Fundamental of MOSFET

1. Depletion MOSFET and Digital MOSFET Circuits
Dr. Varun Kumar
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2. Outlines
1 Depletion MOSFET
2 Comparison of p-with n-channel FETs
3 Circuit symbol of MOSFET
4 Digital MOSFET circuits
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3. Depletion type MOSFET
⇒ An n-channel depletion-type MOSFET.
⇒ On a lightly doped p-type substrate, two highly doped n+ regions are
diffused.
⇒ These two n+ sections are called as source and drain. Both terminals
are separated by a distance 5 − 10µm
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4. Continued–
⇒ The field will end on “induces” positive charges on the semiconductor.
⇒ When negative gate voltage increases, the induced +Ve charge in the
semiconductor also increases.
⇒ Region beneath the oxide now has n-type carriers, the conductivity
decreases, and the current flows from source to drain through the
induced channel.
⇒ Here the drain current is “depleted” by -Ve gate voltage. Hence, this
device is called an depletion type MOS.
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5. Continued–
⇒ (a) The drain characteristics
⇒ (b) The transfer curve (for VDS = 10V ) for n-channel MOSFET,
which may be used as enhancement or depletion type MOSFET.
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6. Comparison of p-with n-channel FETs
⇒ p-channel enhancement FET is popular in MOS, due to its easier
production.
⇒ Most of the contaminant in MOS fabrications are mobile ions.
⇒ +Ve charge are trapped in oxide layer between gate and substrate.
⇒ The hole mobility in silicon at normal field is 500Cm2/V-s, whereas
electron mobility is 1300Cm2/V-s in
⇒ p-channel device will have more ON resistance on an equivalent
n-channel on same condition.
⇒ p-channel requires more than double area for maintaining the same
resistance.
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7. Circuit symbol
⇒ Above one are p-channel MOSFET.
⇒ (a) and (b) are either depletion or enhancement type MOSFET.
⇒ (c) represents only the enhancement type MOSFET.
⇒ In (a), the substrate is understood to be connected internally to the
source
⇒ For n-channel MOSFET, the direction of arrow will be reverse.
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8. Digital MOSFET circuits
1 MOS inverter or NOT gate
⇒ (a) MOS inverter (b) Truth table
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9. Continued–
2 NAND gate
⇒ (a) MOS NAND gate (b) Truth table
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10. Continued–
3 NOR gate
⇒ (a) MOS NOR gate (b) Truth table
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