The unijunction transistor (UJT) is a special semiconductor device with just one junction. It works as an electric switch, not an amplifier like other transistors. UJTs are great for making oscillators, pulse generators, and for triggering SCRs.
In the 1960s and 1970s, the UJT was a hit because it was cheap and had special negative resistance. It was used in many things like oscillators, sawtooth wave generators, and power supplies. Even now, the UJT is still used in some electronics.
General Electric brought out the first unijunction transistor (UJTs) in the 1950s. They were exploring germanium tetrode transistors then. The 2N2646, a silicon UJT they made, was a big step forward for these devices.
Introduction to Unijunction Transistors (UJTs)
Definition and Key Characteristics
A unijunction transistor (UJT) is a special kind of three-terminal device. It has one junction and works mainly as a switch. You won’t find it amplifying signals like other transistors. Instead, it’s used for specific jobs like making oscillations, generating pulses, and initializing other circuits.
Historical Development and Inventors
In 1953, the unijunction transistor was created at General Electric. It emerged from research on germanium tetrode transistors. The first ones made were using silicon. They usually had the part number 2N2646.
Construction and Components
The unijunction transistor (UJT) is made of a lightly doped n-type silicon bar. It has an electrical connection at each end, making the Base-1 (B1) and Base-2 (B2) terminals. A p-type emitter region is near the B2 terminal, diffused into the n-type bar. This section creates the emitter (E) terminal.
Semiconductor Materials and Doping
The UJT has three main parts: an emitter (E) and two bases (B1 and B2). Its base is a thinly doped n-type silicon bar. On both ends of the bar are ohmic contacts B1 and B2. The emitter, on the other hand, is heavily doped with p-type material.
Terminal Configuration: Emitter, Base1, and Base2
A UJT resembles an N-channel JFET in shape. However, in the UJT, the emitter junction is forward-biased. In comparison, a JFET is used with the gate junction reverse-biased. The UJT acts as a device that controls current with a negative resistance effect.
Structure and Internal Layers
This UJT structure starts with a lightly doped n-type silicon bar. At each of the bar’s ends, there is an electrical connection. These points form the Base-1 (B1) and Base-2 (B2) terminals. Just beside the B2, a p-type emitter section is added into the bar. This step creates the emitter (E) terminal.
Operating Principles of UJTs
When there’s no voltage difference between the emitter and the base leads, only a tiny current flows. But when a large enough voltage, called the trigger voltage, is reached, something interesting happens. A big current flows from the emitter to B2 along with the B1 to B2 current. This happens because the added current reduces the resistance in the base, near the emitter.
Unijunction Transistors: Principles and Uses
Principle of Operation: Emitter Open and Emitter Positive
When used with the emitter open, a gradient along the n-type silicon bar forms. The voltage V1 between the emitter and B1 causes a reverse bias. This stops the emitter current.
Applying a positive voltage to the emitter makes the pn-junction forward-biased. This action lets holes move from the p-type emitter to the n-type bar. It makes the region more conductive by reducing the voltage drop and lets the emitter current rise.
Intrinsic Stand-off Ratio and Peak Point Voltage
The intrinsic stand-off ratio (η) is the ratio of V1 to the total VBB. It falls between 0.51 to 0.82. The unijunction transistor’s (UJT) peak point voltage (VP) is VP = ηVBB + VD. Here, VD is the forward voltage drop of the pn-junction.
Intrinsic Stand-off Ratio (ƞ) | Inter-base Resistance (RBB) | Peak Point Voltage (VP) |
---|---|---|
0.51 to 0.82 | 4 kΩ to 10 kΩ | 15.7 V |
The unijunction transistor (UJT) has a unique negative resistance characteristic. This feature allows it to function well as an oscillator. It’s commonly used in pulse generators and saw-tooth wave generators.

Characteristics and Performance Metrics
The relationship between a unijunction transistor’s (UJT’s) emitter voltage (VE) and current (IE) at a fixed VBB is the emitter characteristics. At first, only a small current passes in the cut-off region. This happens because of the leak from B2 to the emitter through minority carriers. When VE rises, IE increases until it reaches a peak at VP and IP.
After the peak, IE suddenly increases with a drop in VE. This behavior shows a negative resistance region in the UJT’s curve.
Negative Resistance Region
The part of the UJT curve between the peak and valley points is the negative resistance region. Here, if IE goes up, VE goes down, showing a negative resistance trait. This unique feature allows the UJT to work well as an oscillator.
Cut-off and Saturation Regions
On the left side of the peak is the cut-off region. Here, the emitter-base junction is reverse-biased. As a result, only a small leakage current flows. The area to the right of the valley point is the saturation region. This is where the UJT works like a forward-biased diode.
Types of Unijunction Transistors
Many unijunction transistors (UJTs) today come in different types. The original UJT design is still widely used. But we also have the complementary UJT (CUJT) and the programmable UJT (PUT). Each type has its own unique features and uses in electronic devices.
Conventional UJT
The first UJT was quite simple in design. It had a bar of n-type material with p-type material added in. This created a specific device parameter, the “intrinsic stand-off ratio” (η). The conventional UJT, like the 2N2646, is a popular option for many applications.
Complementary UJT (CUJT)
The CUJT turns the conventional UJT idea around. It uses a bar of p-type material with n-type diffused in. The 2N61114 is a common CUJT model. This change of structure also changes its defining parameter.
Programmable UJT (PUT)
The PUT is an interesting multi-junction device. With two external resistors, it behaves like a UJT. It’s part of the thyristor family, with four p-n layers. Models like the 2N6027, 2N6028, and BRY39 are examples of PUTs. They add versatility to the UJT concept.

Applications of Unijunction Transistors
In the 1960s and 1970s, hobbyist circuits loved unijunction transistors for relaxation oscillators. They powered variable-rate strobe lights. UJTs are great for triggering thyristors too. They work with SCR and TRIAC devices. These applications let you use a DC voltage to set a device’s “on-period”.
Triggering Circuits for SCRs and Thyristors
UJTs and PUTs are key in triggering thyristors. These include SCRs and TRIACs. By adjusting a DC control voltage, the “on-period” can be fine-tuned. This is very valuable for managing large AC current control.
Timing Circuits and Sawtooth Wave Generation
UJTs are not just for triggering, they’re also used in timing circuits and creating sawtooth waveforms. The Hall effect plays a role. It changes the voltage at the pn-junction of a UJT. This affects the frequency of the relaxation oscillators, a unique feature not seen in PUTs.
Advantages and Limitations
Unijunction transistors (UJTs) have many good points and a few downsides. They are great for various semiconductor devices and switching applications.
Advantages of UJTs
UJTs are known for being low cost and for using low power. They have excellent characteristics and work well in a variety of relaxation oscillators and timing circuits.
Limitations and Considerations
Despite their benefits, UJTs have limits. They can’t work as linear amplifiers. They are also affected by temperature changes and manufacturing variations.
The first types of UJTs are obsolete now. But, the programmable UJT (PUT) is still used in triggering applications and sawtooth wave generation.
Comparison with Other Semiconductor Devices
The unijunction transistor (UJT) and the field-effect transistor (FET) work in different ways. FETs can make signals bigger, but UJTs can’t. UJTs can control big AC power using small signals, which is special.
The UJT works unlike the bipolar junction transistor (BJT) too. BJT amplifies current, but UJT resists it. UJTs are easy with one junction, while BJTs are more complicated with two.
UJTs fit well in certain uses like making pulses and triggering circuits. BJTs are better for things like amplifying, digital tech, and sensing heat because they work with more current and higher speeds.