MOSFETs Explained: Regions, Characteristics, and Applications

The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is the most widely used transistor in analog and digital electronics. Whether you are designing amplifiers, current mirrors, or digital circuits, MOSFET fundamentals are essential. This guide explains MOSFET operation modes, equations, and applications with a focus on analog design and interview preparation.

1. MOSFET Basics

A MOSFET has four terminals:

• Gate (G) – controls the channel conductivity
• Drain (D) – output terminal
• Source (S) – reference terminal
• Body (B) – substrate (often tied to Source)

The gate is insulated from the channel by a thin oxide layer, giving MOSFETs their high input impedance.

2. Regions of Operation

a) Cutoff Region

Condition: V_GS < V_TH
The MOSFET is OFF, no current flows (I_D ≈ 0).

b) Triode (Linear) Region

Condition: V_GS > V_TH and V_DS < (V_GS – V_TH)
Acts like a variable resistor. Current equation:

I_D = μ_nC_ox(W/L) [ (V_GS – V_TH)V_DS – (V_DS²/2) ]

c) Saturation Region

Condition: V_GS > V_TH and V_DS ≥ (V_GS – V_TH)
Acts like a current source. Current equation:

I_D = (1/2) μ_nC_ox(W/L)(V_GS – V_TH)²(1 + λV_DS)

λ is the channel length modulation parameter (finite output resistance).

3. Transfer Characteristics

The transfer curve (I_D vs. V_GS) shows the threshold behavior. For small V_GS, the MOSFET is OFF. Once V_GS exceeds V_TH, current rises quadratically in saturation.

4. Small-Signal Parameters

• Transconductance (g_m): ∂I_D/∂V_GS = μ_nC_ox(W/L)(V_GS – V_TH)
• Output resistance (r_o): 1 / (λI_D)
• Gain: A_v = g_m × r_o

5. Applications of MOSFETs

• Switching: Used in digital circuits (logic gates, processors).
• Amplification: Common source amplifiers, analog front-ends.
• Current Mirrors: Precise biasing circuits in ICs.
• Voltage Regulators: As pass elements in LDOs and DC-DC converters.
• RF Circuits: As mixers, oscillators, and low-noise amplifiers.

6. Interview Questions on MOSFETs

• Why is MOSFET preferred over BJT in modern IC design?
• Explain channel length modulation and its impact on current source behavior.
• How does scaling (W/L) affect transconductance and speed?
• Difference between subthreshold region and cutoff region?

Conclusion

MOSFETs form the backbone of modern electronics. Understanding their regions of operation, small-signal behavior, and real-world non-idealities is crucial for both practical design and interviews.