The document discusses depletion MOSFET and digital MOSFET circuits. It describes a depletion-type n-channel MOSFET structure on a p-type substrate with two n+ source and drain regions separated by a distance. When a negative gate voltage is applied, it induces positive charges in the semiconductor beneath the oxide, decreasing conductivity and allowing current to flow through the induced channel. Digital MOSFET circuits include the MOS inverter, which acts as a NOT gate, the NAND gate, and the NOR gate. P-channel devices require more area than n-channel devices for the same resistance due to differences in hole and electron mobility.

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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