Working principle/characteristics/application fields/equivalent circuit of unijunction transistor

The unijunction transistor is a 3-terminal semiconductor device that, unlike the BJT, has only one pn junction. It is basically designed to be used as a single-stage oscillator circuit for generating pulse signals suitable for digital circuit applications.

Unijunction transistor application areas

Following are the major application areas where unijunction transistors are widely used.

Trigger Circuit

Oscillator Circuit

Voltage/current regulated power supply.

Timer based circuit,

Sawtooth Generator,

Phase control circuit

Bistable Network

Main Features

Easily accessible and inexpensive: The cheap price and easy availability of UJT along with some special features have led to the widespread implementation of this device in many electronic applications.

Low Power Consumption: Due to its low power consumption characteristics under normal operating conditions, this device is considered an incredible breakthrough in the continuous efforts to develop reasonably efficient devices.

Highly Stable, Reliable Operation: When used as an oscillator or delay trigger circuit, the UJT has extremely high reliability and extremely precise output response.

Basic structure of a unijunction transistor

Figure #1

UJT is a three-terminal semiconductor device with a simple structure as shown in the figure above.

In this structure, a block of lightly doped n-type silicon material (with increased resistance properties) provides a pair of pedestal contacts connected to both ends of one surface and an alloy aluminum bar on the opposite rear surface.

The device's pn junction is created at the boundary of the aluminum rod and the n-type silicon block.

This single pn junction so formed is the reason for the device name "single junction". The device was originally called a double (dual) base diode because of the presence of a pair of base contacts.

Note in the above diagram that the aluminum bar has melted/merged closer to the base 1 contact than the base 2 contact on the silicon block, and that the base 2 terminal has also become positive relative to the base 1 terminal by the VBB volts. How these aspects affect the operation of the UJT will be apparent in the following sections

Symbolic representation

The figure below shows the symbolic representation of a unijunction transistor.

Figure #2

Observe that the emitter terminal is at an angle to the line that depicts the block of n-type material. An arrow can be seen pointing in the direction of typical current (hole) flow while the single junction device is in the forward biased, triggered or on state.

Unijunction transistor equivalent circuit

Figure #3

The equivalent UJT circuit can be seen in the figure above. We can see that this equivalent circuit looks relatively simple, it includes several resistors (one fixed resistor, one adjustable resistor) and a single diode.

The resistor RB1 is shown as an adjustable resistor, considering that its value will change with the change of current IE. In fact, in any transistor representing a single junction, RB50 may fluctuate from 0 kΩ to 50 Ω for any equivalent change of IE from 1 to 5 = μA. The base-to-base resistance RBB represents the device resistance between terminals B2 and B0 when IE = 1. In the formula is,

RBB = (RB1 + RB2) | IE = 0

RBB typically ranges between 4 and 10 k. The position of the aluminum bar as shown provides the relative size of RB2 and RB0 when IE = 1. We can estimate the value of VRB1 (when IE = 0) using the voltage divider law as follows:

VRB1 = (RB1 x VBB) / (RB1 + RB2) = ηVBB (IE = 0)

The Greek letter η (eta) is known as the intrinsic turn-off ratio of a unijunction transistor device and is defined by the following equation:

η = RB1 / (RB1 + RB2) (IE = 0) = RB1 / RBB

For the indicated emitter voltage (VE) higher than VRB1 (= ηVBB), the diode’s forward voltage drop VD (0.35 → 0.70 V), the diode will be triggered ON.

Ideally, we can assume a short circuit condition so that IE will start conducting through RB1. Using the formula, the trigger voltage level at the emitter can be expressed as:

VP = ηVBB + VD

Main features and working principle

The characteristics of a representative unijunction transistor with VBB = 10 V are shown in the figure below.

Figure #4

We can see that for the emitter potentials indicated to the left of the peak point, the IE value never exceeds the IEO (in microamps). The current IEO more or less follows the reverse leakage current ICO of a conventional bipolar transistor.

This region is called the cutoff region and is shown in the figure.

Once conduction is achieved at VE = VP, the emitter potential VE decreases as the IE potential increases, exactly as the resistance RB1 decreases with increasing current IE, as described previously.

The above characteristics provide the unijunction transistor with a highly stable negative resistance region, enabling the device to operate and be applied with extremely high reliability.

In the above process, it can be expected that a valley point is eventually reached and any increase in IE beyond this range will cause the device to enter the saturation region.

Figure #3 shows a diode equivalent circuit with similar characteristics approach in the same area.

The decrease in the resistance of the device in the active region is due to the fact that once the device is ignited, the p-type aluminum rod injects holes into the n-type block. This causes the number of holes on the n-type cross section to increase, and the number of free electrons to increase, resulting in an increase in the conductivity (G) of the device and an equivalent decrease in its resistance (R↓ = 1/G↑).

Important parameters

You will find three additional important parameters associated with the unijunction transistor, namely IP, VV and IV. All of these are represented in Figure #4.

These are actually pretty easy to understand. The typical transmitter characteristics that exist can be seen in Figure #5 below.

Figure #5

Here, we can observe that the IEO (μA) is not obvious because the horizontal scale is calibrated in milliamps. Each curve that intersects the vertical axis is the corresponding result of VP. For constant values ​​of η and VD, the VP value varies according to VBB as follows:

Unijunction Transistor Datasheet

The standard technical specification range of UJT can be understood from Figure #5 below.

UJT Pinout Details

The pinout details are also included in the above datasheet. Note that the base pins B1 and B2 are opposite each other, while the emitter pin E is centered between them.

Furthermore, the pedestal pins that should be connected to the higher supply level are located near the branch on the package ring.

How to Use a UJT to Trigger an SCR A relatively popular application of the UJT is to trigger power devices such as SCRs. Figure #6 below depicts the basic components of this type of trigger circuit.

Figure #6: Using UJT to trigger SCR

Figure #7: UJT load line used to trigger external devices such as SCRs

The main timing components consist of R1 and C, while R2 acts like a pull-down resistor for the output trigger voltage.

How to calculate R1

Resistor R1 must be calculated to ensure that the load line defined by R1 passes through the device characteristic in the negative resistance region, i.e., shifted to the right of the peak point but to the left of the valley point, as shown in Figure #7.

If the load line cannot pass to the right of the peak point, the single junction device cannot start.

Once we consider the peak points of IR1 = IP and VE = VP, we can determine the formula for R1 that guarantees the switch turn-on condition.

IP looks logical because the charging current of the capacitor is zero at this point. This means that the capacitor at this particular point is transitioning from charging to discharging.

So, for the above condition, we can write:

Or, to ensure complete shutdown of the SCR:

R1》（V - Vv）/Iv

This means that the resistor R1 must be selected within the range described below:

（V - Vv） / IV 《 R1 《 （V - Vp） / IP

How to calculate R2

The resistor R2 must be small enough to ensure that the voltage VR0 across R2 does not falsely trigger the SCR when IE ≅ 2 A. To do this, VR2 must be calculated as follows:

VR2 ≅ R2V / (R2 + RBB) (when IE ≅ 0)

The capacitor provides the time delay between trigger pulses and determines the length of each pulse.

How to calculate C

Referring to the figure below, once the circuit is powered on, a voltage VE equal to VC will begin to charge to a voltage VV through a time constant τ = R1C.

Figure #8

The general formula for determining the charging period of C in a UJT network is:

vc = Vv + (V - Vv) (1-e-ton/R1C)

From our previous calculations, we already know the voltage across R2 during the above charging period of the capacitor. Now, when vc = vE = Vp, UJT

The device will enter the switch on state, causing the capacitor to discharge through RB1 and R2 at a rate determined by the time constant:

τ = (RB1 + R2)C

The following formula can be used to calculate the discharge time, when

vc = vE

VC ≅ Virtual PE-tons/(RB1+R2)C

The equation becomes a little more complicated due to RB1, which decreases in value as emitter current increases, as well as other aspects of the circuit such as R and V, which also affect the overall discharge rate of C.

Nonetheless, if we refer to the equivalent circuit given in Figure #8(b) above, typically R1 and RB2 are values ​​such that the Thevenin network configured around capacitor C may be slightly affected by the resistance of R1, RB2. Although the voltage V appears to be quite large, the resistor divider that assists the Thevenin voltage can usually be ignored and eliminated, as shown in the simplified equivalent diagram below:

Therefore, the simplified version above helps us get the following formula, the discharge phase of capacitor C when VR2 is at its peak.

VR2 ≅ R2（Vp - 0.7） / R2 + RB1