Most Important Points | Field Effect Transistor | CompetitiveExamination

Suitable for the post of Junior Engineer in BSNL, DMRC, DRDO, PTCUL, RRB, UPCL, etc examination.
  • BJT relies on two types of charge i.e. free electrons and holes. This is why called bipolar. While FET is an unipolar device because its operation depends on only one type of charge either electrons or holes i.e. FET has majority carriers but not minority carriers.
  • JFET: P-regions are connected internally to get a single external gate lead.
    • P-type regions are doped heavily.
    • The space between junction is called Channel.
    • Field effect term is related to the depletion layers around each p-region.
    • When Vgs = 0, then maximum drain current flows through n-channel. But as we apply negative voltage the depletion layer expands and the conducting channel becomes narrower.
    • It means JFET is a voltage controlled device because input voltage controls an output current.
    • Because of reverse bias, Ig is approximately zero.
      • Rin = Vgs/Ig nearly equal to infinite. J-FET has almost infinite input resistance (MΩ).
    • Source and drain terminals are not interchangeable at high frequencies but interchangeable at low frequencies. Because the capacitance between drain and gate is smaller than the capacitance between gate and the source.
      • Cgd < Cgs
    • Idsg is the maximum current which flows from drain to source with a shorted gate.
      • Vgs = 0, Id = Idss
    • As we increase Vds, the depletion region expands and at Vds = Vp, the depletion layers are almost touching. The narrower conducting channel therefore pinch off or prevents a further increase in current. This is why the current has an upper limit of Idss.
      • Vp = Pinch off voltage.
      • Vbr = breakdown voltage Vds maximum.
    • Between Vp and Vbr JFET acts like a current source of approximately Idss when Vgs = 0.

  • The Ohmic Region: In this region, the drain current increase linearly with the increase in drain to source voltage obeying ohm's law.
    • So In ohmic region, JFET is equivalent to a resistor with value of approximately:
      • Rds = Vp/Idss
    • Suppose we have fixed Vds and now we are increasing Vgs (applying negative gate voltage), a point reach where drain current reduces to almost zero. This voltage is called the gate source cut off voltage (Vgs off).
      • Vgs off = -Vp
      • Two voltages always have the same magnitude because there are values where depletion layers almost touch.
  • The Trans-conductance Curve/Transfer Characteristics: It is a graph between Id and Vgs with constant Vds.
    • The equation of this graph is:
      • Id = Idss (1 - Vgs / (V gs off) )^2
      • Because of square quantity in the equation, JFETs are often called square law devices.
      • Normalized means we have graphing ratios like Id/Idss and Vgs / (Vgs off).
      • Vgs / Vgs off = Half cut off point.
      • Normalized current ratio: Id/Idss = 1/4
      • When the gate voltage is half the cut off voltage, the drain current is one quarter of maximum.
Author: Mr. Amarjeet Singh Jamwal, Assistant Loco Pilot, Ambala Division, INDIAN RAILWAY.

Reference: Albert Malvino and David J Bates, “Electronic Principles”, 7th Edition, TATA McGRAW HILL.

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