Why did the world switch from the vacuum tube to the MOSFET? The answer lies in a phenomenon called the Surface Field Effect.
In the 1930s, the concept was a dream; by 1960, it became a reality through the Si-SiO₂ (Silicon-Silicon Dioxide) system. Today, it is the backbone of every microprocessor and high-efficiency power device in existence.
1. What exactly is the Surface Field Effect?
The "field effect" is a physics principle where an external electric field is used to control the electrical conductivity of a material. In a MOSFET, this happens at the surface of the semiconductor.
- The Mechanism: When voltage is applied to the Gate, an electric field penetrates the oxide layer.
- The Channel: This field attracts charge carriers to the surface of the silicon, creating a thin "conductive bridge" (the channel) between the Source and the Drain.
- The Switch: By simply changing the voltage, you can instantly create or destroy this bridge, making it the perfect digital switch.
MOSFET Cross-Section Diagram
2. Source vs. Drain: Who Does What?
A common confusion is whether the Drain "emits" electrons like a BJT Emitter. It does not.
- Source: The "Entry." It provides (sources) the charge carriers to the channel. In an NMOS, it is usually the negative terminal.
- Drain: The "Exit." It collects (drains) the carriers after they have passed through the channel. In an NMOS, it is the positive terminal.
3. The Governing Equations
To design circuits, engineers use these key formulas to determine how much current ($I_D$) flows through the device.
$I_D = \mu_n C_{ox} \frac{W}{L} [(V_{GS} - V_{th})V_{DS} - \frac{V_{DS}^2}{2}]$
$I_D = \frac{1}{2} \mu_n C_{ox} \frac{W}{L} (V_{GS} - V_{th})^2$
$g_m = \frac{\partial I_D}{\partial V_{GS}} = \mu_n C_{ox} \frac{W}{L} (V_{GS} - V_{th})$
This equation is only valid for saturtaion region. See more details about MOSFET Transconductance
4. MOSFET vs. BJT: The Showdown
| Feature | MOSFET | BJT (Bipolar Junction Transistor) |
|---|---|---|
| Control | Voltage-Controlled (Gate Voltage) | Current-Controlled (Base Current) |
| Input Impedance | Extremely High (Infinite in theory) | Low |
| Terminals | Gate, Source, Drain | Base, Emitter, Collector |
| Efficiency | High (Low power waste) | Moderate (Base current wastes power) |
| Thermal Stability | Excellent | Prone to Thermal Runaway |
Summary
If you are designing modern digital logic (like microprocessors) or high-efficiency power converters, the MOSFET is your gold standard. Its high input impedance means you can control massive amounts of power with almost zero control current.
Use BJTs only when you need very high linearity in specific analog audio applications or when working with extremely low-voltage legacy circuits where the Vth of a MOSFET might be a hindrance.