⚡ MOSFET Basics — The Voltage-Controlled Transistor

🎯 Key Concept

If a BJT controls current using current, a MOSFET controls current using voltage.
This single difference makes MOSFETs faster, cooler, and easier to drive in modern electronics.
That’s why MOSFETs dominate power supplies, motor drives, and microcontroller projects today.

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🔁 MOSFET vs BJT — The Fundamental Difference

📚 Core Theory

The most important difference lies in how the device is controlled.

Control Mechanism

• BJT: Requires base current
• MOSFET: Requires only gate voltage

Almost no current flows into the MOSFET gate — it behaves like a capacitor.

This means:

• No loading of control circuitry
• Extremely high input impedance
• Better efficiency

⚡ Comparison

Feature	BJT	MOSFET
Control Type	Current	Voltage
Input Current	Required	~Zero
Switching Loss	Higher	Lower
Efficiency	Moderate	High

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🧠 Inside a MOSFET — How It Works

📚 Core Theory

A MOSFET has three terminals:

• Gate (G): Control input
• Drain (D): Current enters
• Source (S): Current exits

When a voltage is applied between Gate and Source, an electric field forms.

This electric field:

• Creates a conductive channel
• Allows current to flow from Drain to Source
• Requires no continuous gate current

Remove the gate voltage → channel disappears → current stops.

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🔀 MOSFET Types — NMOS and PMOS

📚 Core Theory

MOSFETs come in two polarities:

N-Channel MOSFET (NMOS)

• Turns ON with positive gate voltage
• Lower resistance
• Higher current capability
• Most commonly used

P-Channel MOSFET (PMOS)

• Turns ON with negative gate voltage
• Used for high-side switching

👉 Beginners almost always start with N-channel MOSFETs.

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⚙️ MOSFET Operating Regions

📚 Core Theory

MOSFETs operate in two main regions:

1️⃣ Linear (Triode) Region

• Acts like a variable resistor
• Used in analog applications
• Voltage drop depends on gate voltage

2️⃣ Saturation Region

• Acts like a controlled current source
• Used in amplifiers

⚠️ For switching, we avoid both and drive the MOSFET fully ON or fully OFF.

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🚦 The Sweet Spot — MOSFET as a Switch

📚 Core Theory

When fully ON, a MOSFET has a very small on-resistance:

$$R_{DS(on)} \ll 1\,\Omega$$

Power loss:

$$P = I^2 \times R_{DS(on)}$$

This results in:

• Very low heat generation
• High efficiency
• Excellent power handling

When OFF:

• Practically zero current
• Acts like an open circuit

⚡ Switching Advantage

Parameter	BJT	MOSFET
Voltage Drop ON	~0.2 V	Millivolts
Heat Loss	Higher	Very Low
Drive Power	Required	Negligible

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🧩 Why MOSFETs Are Taking Over

📚 Core Theory

MOSFETs offer huge practical advantages:

• Can be driven directly from microcontroller GPIO
• Switch very fast (MHz range)
• Handle very high currents
• Generate less heat
• Enable compact and efficient designs

This is why MOSFETs dominate:

• DC-DC converters
• Motor controllers
• Battery systems
• Power amplifiers

🚀 Practical Warning

MOSFET gates are ESD sensitive. Always:

• Use gate resistors
• Avoid floating gates
• Handle carefully

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🚀 Key Takeaway

🚀 Key Takeaway

• MOSFETs are voltage-controlled devices
• Gate draws almost no current
• Extremely efficient for switching
• NMOS is beginner-friendly
• Mastering MOSFETs unlocks modern power electronics

Final Insight:
⚡ MOSFETs turn voltage into effortless power control — that’s why modern electronics runs on them.