MOSFET Body Structure (Physical Construction)

MOSFET Body Structure (Physical Construction)

A MOSFET (Metal–Oxide–Semiconductor Field-Effect Transistor) is built on a semiconductor substrate with specially doped regions.

1. Basic Structure (NMOS Example)

• Substrate (Body/Bulk): p-type
• Source (S): n+ region
• Drain (D): n+ region
• Gate (G): Metal or polysilicon
• Oxide Layer (SiO₂): Insulating layer

Visual Representation

2. Working Principle

• V_GS = 0: No channel → OFF
• V_GS > V_T: Channel forms (inversion layer)
• Current flows from drain to source

3. PMOS Structure

• Substrate → n-type
• Source/Drain → p+ regions
• Current carried by holes

Note: Polarity is reversed compared to NMOS.

4. Difference Between Source and Drain

Feature	Source	Drain
Function	Supplies charge carriers	Collects charge carriers
Carrier Flow	Carriers enter the channel	Carriers leave the channel
Voltage (NMOS)	Lower potential	Higher potential
Physical Structure	Same as drain (symmetrical)	Same as source
Key Note	In MOSFET, source and drain are physically identical; their role depends on biasing

Source = entry point of carriers Drain = exit point of carriers

5. Body (Bulk) Terminal Importance

• Usually connected to source
• If not → body effect occurs
• Threshold voltage increases with V_SB

V_T increases when V_SB increases

6. Parasitic Components

• Body Diode: Between source-body and drain-body
• Capacitances:
  • C_gs (Gate–Source)
  • C_gd (Gate–Drain)
  • C_gb (Gate–Body)

Summary

• Channel forms due to inversion layer
• Gate is insulated → very high input impedance
• Body affects threshold voltage
• Internal diode always present
• MOSFET is voltage-controlled

Metal + Oxide + Semiconductor → controls current without direct contact

Parent Topics