Unijunction Transistor (UJT) Characteristics

The Unijunction Transistor

• The Unijunction Transistor (UJT) has three terminals.
• Terminals are labeled Emitter (E), Base 1 (B1), and Base 2 (B2).
• The UJT symbol is different from that of a JFET because the arrow is at an angle.
• The UJT has only one PN junction.
• The UJT's characteristics differ from those of BJTs and FETs.
• The base resistance is 5000 to 10,000 ohms between terminals B1 and B2.
• Without emitter current, voltage applied between B1 and B2 drops uniformly.
• With 10 volts applied, the voltage at a PN junction halfway is 5 volts.
• Less than 5 volts applied to the emitter reverse biases the PN junction.
• More than 5 volts forward biases the PN junction.
• With 10 volts across the base, the junction switches from high to low resistance if 5 or more volts are applied to the emitter.
• A UJT can switch on at different emitter voltages by varying the supply voltage across the base.
• UJTs have very small reversed emitter current in the cut-off region if the emitter is reversed biased.
• When the emitter voltage reaches the forward bias (turn-on) value ("Peak point"), the forward emitter current rapidly increases, with a fall in emitter voltage, in the negative resistance region.
• in the saturation region, the device acts as a positive resistor, with emitter current rising slowly with emitter voltage.

UJT Application

• The UJT can be used as a trigger device for SCRs and triacs.
• Other applications include non-sinusoidal oscillators, sawtooth generators, phase control, and timing circuits.
• A UJT relaxation oscillator serves as one example application.
• With applied DC power, capacitor C charges exponentially through R₁ until it reaches the peak-point voltage, Vp.
• At Vp, the PN junction becomes forward biased, and the emitter characteristic enters the negative resistance region where VE decreases and IE increases.
• The capacitor quickly discharges through the forward-biased junction and R2.
• When the capacitor voltage decreases to the valley-point voltage, Vv, the UJT turns off.
• The capacitor then begins to charge, and the cycle repeats, as shown in the emitter voltage waveform.
• During capacitor discharge, the UJT conducts, developing a voltage across R2.

The Programmable Unijunction Transistor

• The Programmable Unijunction Transistor (PUT) is a type of thyristor, unlike the UJT in structure.
• The PUT can replace the UJT in some oscillator applications.
• The PUT resembles an SCR, but its anode-to-gate voltage can turn the device both on and off.
• The PUT's structure is similar to that of an SCR (four-layer), with the gate brought out.
• The gate connects to the N region adjacent to the anode and the PN junction controls the on and off states.
• The gate can be biased to a desired voltage using an external voltage divider.
• When the anode voltage exceeds this programmed level, the PUT turns on.
• A plot of the anode-to-cathode voltage, VAK, versus anode current, In, reveals a characteristic curve similar to the UJT.
• The PUT therefore replaces the UJT in many applications, such as the relaxation oscillator.

The Phototransistor

• The phototransistor has a light-sensitive, collector-base PN junction.
• It is exposed to incident light through a lens opening.
• With no incident light, a small thermally-generated collector-to-emitter leakage current, ICEO (dark current), is present, typically in the nA range.
• When light strikes the collector-base PN junction, a base current, Ix, is produced, directly proportional to light intensity.
• This action produces a collector current that increases with Ix.
• Except for base current generation, the phototransistor behaves as a conventional BJT.
• There is no electrical connection to the base in many cases.
• Base current photo-generation occurs in the collector-base region.
• The larger the physical area of this region, the greater the base current generated.
• Phototransistors are designed with a large area to the incident light.
• A phototransistor can be a two-lead or three-lead device.
• In the three-lead configuration, the base lead is brought out, allowing use as a conventional BJT with or without light-sensitivity.
• In the two-lead configuration, the base is not electrically available, and the device can only be used with light as the input.
• In many applications, the phototransistor is used in a two-lead version.
• The Phototransistor bias circuit with current curves shows a basic bias circuit and typical collector characteristic curves.
• Each individual curve on the graph corresponds to a certain value of light intensity (in this case, the units are mW/cm²) and that the collector current increases with light intensity.