MOSFET Structure

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MOSFET Structure
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• 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

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Symbols
p Channel MOSFET
n Channel MOSFET
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Current-Voltage Characteristic
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Channel Formation
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Analysis Low VDS(A)
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Intermediate VDS(B)
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Increased VDS
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Analysis Intermediate VDS
First Order Approximation Gate to Channel Voltage
VGS-VDS/2
Extra term!
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Large VDS Saturation (C)
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Analysis Saturation (C)
Pinch-off
Substitute for VDS(sat) in equation for IDS to
get IDS(sat)
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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.