Uni-junction Transistor

Definition

A Uni-junction Transistor (UJT) is a three-terminal semiconductor switching device that contains only one PN junction. Unlike bipolar junction transistors (BJTs) or field-effect transistors (FETs), the UJT operates primarily as a voltage-controlled switch, exhibiting a unique negative resistance characteristic that makes it highly effective in relaxation oscillators, timing circuits, and trigger devices for thyristors.

Main Content

1. Physical Construction and Symbol

Internal Structure: The UJT consists of a lightly doped N-type silicon bar mounted on a ceramic base. Two ohmic contacts are made at the ends of this bar, designated as Base 1 ($B_1$) and Base 2 ($B_2$). A heavily doped P-type alloy region is infused into the side of the silicon bar closer to $B_2$ to form a single PN junction. The terminal connected to this P-region is called the Emitter ($E$).
Schematic Symbol: The circuit symbol of a UJT resembles a junction field-effect transistor (JFET), but with a distinct angled arrow pointing toward the N-type channel, indicating the direction of conventional current flow from the P-type emitter to the N-type silicon bar.

          Base 2 (B2)
               |
               |
             |\|
    E --------\ \_______ Base 1 (B1)
              | /
             |/|
               |
               |

2. Equivalent Circuit Model

Internal Resistances: Internally, the N-type silicon bar acts as a simple resistive voltage divider. The total resistance of the bar between the two bases when the emitter is open-circuited is called the inter-base resistance ($R_{BB}$). It is divided into two parts: $r_{B2}$ (the resistance from $B_2$ to the PN junction) and $r_{B1}$ (the resistance from $B_1$ to the PN junction). Therefore, $R_{BB} = r_{B1} + r_{B2}$.
Intrinsic Standoff Ratio ($\eta$): The voltage drop across $r_{B1}$ determines the voltage required to turn the device on. This is governed by the Intrinsic Standoff Ratio ($\eta$), defined mathematically as: $\eta = \frac{r_{B1}}{r_{B1} + r_{B2}} = \frac{r_{B1}}{R_{BB}}$ Typically, $\eta$ ranges between $0.5$ and $0.8$. The voltage at the internal junction node point is given by $\eta V_{BB}$, where $V_{BB}$ is the external voltage applied between $B_2$ and $B_1$.

               B2
               |
               |
              [ ] r_B2
               |
        A Node +-------|<-------- E
               |    Diode (D)
              [ ] r_B1
               |
               |
               B1

3. V-I Characteristics and Negative Resistance

Peak and Valley Points: The static emitter characteristic curve (voltage $V_E$ vs. current $I_E$) is divided into three distinct operational regions. It starts in the Cut-off region where the emitter voltage is below the triggering threshold. Once $V_E$ reaches the Peak Point Voltage ($V_P$), the PN junction becomes forward-biased, and the emitter starts conducting. As the emitter current increases, the emitter voltage decreases until it reaches the Valley Point ($V_V$).
Negative Resistance Region: The area between the Peak Point and the Valley Point is the negative resistance region. In this region, a rise in emitter current ($I_E$) causes a drop in emitter voltage ($V_E$) because the injected holes decrease the resistance ($r_{B1}$) of the lower half of the silicon bar. Beyond the valley point, the device enters the saturation region, where it behaves like a normal forward-biased diode.

Emitter Voltage (VE)
   ^
   |      Peak Point (VP, IP)
   |       /\
   |      /  \
   |     /    \  Negative Resistance Region
   |    /      \
   |   /        \___ Valley Point (VV, IV)
   |  /             \_________ Saturation Region
   | / Cut-off
   +-------------------------------------> Emitter Current (IE)

Working / Process

1. Cut-off Region (Off-State)

Initial State: When an external voltage $V_{BB}$ is applied between $B_2$ and $B_1$ (making $B_2$ positive with respect to $B_1$), a voltage gradient is established along the N-type silicon bar. The voltage at the internal junction point $A$ becomes $\eta V_{BB}$.
Diode Condition: If the emitter terminal voltage $V_E$ is less than the internal node voltage plus the diode forward voltage drop ($V_D \approx 0.7V$), the internal PN junction diode remains reverse-biased. No emitter current flows except for a very small reverse leakage current, keeping the UJT in its "Off" state.

2. Triggering and Negative Resistance State

Reaching Threshold: As the external emitter voltage $V_E$ is increased gradually and reaches the Peak Point Voltage ($V_P = \eta V_{BB} + V_D$), the PN junction diode becomes forward-biased.
Resistance Reduction: Holes are injected from the heavily doped P-type emitter into the N-type channel between the emitter and $B_1$. This influx of charge carriers floods the region, dramatically increasing conductivity and lowering the resistance $r_{B1}$.
Voltage Drop: Because $r_{B1}$ drops rapidly, the voltage drop across it also falls. This causes the emitter voltage $V_E$ to decrease even as the emitter current $I_E$ increases, establishing the negative resistance characteristic of the device.

3. Saturation Region (On-State)

Stable Conduction: As the emitter current continues to rise, it eventually reaches the Valley Current ($I_V$). At this stage, the semiconductor material is fully saturated with charge carriers.
Diode-Like Behavior: Beyond the Valley Point ($V_V$), any further increase in emitter current is accompanied by a slight, proportional increase in emitter voltage. In this state, the UJT is fully turned "On" and behaves exactly like a standard forward-biased silicon diode with a very low internal resistance.

Advantages / Applications

Stable Triggering Control: The UJT has a highly stable triggering voltage that is relatively independent of temperature fluctuations, making it exceptionally reliable for precise switching operations.
Low Power Consumption: Under normal standby conditions (Cut-off region), the UJT draws very little leakage current, which makes it highly energy-efficient in standby states.
UJT Relaxation Oscillator: The most popular application of a UJT is as a relaxation oscillator. By combining it with a simple RC (resistor-capacitor) network, it can generate precise, continuous sawtooth waveforms used in time-base generators and horizontal deflection circuits.
Thyristor Triggering Circuits: Because the UJT can deliver a high-amplitude, rapid pulse of current when triggered, it is widely used to switch on Silicon Controlled Rectifiers (SCRs) and Triacs in power control circuits.

Summary

The Uni-junction Transistor (UJT) is a specialized, single-junction three-terminal semiconductor device that acts as a voltage-controlled switch. Operating by feeding a trigger voltage to its emitter, it leverages a unique negative resistance region to transition rapidly from an off-state to a highly conductive on-state, making it an essential component for relaxation oscillators, pulse generators, and thyristor triggering systems.