Unijunction Transistor

Unijunction Transistor, or UJT for short, is another solid-state three terminal device that can be used in door impact, timing circuits and trigger generator applications to replace and control thyristors and triages for AC power control type applications.

Like diodes, unijunction transistors are made of separate P-type and N-type semiconductor materials that form a single (hence the Uni-Junction name) PN-connection within the main conductive N-type channel of the device.

Although Unijunction Transistor has the name of a transistor, it is not used to amplify a signal. Since it is instead used as an ON-OFF switching transistor, switching properties are very different from that of a traditional bipolar or field-acting transistor. UJTs have one-way conductivity and negative impedance properties that act more like a variable voltage divider during failure.

Like N-channel FET's, UJT consists of a single solid N-type semiconductor material that forms the mainstream transport channel with two external connections marked Base 2 ( B2) and Base 1 (B1). The third link, mixed with Transmitter (E), is located along the channel. The emitter terminal is represented by an arrow pointing from type P emitter to N-type base.

The emitter rectifier p-n connection of the Unijunction transistor is created by fusing the P-type material into the N-type silicone channel. However, P-channel UJTs with N-type emitter terminals are also available, but they are rarely used.

The Emitter connection is positioned along the channel, closer to terminal B2 than B1. The UJT symbol uses an arrow pointing to the base, indicating that the emitter terminal is positive and that the silicone bar is negative material. Below is the symbol, structure and equivalent circuit of UJT.

Unijunction Transistor Symbol and Structure

Unijunction Transistor
Unijunction Transistor Symbol and Structure

Note that the single-joint transistor symbol is very much like that of the effective transistor or JFET receiving the connection, except that it has a bent arrow representing emitter(E) input. Although similar in terms of ohmic channels, JFETs and UFTs work very differently and should not be confused.

So how does it work? From the equivalent circuit above, we can see that the N-type channel consists mainly of two resistance RB2 and rb1 serially with an equivalent (ideal) diode, D represents the p-n combination connected to the Central points. This emitter pn connection is fixed in position along the ohmic channel during production and therefore cannot be changed.

Resistance is given between Rb1, emitter, E and Terminal B1, while resistance is given between Rb2, emitter, E and Terminal B2. Since the physical position of the P-n connection is closer to terminal B2 than B1, the recissive value of rb2 will be less than RB1.

The total resistance (Ohmik resistance) of the silicon rod will depend on the actual level of doping of semiconductors and the physical dimensions of the N-type silicone channel, but can be represented by RBB. If measured with an ohmmeter, this static resistance is typically measured somewhere between 4kΩ and 10kΩ for the most common UJTs, such as 2N1671, 2N2646 or 2N2647.

These two series of resistors form a voltage dividing network between the two base terminals of the single-link transistor. Since this channel extends from B2 to B1, the potential at any point along the channel will be proportional when a voltage is applied to the device. Location between terminals B2 and B1. The level of the voltage gradient therefore depends on the amount of the supply voltage.

When used in a circuit, terminal B1 connects to the ground and acts as an introduction to the Transmitter device. Let's say that a VBB voltage is applied to the UJT between B2 and B1, so that B2 has a positive polarity compared to B1. With zero Transmitter input applied, the voltage developed along the RB1 (low resistance) of the resistant voltage divider can be calculated as follows:

Unijunction Transistor RB1 Voltage

Unijunction Transistor
Unijunction Transistor RB1 Voltage

For a single-joint transistor, the RBB resistance ratio of RB1 shown above is called the internal separation rate and is indicated by the Greek symbol: η (eta). Typical standard η values range from 0.5 to 0.8 for the most common UJTs.

If a small positive input voltage lower than the voltage developed throughout the resistance is applied to the emitter input terminal RB1 (ηVBB), the diode p-n connection is inverted. Therefore, it offers a very high impedance and does not transmit the device. UJT is turned "OFF" and zero current flows.

However, when the emitter input voltage increases and is larger than VRB1 (or ηVBB + 0.7V, where 0.7V equals the voltage drop of the p-n connection diode), the p-n connection is polarized forward and begins transmission with a single connection transistor.

The effect of the additional emitter current flowing to the base reduces the resistant part of the channel between the emitter junction and terminal B1. This decrease in the value of RB1 resistance to a very low value means that the emitter connection becomes biased further forward and results in a larger flow of current. The effect of this results in negative resistance in the emitter terminal.

Likewise, if the input voltage applied between emitter and terminal B1 decreases to a sub-fault value, the resistance value of the RB1 will increase to a high value. Then the Unijunction Transistor can be considered a voltage failure device.

Thus, we can see that the resistance offered by RB1 is variable and depends on the value of the emitter current, IE. Then pressing the transmitter connection forward relative to B1 causes more currents to flow, reducing the resistance between emitter E and B1.

In other words, current flow to UJT's Transmitter causes the resistance value of RB1 to decrease and the voltage above it decreases, vrb1 should also decrease and allow more currents to flow, creating a negative resistance condition.

Unijunction Transistor Applications

Now that we know how a single-joint transistor works, what can they be used for? The most common application of a connection transistor is a triggering device for SCRs and Triassices, but other UJT applications include saw-tooth generators, simple oscillators, phase control and timing circuits. The simplest of all UJT circuits is the Relaxation Oscilator, which produces non-sinusoidal waveforms.

In a basic and typical UJT loosened oscillator circuit, the transmitter terminal of the unijunction transistor is connected to a serially connected resistance and capacitor connection, RC circuit, as shown below.

Unijunction Transistor Relaxation Oscelator

Unijunction Transistor

When a voltage (Vs) is applied first, the combined transistor becomes "OFF" and the C1 capacitor is completely discharged but begins to charge exponentially through R3 resistance. When the UJT Transmitter is connected to the capacitor, when the charging voltage along the capacitor is greater than the Vc diode voltage drop value, the p-n connection acts like a normal diode and becomes forward-sided by triggering the UJT to the transmission. Single connection transistor "ON". At this point, the Transmitter collapses from Transmitter to B1 as it transitions to low impedance saturation while the transmitter flows through R1.

Since the omic value of resistance R1 is very low, the capacitor discharges rapidly through UJT and a rapidly rising voltage pulse appears in R1. In addition, since the capacitor discharges faster than it charges over R3 resistance via UJT, the discharge time is much shorter than the charging time as the capacitor is discharged through the low-resistance UJT.

When the voltage on the capacitor falls below the holding point of the pn connection (VOFF), the UJT becomes "OFF" and no current flows into the Transmitter connection. Thus, the capacitor is charged once again through resistance R3 and von and VOFF are repeated continuously, etc. when there is a supply voltage between this charging and discharge process.

UJT Oscillator WaveForms

Unijunction Transistor

Then we can see that the single-joint oscitor is constantly "ON" and "OFF" without any feedback. The operating frequency of the oscilator, capacitor C1 and η value and serial charging resistance are directly affected by the value of the R3. The output pulse shape produced from terminal Base1 (B1) is a saw tooth waveform, and to regulate the time period, you only need to change the omic value of the resistance, R3, since it adjusts the RC time constant to charge the capacitor.

The T-time of the saw gear waveform will be given as the charging time plus the discharge time of the capacitor. In ejaculation time, τ1 is usually very short compared to the larger RC charging time. The oscillation time of τ2 is roughly equivalent to T ≅ τ2. The oscillation frequency is therefore given with ε = 1/T.

UJT Oscillator Example

The datasheet of a 2n2646 single-function transistor gives the intrinsic standby rate of η 0.65. If a 100nF capacitor is used to produce timing pulses, let's calculate the timing resistance required to produce an oscillation frequency of 100hz.

  • The timing period is given as follows:
Unijunction Transistor
  • The value of the timing resistance R3 is calculated as follows:
Unijunction Transistor

Then, in this simple example, the required charging resistance value is calculated as 95.3kΩ to the nearest preferred value. However, since the resistance value of R3 can be too large or too small, there are certain conditions that are required for the correct operation of the UJT relaxation oscillator.

For example, if the R3 value is too large (Megohms), the capacitor may not be charging enough to trigger the transmission of Unijunction. But at the same time, when the capacitor is discharged, it must be large enough to allow the UJT to become "OFF".

Similarly, if the R3 value is too small (several hundred Ohms), the current flowing into the Transmitter terminal once triggered may be large enough to drive the device into the saturation zone, preventing it from becoming completely "OFF". In both cases, the unijunction oscillator circuit will fail to release.

UJT Speed Control Circuit

A typical application of the above unijunction transistor circuit is to create a series of pulses to ignite and control a thristor. Using UJT as a phase control trigger circuit with a SCR or Triyak, we can adjust the speed of a universal AC or DC motor as shown.

Unijunction Transistor Speed Control

Unijunction Transistor

Using the circuit above, we can control the speed of a universal serial motor (or whatever kind of load we want, heaters, lamps, etc.) by regulating the current flowing from the SCR. To control the speed of the motors, you just need to change the frequency of the resulting saw bump by changing the value of the ponciometer.