Unijunction transistor and its characteristics

Unijunction transistor and its characteristics:

UJT, unijunction transistor is the three terminal conductor switching device. If it is triggered the emitter current increases regeneratively until it is limited by emitter power supply. Due to this special character. UJT is not used as a transistor amplifier but it is used in a switching and timing applications.

Construction of UJT

UJT consist of n-type silicon bar with electric connectors available on either side. These electric connectors each comprise a base lead and are represented by \(B_1\) and \(B_2\) in the figure below. The emitter P which is situated closer to \(B_2\) than \(B_1\),forms p-n junction with the bar as shown in the figure below:

So,

• UJT has p-n junction and three leads.
• It is double based p-n junction diode.
• The emitter is doped heavily with many holes but the base is lightly doped, so the resistance between the base terminals is very high 5 to 10 KHz as emitter is open.

Operation of UJT

With the emitter open, when \(V_{BB}\) is applied between the base \(B_1\) and \(B_2\), voltage gradient appears on the n-bar. Here more than half of voltage supply appears between emitter and \(B_1\) as the base \(B_2\) is nearer to P as compared to \(B_1\). This creates the reverse biased condition and cut off emitter current, In practice, a small leakage current appears due to the minority of carriers of \(B_2\) to the emitter.

1. Consider that positive voltage is applied at the emitter. Let it be increased, then it overcomes the reverse voltage \(V_1\) and becomes forward biased. Now, the holes are emitted to n-region and are slowly attracted towards \(B_1\). This process decreases the resistance of this section in the bar and the voltage drop increases the emitter current. So more holes are ejected and saturation condition is obtained. Then this emitter current is limited by emitter power supply and the device now is said to be in the ‘ON’ state.
2. When the negative voltage is applied to the emitter the p-n junction is reverse biased and the emitter current is cut-off. The device now is said to be in the ‘OFF’ state.

Now, if \(R_{BB}\)= Total resistance of the bar

Then clearly for no voltage applied condition,

$$R_{BB}=R_{B_1}+R_{B_2}$$The voltage drop across \(R_B\),

$$V_1=\frac{R_{B_1}}{R_{B_1}+R_{B_2}}.V_{BB}=\eta V_{BB}$$

Where, \(\eta\)=intrinsic stand off ratio.

UJT is lower power absorbing device under normal conditions.

The circuit arrangement to study the characteristics of UJT is given below:

During the study of characteristics of UJT we fix the voltage \(V_{BB}\) between the bases of it and vary with the emitter voltage \(V_E\) and note the corresponding emitter current \(I_E\). The curve is drawn between emitter voltage and the emitter current at fixed \(V_{BB}\) between the bases. Hence the characteristics of UJT and is drawn below.

The region of the curve are as follows:

1. Cut-off region

In the cut-off region, initially as \(V_E\) increases from zero, slight leakage current flows from terminal \(B_2\) to the emitter. The current is due to minority of carriers in the reverse biased diode.

2. The peak point

As the value of \(V_E\) increases from certain value, forward \(I_E\) begins to flow and as the voltage increases to \(V_p\) at point P the corresponding value of \(I_E\) reaches \(I_p\). Here, the point P is called peak point.

3. Negative resistance region

After the peak point P, sudden increases in \(I_E\) results in falling of \(V_E\) and this region lasts until the valley point V, where the corresponding value of current is noted to be \(I_v\). The value of valley point is \(V_v\).

4. Saturation region

After the valley point V the device is driven to saturation state.

Figure

This figure shows the family of \(\frac{V_E}{I_E}\) characteristics of a UJT at different voltages between the bases \(V_{BB}\). From the figure we can see that peak voltage fall steadily with reducing \(V_{BB}\) and so does the valley point voltage \(V_v\). The difference \(V_p-V_v\) is a measure of the switching efficiency of UJT and can seen to fail as \(V_{BB}\) decreases.

References:

(1)Theraja, B.L. Basic Electronics. N.p.: S.Chand, n.d. Print.

(2)C.L.Arora. Refresher Course in Physics. Vol. II and III. N.p.: S.Chand, 2006. Print.

(3)Malvino. Electronic Principles. N.p.: Tata McGraw-Hill, n.d. Print.

(4)N.Nelkon and P.Parker. Advanced Level Physics. 5th ed. N.p.: Arnold Heinemann, n.d. Print.

(5)Priti Bhakta Adhikari,Diya Nidhi Chaatkuli, Ishowr Prasad Koirala. A Textbook of Physics (2nd Year). N.p.: Sukunda Pustak Bhawan, 2070. Print.