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Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is also called as Insulated Gate Field Effect Transistor (IGFET).

  1. The Depletion Mode MOSFET (DMOS):
    • DMOS has a piece of n-material with insulated gate on left and p-region on the right.
    • A thin layer of SiO2 is deposited on the left side of the channel.
    • SiO2 is an insulator similar to glass.
    • The p-region is called substrate or body.
    • Because of insulated gate, negligible gate current flows even when the gate voltage is positive.
    • Like JFET it is normally ON device.
    • Suppose entire structure is like a capacitor in which gate and channel acts as plates and SiO2 layer is dielectric.
    • When We give negative charge to one plate, other plate get positively charged and vice versa.
    • Negative gate voltage, induces a positive charge in the channel. As a result, the channel is depleted of free electrons, and Id reduces. Because of depletion of free electrons this mode is called depletion mode.
    • Positive gate voltage increases the number of free electrons passing through the channel. This increases i.e. enhances the conducting of the channel. Because of this, positive gate operation is called Enhancement mode.
  2. The Enhancement mode MOSFET (E-MOSFET):
    • The P-substrate is extended all the way to the SiO2 layer. So with zero gate voltage no current flow from source to drain. For this reasons, an E-MOSFET is normally OFF when the gate voltage is zero.
    • When we apply positive voltage at gate it attracts free electrons into the p-region. These free electrons recombine with the holes next to SiO2 layer. When enough voltage is applied at gate, all the holes touching SiO2 layer are filled and free electrons start to flow from source to drain. This thin conducting layer of n-material next to the silicon dioxide is called n-type inversion layer.
    • The minimum Vgs that creates n-type inversion layer is called the Threshold voltage Vgs(th).
    • When Vgs is greater than Vgs(th), the device turns ON and the drain current is controlled by the gate voltage.
    • E-MOSFET are often used in class AB amplifier and also used in switching devices.
  3. CMOS:
    • Q1 is P-Channel MOSFET and Q2 is n-Channel MOSFET.
    • When Vin is high, Q1 is OFF and Q2 is ON. Shorted Q2 pulls the output voltage to the ground.
    • When Vin is low, Q1 is ON and Q2 is OFF. Shorted Q1 pulls the output voltage upto +Vdd.
    • Since the output voltage is inverted, the circuit is called CMOS Inverter.
    • When the input voltage is zero, the output voltage is high and when the input voltage is high, the output voltage is low.
    • Between two extremes, there is cross over point where input voltage equals Vdd/2. At this point, both MOSFETs have equal resistances and output voltage equal Vdd/2.

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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