![]() After a certain time, if the applied voltage increases to the emitter lead, it will reach out at V PEAK. When the transistor reaches the triggering voltage, V TRIG, Unijunction Transistor (UJT) will turn on. The applied voltage hasn’t reached the triggering voltage, thus making transistor to be in off state. 4 – Characteristics of Unijunction Transistor (UJT) CutoffĬutoff region is the area where the Unijunction Transistor (UJT) doesn’t get sufficient voltage to turn on. The characteristics of Unijunction Transistor (UJT) can be explained by three parameters:įig. Characteristics Curve of Unijunction Transistor (UJT) This shows that RB1 depends on the emitter current and it is variable. The Unijunction Transistor (UJT) will act as voltage breakdown device, when the input applied between emitter and base 1 reduces below breakdown value i.e., RB1 increases to a higher value. Increase in emitter current reduces the resistance between emitter and Base 1, resulting in negative resistance at emitter terminal. This results in larger flow of emitter current from emitter region to base region. ![]() The forward biased is achieved when voltage applied across emitter terminal is increased and becomes more than VRB1. The UJT begins to conduct when the PN junction is forward biased. Thus a very high impedance is developed prompting device to move into non-conducting state i.e., it will be switched off and no current flows through it. The PN junction is reverse biased when small amount of voltage which is less than voltage developed across resistance R B1 (ηV BB) is applied across the terminal emitter (E). Most UJT’s have η value ranging from 0.5 to 0.8. The important parameter of Unijunction Transistor is ‘intrinsic stand-off ratio’ (η), which is resistive ratio of R B1 to R BB. ![]() When the emitter input is zero, the voltage across resistance RB1 of the voltage divider circuit is calculated by The terminal B 2 will be positive biased to B 1, when a voltage (V BB) applied across the terminals B 1 and B 2. The Emitter (E) will act as input when employed in a circuit, as the terminal B 1 will be grounded. When a voltage is applied across the semiconductor device, the potential will be in proportion to the position of base points along the channel. #Qucs unijunction transistor oscillator seriesSince the PN junction is more close to B 2, the value of RB2 will be less than the variable resistance RB1.Ī voltage divider network is formed by the series resistances RB2 and RB1. The variable resistance RB1 is provided between the terminals Emitter (E) and Base 1 (B 1), the RB2 between the terminals Emitter (E) and Base 2 (B 2). 3 – Simplified Equivalent Circuit of Unijunction Transistor (UJT) The emitter PN junction is fixed along the ohmic channel during its manufacturing process.įig. The simplified equivalent circuit (at Figure 3 below) shows that N-type channel consists of two resistors RB2 and RB1 in series with an equivalent diode, D representing the PN junction. How does a Unijunction Transistor (UJT) works The symbols of both UJT and JFET resemble the same except the emitter arrowhead represents the direction in which conventional current flow, but they operate differently. The emitter junction is placed such that it is more close to terminal Base 2 than Base 1. 2 – Basic Construction & Symbol of Unijunction Transistor (UJT) ![]() The ohmic contact on either ends of the silicon bar is termed as Base 1 (B 1) and Base 2 (B 2) and P-type terminal is named as emitter.įig. In Unijunction Transistor, the PN Junction is formed by lightly doped N type silicon bar with heavily doped P type material on one side. Symbol and Construction of Unijunction Transistor (UJT) It exhibits negative resistance in its characteristics which employs it as relaxation oscillators in variety of applications. The unique switching characteristics of UJT makes it different from conventional BJT’s and FET’s by acting as switching transistor instead of amplifying the signals. 1 – Introduction to Unijunction Transistor (UJT) ![]() The Unijunction Transistor is also known as Double Base Diode.įig. The Unijunction Transistor is a simple device that consists of a bar of n-type silicon material with a non-rectifying contact at either end (base 1 and base 2), and with a rectifying contact (emitter) alloyed into the bar part way along its length, to form the only junction within the device (hence the name ‘Unijunction’). It is a three terminal semiconductor switching device. This post will give you a detailed idea about UJT, its basic construction, symbol, how it works, its characteristics curve, various applications, advantages and disadvantages. In spite of having transistor added to its name, a Unijunction Transistor (UJT) working and characteristic curve varies completely from other counterparts. ![]()
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