Test: Uni Junction Transistor (UJT) - Electronics and Communication Engineering (ECE) MCQ

A unijunction transistor consists of three terminals. It is a semiconductor device that displays negative resistance and switching characteristics. It is used as a relaxation oscillator in phase control applications.

The intrinsic stand-off ratio(η) is equal to the ratio R_B1 / R_BB. Standard values of η range from 0.5 to 0.8.
Given R_B1 = 10kΩ and R_BB 
= 15kΩ:
η = R_B1 / R_BB 
= 10 / 15
= 0.67

The unijunction transistor consists of a single solid piece of N-type channel with its two outer connections marked as Base 2 and Base 1. The third connection is the Emitter and it is located along the channel. The emitter lies closer to base 2 as compared to base 1.

Unlike bipolar transistors or field effect transistors these unijunction transistors cannot be used to amplify a circuit. The unijunction transistors are used as an on-off switching transistor. They have unidirectional conductivity.

The generalized value for the voltage across resistor R_B1 is (R_B1 / R_B1 + R_B2) × V_BB. Where, R_B1 and R_B2 are the resistance across two terminals B₁ and B₂. The voltage V_BB is the voltage across the two terminals.

The core conducting component is the N-type channel of the transistor. Unijunction transistor consists of a single solid piece of N-type semiconductor material forming the main current carrying channel.

As soon as the transistor reaches the triggering voltage the unijunction transistor will turn on. If the applied voltage increases to the emitter lead, the peak voltage is achieved. The voltage drops from peak voltage to Valley Point. This happens in spite of the current increasing; this is because of the negative resistance.

The device only starts to conduct when the forward anode to cathode voltage exceeds the applied gate to cathode voltage.

Uni Junction Transistor (UJT):

• The uni-junction transistor is a solid-state three-terminal device that can be used in gate pulse, timing circuits, and trigger generator applications to switch and control both thyristors and triacs for AC power control type applications.
• When a UJT is used for triggering an SCR, the wave shape of the voltage obtained from the UJT circuit is a saw-tooth wave.
• The UJT is a three-terminal, semiconductor device which exhibits negative resistance and switching characteristics for use as a relaxation oscillator in phase control applications.
• The device has only one junction, so it is called the unijunction device
• The device, because of one P-N junction, is quite similar to a diode but it differs from an ordinary diode as it has three terminals
• The structure of a UJT is quite similar to that of an N-channel JFET
• The main difference is that P-type (gate) material surrounds the N-type (channel) material in the case of JFET and the gate surface of the JFE T is much larger than the emitter junction of UJT
• In a unijunction transistor, the emitter is heavily doped while the N-region is lightly doped, so the resistance between the base terminals is relatively high, typically 4 to 10 kilo Ohm when the emitter is open
• The N-type silicon bar has a high resistance and the resistance between emitter and base1 is larger than that between emitter and base2; It is because emitter is closer to base2 than base1
• UJT is operated with emitter junction forward- biased while the JFET is normally operated with the gate junction reverse-biased
• UJT does not have the ability to amplify but it has the ability to control a large AC power with a small signal. So that it is not used for amplification purposes. 

Applications of UJT:

• Uses negative oscillator resistance property for the generation of the sawtooth waveform
• Used as a Relaxation oscillator.
• Used as a Voltage regulator.
• Used as a switching circuit.
• Widely used as a triggering device for silicon control rectifiers (SCR).
• Used as a phase control circuit.
• Used as a timing circuit.
• Used in a sawtooth generator.
• Used to generate magnetic flux.

Unipolar Junction Transistor (UJT)

A UJT is a three-terminal semiconductor switching device. This device has a unique characteristic that when it is triggered, the emitter current increases regeneratively until it is limited by the emitter power supply. Due to this characteristic, the UJT can be employed in a variety of applications e.g., switching, pulse generator, sawtooth generator etc.

Construction: The figure shows the structure of a unijunction transistor. It consists of an n-type silicon bar with an electrical connection on each end. The leads to these connections are called base leads base. One B₁ and base two B₂. Partway along the bar between the two bases, nearer to B₂ than B₁, a p-n junction is formed between a p-type emitter and the bar. The lead to this junction is called the emitter lead E.

• Since the device has one p-n junction and three leads, it is commonly called UJT.
• With only one p-n junction, a device is really a form of diode because of the two bases taken from one section of the diode, the device is also called a double-based diode.
• The emitter is heavily doped having many holes. The n-region is lightly doped.

Characteristics of UJT

• The figure shows the curve between emitter voltage (V_E) and emitter current (I_E) Of a UJT at a given voltage VBB between the bases. These is known as the emitter characteristics of UJT.
• Initially, in the cut-off region, as V_E increases from zero, slight leakage current flows from terminal B₂ to the emitter. This current is due to minority carriers in the reverse-biased diode.
• Above a certain value of V_E, forward I_E begins to flow, increasing until the peak voltage V_p and current I_p are reached at point P.
• After the peak point P. an attempt to increase V_E is followed by a sudden increase in emitter current I_E with a corresponding decrease in V_E. This is a negative resistance portion of the curve because of an increase in I_E. V_E decreases. The device, therefore, has a negative resistance region that is stable enough to be used with a great deal of reliability in many areas, e.g., trigger circuits, sawtooth generators, biasing circuits.
• The negative portion of the curve lasts until valley point V is reached with valley point voltage V_v and valley-point current I_v. After the valley point, the device is driven to saturation.

UJT Relaxation Oscillator

A UJT relaxation oscillator consists of a capacitor C and a resistor R connected to the emitter of the UJT, as shown in Fig. Base B₂ of the UJT is connected to the power-supply positive through resistor R₂, and base B₁ is connected to the ground through resistor R, Usually, R₁ and R₂ are low-value resistors, a typical value being 47 ohms each. These are used to limit the current through the UJT and protect it. We can take three outputs from the UJT relaxation oscillator. These are Vo₁, V_o2, and V_o3, respectively.

The Unijunction transistor is 2-layer, the 3-terminal solid-state switching device (silicon). It is also called as the double-base diode. The device has a unique characteristic that when it is triggered, its emitter current increases regeneratively (due to negative resistance characteristic) until it is restricted by an emitter power supply. Since the device has one PN junction and three leads it is commonly called UJT.

A unijunction transistor (UJT) is a three-terminal semiconductor device having one p-n junction. It is mostly used as a switch and commonly employed in timing circuits, SCR/Triac triggering circuits, oscillators, and so on. It consists of an n-type silicon semiconductor bar having ohmic contacts at each end. The two end connections are called base-1 and base-2, which are labeled as B1 and B2, respectively. This n-type semiconductor is lightly doped. Near terminal B2, a small heavily doped p-region is alloyed which forms a p-n junction with the bar. The terminal to this junction is called emitter (E). The resistance of the silicon bar is called InterBase resistance RBB, i.e. the resistance between the terminals B1 and B2.

Internally UJT acts like a voltage divider consisting of two series resistors R_B1 and R_B2, while the P-N junction acts like a diode D driving the junction of R_B1 and R_B2. 

Since P-region is heavily doped as compared to N-region, holes are injected from P-region to the lower half of UJT. As N-region is lightly doped the holes injected have a longer lifetime The flooding of the lower half of UJT with holes drastically lowers the resistance R_B1. Because of this, the drop across R_B1 reduces to a low value, and the emitter current increases. 

A unijunction transistor (UJT) is a three-terminal semiconductor device having one p-n junction. It is mostly used as a switch and commonly employed in timing circuits, SCR/Triac triggering circuits, oscillators, and so on. It consists of an n-type silicon semiconductor bar having ohmic contacts at each end. The two end connections are called base-1 and base-2, which are labeled as B1 and B2, respectively. This n-type semiconductor is lightly doped. Near the terminal B2, a small heavily doped p-region is alloyed which forms a p-n junction with the bar. The terminal to this junction is called emitter (E).

The total resistance of the silicon bar from terminal B₂ to B₁ with emitter open is called the InterBase resistance RBB. Thus,

RBB = RB₁ + RB₂, with emitter open

where RBB is typically in the range of 4 kΩ to 10 kΩ and RB₁ is usually greater than RB₂ (RB₂ = 60% of RBB. Hence the resistance of the base terminal is very high in UJT.

A junction transistor is an analogous to a vacuum triode. The main difference between them is that a transistor is a current device while a vacuum triode is a voltage device. The advantages of a transistor over a vacuum triode are long life, high efficiency, light weight, smaller in size, less power consumption.

In most of the transistors, the collector is made larger than emitter region. This is due to the fact that collector has to dissipate much greater power. The collector and emitter cannot be interchanged.