Lecture on MOSFETs (Metal Oxide Semiconductor Field Effect Transistors)

Introduction to Transistors

• Transistors are essential components in modern electronics.
• Billions of transistors are in use in everyday devices.
• Focus on Field Effect Transistors (FETs), specifically MOSFETs.

Basics of Current and Semiconductors

• Current Flow:
  • Flows from positive to negative terminal of a battery.
  • Electric field flows similarly; however, electrons flow in the opposite direction.
• Semiconductor Materials:
  • Made from materials like silicon.
  • Conductivity lies between conductors and insulators.
• Doping:
  • Impurities are introduced to pure crystals to enhance conductivity.
  • N-type: Pentavalent impurities, electrons are majority carriers.
  • P-type: Trivalent impurities, holes are majority carriers.
• Depletion Region:
  • Formed at junctions of n-type and p-type.
  • Forward and reverse bias influences the depletion layer.

Introduction to MOSFETs

• Types: Enhancement type and depletion type.
• Focus on N-channel MOSFET, specifically the enhancement type.

Structure of N-channel Enhancement Type MOSFET

• Components:
  • Yellow regions: n-type semiconductors
  • Blue regions: p-type semiconductors
• Terminals:
  • Source, Drain, Gate, and Substrate/Body
  • Source and substrate are internally connected.

Working Principle of MOSFET

• Voltage Connections:
  • Vds: Voltage between drain and source, controls current flow.
  • Vgs: Voltage between gate and source, creates a channel for electron flow.
• Channel Creation:
  • Gate voltage attracts electrons, forming a channel (n-type region) between source and drain.
  • Controlled by the gate voltage, known as the threshold voltage.

Regions of Operation

• Cutoff Region: No current flows due to depletion region.
• Ohmic/Linear Region: Current follows Ohm's law; increases linearly with voltage.
• Saturation Region:
  • Channel pinching occurs, flow becomes restricted.
  • Saturation current reached, further voltage increase does not increase current.
  • To increase current, further increase the gate voltage.

MOSFET Characteristics

• Voltage Control Device:
  • Gate voltage controls current flow from drain to source.
• Graph Characteristics:
  • Drain characteristics and transfer characteristics (constant Vds) are important to understand.

Depletion Type MOSFET

• Differences from Enhancement Type:
  • Channel present by default.
  • Requires negative gate voltage to turn off.

Circuit Symbols

• Symbols:
  • Four terminals: source, gate, drain, substrate.
  • Enhancement type and depletion type distinguished by line styles.
  • N-channel and P-channel indicated by arrow direction.

Conclusion

• MOSFETs are integral to electronic devices, controlling current with voltage.
• Suggested further learning: mobile charger functionality, mobile orientation detection.

Thank you for watching.