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Download TYPES OF FIELD EFFECT TRANSISTORS
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TYPES OF FIELD EFFECT TRANSISTORS Junction field effect transistor JFET Metal Oxide semiconductor field effect transistor ( MOSFET ) Junction field effect transistor JFET Is a three terminal semi conductor device in which current conduction is by one type of carrier i.e electrons or holes. The output characteristics are controlled by input voltage and not by input current. CONSTRUCTION A JFET consists of p type or N type silicon bar containing two PN junction at the sides. the bar forms the conducting channel for the charge carriers.. If the bar is of n type it is called n channel JFET. And if the bar is Ptype it is called p channel JFET. the two pn junctions forming diodes are connected internally and common terminal called gate. is taken out. other terminal are source and drain taken out from the bar. WORKING PRINICIPLE When a voltage Vds is applied between drain and source terminals and the voltage on the gate is zero. the two pn junction at the side of the bar establish depletion layers. The electrons will flow from source to drain through a channel between the depletion layers. the size of these layers determined the width of the channel and hence the current conduction through the bar. When a reverse voltage VGS is applied between the Gate and source, width of the depletion layer is increased this reduces width of conducting channel , their by increasing the resistance of n type bar .the current from source to drain is decreased. if the reverse voltage on the gate is decreased width of the depletion layer also decreases. this increases the width of the conducting channel and hence source to drain current OUTPUT CHARACTERISTICS OF JFET The curve between drain current (Id) and drain source voltage (Vds)of a JFET at constant gate source voltage (VGS) is known as output characteristics of JFET .keeping Vgs fixed at some value 1 V the drain source voltage in steps. Corresponding to each value of Vds the drain current Id is noted. A plot of these values gives the output characteristics of JFET at Vgs = 1V repeating similar procedure output charaecteristics at other gate –source voltages . At first the drain current Id rises rapidly with drain surce voltage Vds but then becomes constant. The drain source voltage above which drain current becomes constant is known as pinch of f voltage.OA is the pinch off voltage. Shorted gate drain current (Idss) It is the drain current with source short circuited to gate Vgs =0 and drain voltage Vds equal to pinch pff voltage.it is called zero bias current. The drain current raise rapidly at first and then levels off at pinch off voltageVp. The drain current has now reached the maximum value Idss when Vds is incresed beyond Vp the depletion layers expand at the top of the channel.the channel now acts as current limiter and holds drain current constant at Idss Pinch off voltage Vp It is the minimum drain source voltage at which the drain current essentially becomes constant Gate – source cut off voltage Vgs (off) It is the gata source voltage where the channel is completely cut off and the drain current becomes zero. Expression for Drain Curent The relation between Idss and Vp .We note that gate – source cut off voltage VGS off on the transfer characteristics is equal to pinch off voltage vp on the drain characteristics Vp = VGS (off) JFET has VGS = -4 V Vp =4 ID= Idss (1-VGS/VGS(off))2 Id =drain current at given Vgs Idss = shorted -gate drain voltage Vgs = gate – source voltage Vgs (off) = gate –source cut off voltage