UJT -Uni Junction Transistor
Table of Contents
UJT Full Form – Uni Junction Transistor
Uni Junction Transistor
The Double-base diode, also known as the Unijunction Transistor (abbreviated as UJT), is a 2-layer, 3-terminal solid-state (silicon) Switching device. The device has the Distinctive property that, upon Activation, the emitter current grows Regeneratively (as a result of the negative resistance Characteristic) until it is Constrained by the emitter power supply. The device is Frequently referred to as a UJT because it has three leads and one pn Junction.
1. Construction of a UJT
Figure illustrates the Unijunction Transistor’s Fundamental design. A small piece of heavily doped P-type material is alloyed to one side of a bar of lightly doped N-type silicon to create a single P-N junction. The term “unijunction” refers to a single P-N junction. As shown, the silicon bar has two ohmic contacts at its ends that are called base-1 (B1) and base-2 (B2), and the P-type region is known as the emitter (E). Because symmetrical units do not offer the best Electrical Characteristics for the Majority of applications, the emitter junction is typically placed closer to base-2 (B2) than base-1 (B1) in order to prevent symmetry in the device.
The symbol for Unijunction transistor is shown in figure. The emitter leg is drawn at an angle to the vertical line representing the N-type material slab and the arrowhead points in the direction of conventional current when the device is Forward-biased, active or in the conducting state. The basic arrangement for the UJT is shown in figure.
A complementary UJT is formed by diffusing an N-type emitter terminal on a P-type base. Except for the polarities of voltage and current, the characteristics of a complementary UJT are exactly the same as those of a conventional UJT.
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.
An N-channel JFET and a UJT have very similar structural characteristics. The primary distinction is that in a JFET, N-type (channel) material is surrounded by P-type (gate) material, and the JFET’s gate surface is considerably larger than the UJT’s emitter junction.
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 base-1 is larger than that between emitter and base-2. It is because emitter is closer to base-2 than base-1.
UJT is operated with emitter junction forward- biased while the JFET is normally operated with the gate junction reverse-biased.
Although UJT can’t amplify, it can control a lot of ac power with a little bit of signal. It has a negative resistance trait, making it suitable for use as an oscillator.
2. UJT parameters
RBBO : It is the resistance between the terminals B1 and B2. In simple words, it is the resistance of the N-Type bar when measured lengthwise. If RB1is resistance of the bar from E to B1 and RB2 is the resistance of the bar from E to B2, then RBBO can be expressed as RBBO= RB1+RB2. The typical range of RBBO is from 4KΩ to 10KΩ.
Intrinsic standoff ratio (η) : It is the ratio of RB1 to the sum of RB1 and RB2. It can be expressed as η = RB1/(RB1+RB2) or η = RB1/RBBO. The typical range of intrinsic standoff ratio is from 0.4 to 0.8
Operation of a UJT
Imagine that the emitter supply voltage is turned down to zero. Then the intrinsic stand-off voltage reverse-biases the emitter diode, as mentioned above. If VB is the barrier voltage of the emitter diode, then the total reverse bias voltage is VA + VB = η VBB + VB. For silicon VB = 0.7 V.
Now let the emitter supply voltage VE be slowly increased. When VE becomes equal to η VBB, IEo will be reduced to zero. With equal voltage levels on each side of the diode, neither reverse nor forward current will flow.
When emitter supply voltage is further increased, the diode becomes forward-biased as soon as it exceeds the total reverse bias voltage (ηVBB + VB). This value of emitter voltage VE is called the peak-point voltage and is denoted by VP. When VE = VP, emitter current IE starts to flow through RB1 to ground, that is B1. This is the minimum current that is required to trigger the UJT. This is called the peak-point emitter current and denoted by IP. Ip is inversely proportional to the interbase voltage, VBB.
Now, charge carriers are introduced into the RB region of the bar when the emitter diode begins to conduct. The resistance of region RB rapidly decreases because of more charge carriers because the resistance of a semiconductor material depends on doping (holes). The voltage drop across RB also decreases with this decrease in resistance, increasing the forward bias of the emitter diode. Additionally, more charge carriers are injected as a result of the larger forward current, which further reduces the resistance of the RB region.
As a result, the emitter current keeps rising until the emitter power supply caps it. The UJT is said to have a negative resistance characteristic because VA decreases as emitter current increases. It can be seen that base-2 (B2) is only utilised when external voltage VBB is applied across it. The active terminals are Terminals E and B1. A suitable positive pulse is typically applied to the emitter to cause the UJT to conduct. Applying a negative trigger pulse will disable it.
The static emitter characteristic (a curve showing the relation between emitter voltage VE and emitter current IE) of a UJT at a given inter base voltage VBB is shown in figure. From figure it is noted that for emitter potentials to the left of peak point, emitter current IE never exceeds IEo . The current IEo corresponds very closely to the reverse leakage current ICo of the conventional BJT.
The cut-off region is the area depicted in the figure. The emitter potential VE begins to decrease as the emitter current IE increases once conduction has been established at VE = VP. This is exactly in line with the reduction in resistance RB required to increase current IE. As a result, this device has a negative resistance region that is stable enough to be used in the applications previously mentioned with a high degree of reliability. Once the valley point is reached, as shown in figure 5.4, any further increase in emitter current IE causes the device to enter the saturation region.
Three other important parameters for the UJT are IP, VV and IV and are defined below:
Peak-Point Emitter Current Ip
It is the emitter current at the peak point. And It represents the minimum current that is required to trigger the device (UJT). It is inversely proportional to the inter base voltage VBB.
Valley Point Voltage VV
The valley point voltage is the emitter voltage at the valley point. The valley voltage increases with the increase in inter base voltage VBB.
Valley Point Current IV
The valley point current is the emitter current at the valley point. It increases with the increase in inter-base voltage VBB.
Special Features of UJT
The special features of a UJT are :
1. A stable triggering voltage (VP)— a fixed fraction of applied inter base voltage VBB.
2. A very low value of triggering current.
3. A high pulse current capability.
4. A negative resistance characteristic.
5. Low cost.
3. Applications of UJT
- Relaxation oscillators.
- Switching Thyristors like SCR, TRIAC etc.
- Magnetic flux sensors.
- Voltage or current limiting circuit.
- Bistable oscillators.
- Voltage or current regulators.
- Phase control circuits.
UJT relaxation oscillator
The circuit diagram of a UJT Relaxation Oscillator is given shown above. R1 and R2 are current Limiting Resistors. Resistor R and Capacitor C determines the frequency of the oscillator.
The Frequency of the UJT Relaxation Oscillator can be Expressed by the Equation
F = 1/ (RC ln(1/(1-η))
Where η is the Intrinsic Standoff ratio and ln stand for natural Logarithm.
When power supply is switched ON the capacitor C starts charging through resistor R. The capacitor keeps on charging until the voltage across it becomes equal to 0.7V plus ηVBB. This voltage is the peak voltage point ―Vp‖ denoted in the Characteristics curve (Fig:2). After this point the emitter to RB1 resistance drops drastically and the capacitor starts discharging through this path. When the capacitor is discharged to the valley point voltage ―Vv‖ (refer Fig : 1) the emitter to RB1 resistance climbs again and the capacitor starts charging. This cycle is repeated and results in a sort of Sawtooth waveform across the capacitor. The saw tooth waveform across the capacitor of a typical UJT Relaxation oscillator is shown in the figure below.