms is mainly used to restrict the flow of current throughout the transistor & the LED. The LED flashing frequency mainly depends on the R1 & C1 values. By using these values, we can make some small experiments to make the preferred LED flashing rate.

For resistor ‘R1’, we can utilize any value from the range of 10K & 1M. In the above circuit, for resistor ‘R1’, a 1M potentiometer or variable resistor is used whereas for a capacitor ‘C1’, you can utilize any value that ranges from 1uF – 100uF.

Where to use 2N2646 UJT/Applications

The applications of 2N2646 Uni Junction Transistor include the following.

  • This UJT transistor is used to make different circuits like SCR firing, phase control, timing, etc
  • It is used in industrial-based applications for general purpose
  • Used to make DIY projects
  • Used in Sawtooth wave generator
  • Relaxation oscillator
  • Voltage detector
  • It is used in different devices which are used for amplifying and switching.
  • Used to make electronics projects

Thus, this is all about an overview of the 2N2646 Uni Junction Transistor datasheet which is mainly designed to use in industrial applications for general purposes. This UJT transistor is very popular as a triggering device but not suitable for controlling power loads. 2N2646 UJT is mainly designed to use in firing circuits for SCRs & not applicable for amplifier circuits such as transistors. This component is ideal in the applications wherever certain least pulse amplitude is necessary & also wherever a trigger current and low emitter leakage current are necessary. 2N2646 UJT is mainly used in low-cost industrial applications.

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