The Unijunction Transistor (UJT) is a three-terminal, single-junction semiconductor device engineered for one primary job: switching. Unlike bipolar junction transistors (BJTs) used for linear amplification, the UJT exploits a negative resistance characteristic to act as a voltage-triggered switch. It remains the foundational component for triggering Silicon Controlled Rectifiers (SCRs) and TRIACs, and for building simple, low-cost relaxation oscillators.

While largely superseded by the Programmable Unijunction Transistor (PUT) and 555 timers in new commercial designs, the UJT is still heavily encountered when repairing legacy industrial motor controls, lighting dimmers, and power supply crowbar circuits. If you are bench-testing older equipment or studying negative resistance, you need to know how to identify, bias, and test it. The safe default part numbers for most bench replacements are the 2N2646 and 2N2647.

UJT Pinout, Symbol, and Core Specifications

The schematic symbol for a UJT features an arrow pointing at a slanted line. The arrow represents the Emitter (E), which is the only doped P-type junction in the device. The slanted line represents the N-type silicon bar, terminated by Base 1 (B1) at the bottom and Base 2 (B2) at the top.

In a standard TO-92 package (flat side facing you, pins pointing down), the typical pinout for the ubiquitous 2N2646 is B1 (left), E (center), B2 (right). However, always verify with a datasheet, as some European BC-series UJTs use a different arrangement.

Bench Tip: Never confuse B1 and B2. The internal silicon bar is physically asymmetrical. Reversing B1 and B2 won't instantly destroy the part, but it will alter the intrinsic standoff ratio ($\eta$), causing your oscillator to run at the wrong frequency or fail to trigger entirely.

Reference Table: Safe Default UJT Part Numbers & Ratings

When sourcing replacements, the 2N2646 is the industry workhorse. If you need a higher standoff ratio or different interbase resistance, the 2N2647 is the direct alternative. Because true UJTs are legacy parts, the 2N6027 (a PUT) is often used as a modern functional substitute, though its pinout and biasing differ.

Part Number Device Type $R_{BB}$ (Interbase Resistance) $\eta$ (Intrinsic Standoff Ratio) Max $V_{B2B1}$ Max $I_E$ (Emitter Current)
2N2646 N-Base UJT 4.7 k$\Omega$ - 9.1 k$\Omega$ 0.56 - 0.75 35 V 2.0 A (Pulse)
2N2647 N-Base UJT 7.0 k$\Omega$ - 15.0 k$\Omega$ 0.68 - 0.82 35 V 2.0 A (Pulse)
B13 / B30 N-Base UJT 4.0 k$\Omega$ - 12.0 k$\Omega$ 0.50 - 0.80 30 V 1.5 A (Pulse)
2N6027 PUT (Modern Sub) N/A (Programmable via $R_1, R_2$) Programmable 40 V 2.0 A (Pulse)

Data sourced from historical ON Semiconductor and Components101 datasheets.

Biasing, Selection, and the Relaxation Oscillator Circuit

To select and bias a UJT, you must understand the intrinsic standoff ratio ($\eta$). This is a fixed internal voltage divider ratio determined by the physical placement of the Emitter junction along the N-type base bar. The voltage at the Emitter required to "fire" or trigger the UJT is the Peak Point Voltage ($V_P$):

$V_P = (\eta \times V_{BB}) + V_D$

Where $V_{BB}$ is the voltage applied across B2 and B1, and $V_D$ is the forward voltage drop of the P-N junction (typically 0.5V to 0.7V). You select a UJT based on its $\eta$ to match your required trigger threshold, and its $R_{BB}$ to ensure your timing resistors fall within a workable range.

Complete Application: 12V Relaxation Oscillator

The most common UJT application is the relaxation oscillator, which generates a sawtooth waveform on the capacitor and sharp positive pulses at B1. This circuit is ideal for triggering SCRs in phase-control applications.

Component Values for ~90 Hz Output:

  • $V_{BB}$: 12V DC regulated supply
  • $R_E$ (Timing Resistor): 10 k$\Omega$ (Must be low enough to allow $I_E$ to exceed the peak point current, $I_P$, but high enough to stay below the valley current, $I_V$)
  • $C_E$ (Timing Capacitor): 1 $\mu$F (Electrolytic or Film)
  • $R_{B1}$ (Output Pulse Resistor): 100 $\Omega$ (Generates the positive trigger pulse)
  • $R_{B2}$ (Temp Compensation Resistor): 470 $\Omega$ (Stabilizes frequency against temperature drift)

How it works: The capacitor $C_E$ charges through $R_E$ toward 12V. Once the capacitor voltage reaches $V_P$ (approx. 8.3V for a 2N2646 where $\eta \approx 0.63$), the Emitter junction forward-biases. The UJT enters its negative resistance region, dumping the capacitor's stored charge rapidly through B1. This creates a sharp voltage spike across $R_{B1}$. Once the capacitor voltage drops to the Valley Voltage ($V_V$), the UJT snaps back into cutoff, and the cycle repeats.

Frequency Calculation:
$f = \frac{1}{R_E \times C_E \times \ln(\frac{1}{1 - \eta})}$
Assuming $\eta = 0.63$, $R_E = 10,000$, and $C_E = 0.000001$:
$f = \frac{1}{0.01 \times \ln(2.70)} = \frac{1}{0.01 \times 0.993} \approx 100 \text{ Hz}$ (Actual bench frequency will vary slightly due to component tolerances and $V_D$).

Operation Regions and Negative Resistance

The UJT is defined by its negative resistance characteristic—a phenomenon where an increase in voltage across a terminal results in a decrease in current, or vice versa. Understanding the three distinct operating regions is critical for debugging oscillator circuits that fail to start or fail to turn off.

Operation Region Emitter Voltage ($V_E$) Emitter Current ($I_E$) Internal Silicon State
1. Cutoff $V_E < V_P$ $I_E \approx 0$ (Leakage only, $< 5 \mu A$) E-B1 junction is reverse-biased. High impedance. Capacitor is charging.
2. Negative Resistance $V_P > V_E > V_V$ Rapidly increasing (mA range) E-B1 junction forward-biases. Holes inject into N-bar, dropping B1 resistance drastically. Voltage across E-B1 actually falls as current rises.
3. Saturation $V_E \approx V_V$ (Valley Voltage) $I_E$ limited only by external $R_E$ The N-bar is fully flooded with charge carriers. The UJT acts like a closed switch with a low forward voltage drop (typically 1.5V to 2.5V).

If your relaxation oscillator "hangs" and the capacitor stays charged, your timing resistor $R_E$ is too large, starving the UJT of the necessary peak point current ($I_P$) to push it through the negative resistance region into saturation. If the oscillator refuses to turn off and $V_E$ stays low, $R_E$ is too small, supplying more current than the valley current ($I_V$), keeping the UJT permanently latched in saturation.

Bench Testing: How a UJT Fails and How to Test It

UJTs are rugged, but they fail in two predictable ways. Overcurrent through the Emitter (usually from a shorted timing capacitor or excessive $V_{BB}$) will melt the delicate P-N junction, resulting in a dead short between E and B1. Overvoltage across the base terminals ($V_{B2B1}$ exceeding 35V) will burn out the internal silicon bar, resulting in an open circuit between B1 and B2.

You can definitively test a UJT on the bench using a standard digital multimeter (DMM). For deeper theory on semiconductor junction testing, refer to the UJT tutorials on Electronics Tutorials.

Numbered Multimeter Testing Steps

  1. Test Interbase Resistance ($R_{BB}$): Set your DMM to Ohms ($\Omega$). Place probes across B1 and B2 (polarity doesn't matter). You should read a resistance between 4 k$\Omega$ and 12 k$\Omega$ (depending on the specific part number). If you read OL (Open Line) or 0 $\Omega$, the silicon bar is destroyed. Discard the part.
  2. Test Emitter to B1 Junction: Set your DMM to Diode Test mode. Place the red probe on the Emitter and the black probe on B1. You should read a forward voltage drop between 0.4V and 0.9V (typically around 0.55V). Reverse the probes; the meter should read OL.
  3. Test Emitter to B2 Junction: Keep the DMM in Diode Test mode. Place the red probe on the Emitter and the black probe on B2. You should read a similar forward voltage drop (0.4V to 0.9V). Reverse the probes; it should read OL.
  4. Verify Standoff Ratio (Advanced): If the diode tests pass but the circuit still misbehaves, power the UJT on a breadboard. Apply 10V across B2 (+) and B1 (-). Measure the DC voltage from B1 to Emitter. It should read $\eta \times 10V$ (e.g., ~6.5V for a 2N2646). If it reads 0V or 10V, the internal junction is compromised even if the diode test passed.
Safety Warning: When testing UJTs in-circuit (soldered to a PCB), you must completely de-energize the board and discharge all timing and filter capacitors. The $R_{BB}$ resistance reading will be wildly inaccurate if parallel PCB traces or shunt resistors are connected across B1 and B2. For accurate diagnostics, desolder at least the B2 pin.

UJT vs. PUT vs. 555 Timer: What to Use Today

If you are designing a new circuit from scratch, or if you cannot source a NOS (New Old Stock) 2N2646, you need to know your alternatives. The semiconductor guides at All About Circuits outline the transition from UJTs to modern timing ICs.

Criterion True UJT (2N2646) PUT (2N6027) 555 Timer (NE555)
Availability (2026) Poor (Mostly NOS / Surplus) Fair (Still in limited production) Excellent (Massively produced)
Trigger Voltage Fixed by internal $\eta$ Programmable via external $R_1, R_2$ Fixed at 1/3 and 2/3 $V_{CC}$
Component Count Low (1 UJT, 2 R, 1 C) Medium (1 PUT, 4 R, 1 C) Medium (1 IC, 2 R, 1-2 C)
Max Frequency ~100 kHz ~100 kHz ~500 kHz (Standard bipolar)
Best Application Repairing legacy SCR triggers Custom threshold crowbar circuits Precision PWM, astable clocks

Choose the UJT only when you are repairing legacy equipment that requires an exact BOM match, or when you need a high-current, ultra-fast rise-time pulse to directly gate a sensitive TRIAC without a buffer transistor. Choose the PUT if you need UJT-like negative resistance behavior but require a specific, non-standard trigger voltage that the fixed $\eta$ of a 2N2646 cannot provide. Choose the 555 Timer for 95% of new hobbyist and industrial timing applications where component availability, frequency stability, and precise duty-cycle control are paramount.