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Which one of the following consists of a layer of metal at the top named gate, a silicon dioxide layer below it, and a semiconductor substrate as the bottom-most layer ?

Which one of the following consists of a layer of metal at the top named gate, a silicon dioxide layer below it, and a semiconductor substrate as the bottom-most layer ? Correct Answer MOSFET

Junction FET (JFET):

  • It can be fabricated with either an N-channel or P-channel though N-channel is generally preferred.
  • For fabricating an N-channel JFET, first, a narrow bar of N-type semiconductor material is taken and then two P-type junctions are diffused on opposite sides of its middle part.

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  • These junctions form two P-N diodes or gates and the area between these gates is called a channel.
  • The two P-regions are internally connected and a single lead is brought out which is called gate terminal.
  • Ohmic contacts (direct electrical connections) are made at the two ends of the bar-one lead is called source terminal S and the other drain terminal D.
     

MOSFET (Enhancement-only):

  • It consists of a layer of metal at the top named gate, a silicon dioxide layer below it, and a semiconductor substrate as the bottom-most layer.

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  • This MOSFET operates only in the enhancement mode and has no depletion mode.
  • It works with large positive gate voltages only.
  • It differs in construction from the DE MOSFET in that structurally there exists no channel between the drain and source.
     

Depletion-enhancement MOSFET or DE MOSFET:

  • Here, space between source and drain regions is diffused by n-type impurities.
  • It can be operated in both depletion mode and enhancement mode by changing the polarity of VGS.
  • When negative gate-to-source voltage is applied, the N-channel DE MOSFET operates in the depletion mode.
  • However, with positive gate voltage, it operates in the enhancement mode.
  • Since a channel exists between drain and source, ID flows even when VGS = 0.
  • DE MOSFET is known as normally-ON MOSFET.

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BJT (Bipolar Junction Transistor):

  • The bipolar junction transistor consists of two back-to-back P-N junctions manufactured in a single piece of a semiconductor crystal.
  • These two junctions give rise to three regions called emitter, base, and collector.

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