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MOSFET Structure - PowerPoint PPT Presentation


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Gate Oxide. Source. Gate. Drain. Field Oxide. n +. L. p-Si. Bulk (Substrate). MOSFET Structure. Importance for LSI/VLSI Low fabrication cost Small size Low power consumption Applications Microprocessors Memories Power Devices Basic Properties Unipolar device

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mosfet structure

Gate Oxide

Source

Gate

Drain

Field Oxide

n+

L

p-Si

Bulk (Substrate)

MOSFET Structure
slide2
Importance for LSI/VLSI
    • Low fabrication cost
    • Small size
    • Low power consumption
  • Applications
    • Microprocessors
    • Memories
    • Power Devices
  • Basic Properties
    • Unipolar device
    • Very high input impedance
    • Capable of power gain
    • 3/4 terminal device, G, S, D, B
    • Two possible device types: enhancement mode; depletion mode
    • Two possible channel types: n-channel; p-channel
symbols

D

D

B

B

G

G

S

S

Symbols

p Channel MOSFET

n Channel MOSFET

channel formation

S

D

VG

+VDS

p-Si

n-Channel

B

Channel Formation
intermediate v ds b

Source

Channel

Drain

VT

VG

VG-channel

VDS

VDS/2

Intermediate VDS(B)
increased v ds

S

D

VG

+VDS

p-Si

n-Channel

B

Increased VDS
analysis intermediate v ds
Analysis: Intermediate VDS
  • First Order Approximation
    • Gate to Channel Voltage = VGS-VDS/2

Extra term!

large v ds saturation c

Source

Channel

Drain

VG-channel

VDS

VG

VT

Pinch-off

Large VDS: Saturation (C)
analysis saturation c
Analysis: Saturation (C)

Pinch-off

Substitute for VDS(sat) in equation for IDS to get IDS(sat)

avalanche and punch through d
Avalanche and Punch-Through(D)
  • For very large VDS, IDS increases rapidly due to drain junction avalanche.
  • Can give rise to parasitic bipolar action.
  • In short channel transistors, the drain depletion region may reach the source depletion region giving rise to ‘Punch Through’.