![]() ![]() SaturationSaturation region is the part of characteristic curve in which the current and voltage bothincrease, if the applied voltage to emitter terminal increases. The voltage drops from VPEAK to Valley Point even though the current increases (negative resistance). After a certain time, if the voltage applied to the emitter lead increases, it will reach out at VPEAK. Negative Resistance regionWhen the emitter reaches the triggering voltage, VTRIG, Unijunction Transistor (UJT) will turn on. The applied voltage VE hasn’t reached the triggering point, thus making transistor to remain in offstate. The curve has three regions: (1) Cutoff region (2) Negative Resistance region, (3) Saturation region.Ĭutoff regionCutoff region is the area where the Unijunction Transistor (UJT) doesn’t get sufficient voltage to turnon. The figure shows Emitter current IEon the x-axis and VE on the y-axis. Characteristics Curve of Unijunction Transistor (UJT)The characteristics of Unijunction Transistor (UJT) is given below. Thisshows that RB1 depends on the emitter current and it is variable. The Unijunction Transistor (UJT) will act as voltage breakdown device, when the input appliedbetween emitter and base 1 reduces below breakdown value i.e., RB1 increases to a higher value. As a result a largecurrent flows between base 1 and base 2. This reduces the resistance between emitter andBase 2, resulting in a reduction in total resistance between Base 1 and base 2. Increase in emitter current results in injection of large number of conductionelectrons in the region between emitter and Base 2. This results in larger flow of emitter current from emitterregion to base region. This is achieved when voltage applied to the emitter terminal The UJT begins to conduct when the PNjunction is forward biased. The PN junction is reverse biased if theηvoltage applied at the emitter is less than the sum of voltage developed across resistance RB2 ( VηBB)and the voltage drop across a forward biased PN junction,Ī very highimpedance is developed in this situation prompting device to move into non-conducting state i.e., itwill be switched off and no current flows through it. Most UJT’s have value ranging from 0.5 to 0.8. The ‘intrinsic stand-off ratio’ ( ) of the Unijunction Transistor is defined applied as the ratioηof RB2to RBB. When the emitter input is zero, the voltage across resistance RB2 of the voltage divider circuitis calculated by ![]() Theterminal B1 is positively biased with respect to B2 when a voltage (VBB) applied across the terminals B1Īnd B2. When a voltage is applied across the semiconductor device,the potential will be in proportion to the position of base points along the channel.The Emitter (E) acts as input when employed in a circuit, as the terminal B2 is grounded. Since the PN junction iscloser to B1, the value of RB1 will be less than the variableresistance RB2.Ī voltage divider network is formed by the seriescombination of resistances RB2 and RB1. The variable resistance RB2 is provided between theterminals Emitter (E) and Base 2 (B1), the RB1 between theterminals Emitter (E) and Base 1 (B1). The emitter PN junction is fixed along the ohmicchannel during its manufacturing process. QUCS UNIJUNCTION TRANSISTOR OSCILLATOR SERIESHow does a Unijunction Transistor (UJT) workThe simplified equivalent circuit (at Figure 3 below)shows that N-type channel consists of two resistors RB2 and RB1in series with an equivalent diode, D representing the PNjunction. The ohmiccontact on either ends of the silicon bar is termed as Base 1 (B1) andBase 2 (B2) and P-type terminal is named as emitter.The emitter junction is placed such that it is closer to Base 1than to Base 2.The symbol of UJT is similar to that of JFET except thatthe emitter arrowhead for UJT is bent in the direction in whichconventional current flows. Construction of Unijunction Transistor (UJT)A Unijunction Transistor is constructed by forming a pn-Junction between a lightly doped N type silicon bar and a heavilydoped P type material on one side as shown in the figure. If finds use in relaxation oscillators in variety of applications. The unijunction transistor exhibits negative resistance inits characteristics. Since there is only one rectifying junction within thedevice, it is called a ‘Unijunction’ transistor. A third terminal is connected with a heavily doped p-type material alloyed into the bar part wayalong its length, and is known as the emitter. It consistsof a bar of n-type silicon material with a terminal attached at its two ends known as base 1 and base2. A Unijunction Transistor (UJT) is a three terminal semiconductor switching device. ![]()
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