As the name implies, it has only one PN junction but it has a three terminal silicon diode. It differs from an ordinary diode in the sense that it has three leads and also its difference from BJT and FET is that it has no ability to amplify. It has the ability to control large ac power with a small input signal. It also has the negative resistance characteristics which makes it useful as an oscillator.
- It is a three terminal device, having two layers. It consists of a slab of lightly doped n type silicon material. The two base contacts are attached to both the ends of this n type surface. These are denoted as B1 and B2 respectively.
- A p type material is used to form a p-n junction at the boundary of the aluminum rod and n type silicon slab. The third terminal called emitter (E) is taken out from this p-type material.
- The n-type is lightly doped while p type is heavily doped.
- As n type is lightly doped, it provides high resistivity and p type as heavily doped; the symbolic representation of UJT is shown in the Fig.
- The emitter is shown by an arrow which is at an angle to the vertical line representation n type material.
- This arrow indicates the direction of flow of conventional current when the UJT is forward biased.
EQUIVALENT CIRCUIT OF UJT
The internal resistances of the two bases are represented as RB1 and RB2 In the actual construction, the terminal E is closer to B2 as compared to B1. Hence resistance RB1 is more than the resistance RB2. The p-n junction is represented by a normal diode with VD as the drop across it. When the emitter diode is not conducting then the resistance between the two bases B1 and B2 is called interbase resistance denoted as RBB.
Its value ranges between 4k and 12K.
Intrinsic Stand off Ratio:
Consider UJT as shown in the to which supply VBB is connected with IE=0 i.e. emitter diode is not conducting,
Then the voltage drop across RB1 can be obtained by using potential divider rule.
The typical range of is from 0.5 to 0.8. The voltage VRB1 is called intrinsic stand off voltage because it keeps the emitter diode reversed biased for all the emitter voltages less than VRB1
PRINCIPLE OF OPERATION:
While operating an UJT, the supply VBB is applied between B1 and B2 whilethe variable emitter voltage VE is applied across the emitter terminals. This arrangement is shown in the Fig
Let us see the effect of change in VE. The potential of A is decided by and is equal to VBB.
As long as VE is less than VA, the p-n junction is reverse biased. Hence emitter current IE will not flow. Thus UJT is said to be OFF.
Case 2 : VE> Vp
The diode drop VD is generally between 0.3 to 0.7 V. Hence we can write,
Vp = VA+VD= VBB+ VD
When V becomes equal to or greater than Vp the p-n junction becomes forward biased and current IE flows. The UJT is said to be ON.
The graph of emitter current against emitter voltage plotted for a particularvalue of VBB is called the characteristics of UJT. For a particular fixed value of VBB such characteristics is shown in the Fig.
The characteristics can be divided into three main regions which are,
1. Cut off region: The emitter voltage VE is less than Vp and the p-n junction is reverse biased. A small amount of reverse saturation current flows through the device, which is negligibly small of the order of µA. This condition remains till the peak point.
2. Negative resistance region: When the emitter voltage VE becomes equal to Vp the p-n junction becomes forward biased and IE starts flowing. The voltage across the device decreases in this region, though the current through the device increases. Hence the region is called negative resistance region. This decreases time resistance RB1. This region is stable and used in many applications. This region continues fill valley point.
3. Saturation region: Increase in IE further valley point current IV drives the device
in the saturation region. The voltage corresponding to valley point is called valley point voltage denoted as Vv. In this region, further decrease in voltage does not take place. The characteristic is similar to that of a semiconductor diode, in this region. The active region i.e. negative resistance regions, the holes which are large in number on p-side, get injected into n-side. This causes increase in free electrons in the n-type slab. This increases the conductivity i.e. decreases the resistivity. Hence the resistance R decreases in this region. As the VBB increases, the potential VP corresponding to peak point will increase.
- The UJT is mainly used in the triggering of other devices such as SCR.
- It is also used in the sawtooth wave generators and some timing circuits.
- The most popular application of UJT is as a relaxation oscillator to obtain short pulses for triggering of SCRs.