Unijunction Transistors — UJT and Programmable UJT Devices

Unijunction Transistors (UJTs) are three-terminal semiconductor devices with a single PN junction and two base contacts, designed specifically for relaxation oscillator and thyristor triggering applications. While BJTs and MOSFETs dominate most discrete transistor sockets, UJTs occupy a specialised niche where their negative-resistance characteristic enables simple, component-count-efficient timing and pulse-generation circuits without the need for an auxiliary power supply or op-amp.

ADD Components sources both classic UJTs and Programmable UJTs (PUTs) for customers maintaining legacy industrial equipment, educational laboratory instruments, and specialised power-control systems. As a Hong Kong-based electronic components distributor and service provider, ADD offers consolidated procurement with anti-counterfeit verification — particularly valuable for mature part numbers like the 2N2646 and 2N6027, where open-market sourcing carries elevated risk of non-conforming or remarked devices. Доставка транзисторов в Россию осуществляется через отлаженные логистические каналы в Москву, Санкт-Петербург, Екатеринбург и Новосибирск.

UJT Sub-Types

EB1 Unijunction Transistor

The classic UJT structure consists of a lightly doped N-type silicon bar with two ohmic base contacts (B1 and B2) and a single P-type emitter junction positioned closer to B2 than B1. When the emitter voltage reaches the peak-point voltage (a fraction of the interbase voltage determined by the intrinsic standoff ratio eta), the device enters a negative-resistance region where emitter current increases rapidly as voltage falls. This negative-resistance characteristic is what makes the UJT useful as a relaxation oscillator: an RC network on the emitter charges until the peak-point voltage is reached, at which point the UJT fires and discharges the capacitor through B1, producing a sharp pulse suitable for triggering SCRs and TRIACs.

The EB1 designation specifically refers to the primary base-1 terminal variant of the UJT, where the output pulse appears at base-1. The interbase resistance (Rbb) between B2 and B1 typically ranges from 4 kOhm to 12 kOhm, and the intrinsic standoff ratio eta ranges from 0.5 to 0.85 depending on the specific part number. This is the configuration used in the vast majority of UJT application circuits.

The industry-standard 2N2646 and 2N2647 are the most recognised EB1 UJT part numbers. ADD sources these through authorised and traceable supply channels, ensuring that customers maintaining vintage test equipment, phase-control dimmers, and motor speed controllers receive genuine devices with verifiable date codes. We stock both the TO-18 metal can and TO-92 plastic packages where available.

EB2 / Complementary UJT

The EB2 variant refers to the complementary structure where the output pulse is taken at base-2 rather than base-1. In an EB2-configured UJT circuit, the timing capacitor and resistor connect differently, yielding a pulse of opposite polarity at B2. This configuration is less common in general-purpose designs but appears in specialised trigger circuits where the pulse polarity must match the requirements of a specific thyristor gate drive topology.

Complementary UJT devices use a P-type bar with an N-type emitter, reversing all polarities relative to standard N-type bar UJTs. These complementary UJTs (sometimes designated as CUJTs) are significantly rarer in the market than standard UJTs. However, for designs where they are specified, substitution with a standard UJT is not possible without circuit redesign — making traceable sourcing essential.

ADD Components also covers the Programmable Unijunction Transistor (PUT), such as the ON Semiconductor 2N6027 and 2N6028. A PUT is functionally similar to a UJT but uses a four-layer thyristor-like structure where the intrinsic standoff ratio and peak-point voltage are set externally by a resistive voltage divider on the gate terminal, rather than being fixed by the device's internal geometry. This programmability allows one PUT part number to serve across a wide range of timing voltages, reducing inventory diversity. The 2N6027 (TO-92 package) is the most widely deployed PUT in industrial timing and trigger circuits.

Applications Where UJTs Remain Relevant

• Relaxation Oscillators: UJT-based sawtooth and pulse generators with minimal external components for SCR and TRIAC gate drive.

• Thyristor Trigger Circuits: Phase-angle control in AC power regulators, light dimmers, and motor speed controllers where a simple, robust trigger source is preferred over microcontroller-based approaches.

• Timing Circuits: Long-period timers and delay generators where the negative-resistance switching provides a clean, sharp transition.

• Educational Electronics: UJTs remain a standard topic in semiconductor physics and power electronics curricula, driving steady demand from university laboratories and training centres.

• Legacy Equipment Sustainment: Industrial machinery, power supplies, and test equipment manufactured from the 1970s through 1990s that use UJTs in their original design and require like-for-like replacement to maintain calibration and safety certifications.

Representative UJT/PUT Parts We Source

Send Us Your UJT Requirements

Whether you are maintaining legacy industrial thyristor drives, stocking electronics teaching laboratories, or designing new power-control systems that benefit from the simplicity of UJT-based trigger circuits, ADD Components provides genuine, traceable unijunction transistors and programmable UJTs at competitive lead times. Our anti-counterfeit inspection process verifies marking permanence, lead finish integrity, and electrical characteristics before shipment.

Send us your BOM: Info@addcomponents.hk | +852 5334 3091 | WhatsApp: +852 5334 3091

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