MOSFET Structure
MOSFET Structure
The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is the building block of all modern digital chips. It has four terminals:
- Gate: Controls whether current flows (the switch handle)
- Source: Where current enters the channel
- Drain: Where current exits the channel
- Body/Substrate: The silicon bulk (usually connected to source)
The key structure is a gate stack: a metal (or polysilicon) gate electrode separated from the silicon channel by a thin gate oxide insulator (originally SiO₂, now high-k dielectrics like HfO₂).
Key Concept: How Switching Works
In an NMOS transistor: when gate voltage is LOW, no channel exists between source and drain — the switch is OFF. When gate voltage exceeds the threshold voltage (Vt), the electric field inverts the P-type silicon surface to create an N-type channel — the switch is ON, and current flows.
NMOS and PMOS
NMOS and PMOS
MOSFETs come in two complementary types:
| Property | NMOS | PMOS |
|---|---|---|
| Substrate | P-type | N-type |
| Source/Drain | N-type | P-type |
| Carrier | Electrons | Holes |
| Turns ON when | Gate = HIGH | Gate = LOW |
| Speed | Faster (higher mobility) | Slower (~3× lower mobility) |
NMOS is faster because electrons have higher mobility than holes in silicon. However, both types are used together in CMOS (Complementary MOS) logic because the combination offers extremely low static power consumption.
Modern Transistor Evolution
Modern Transistor Evolution
As transistors have shrunk, their structure has evolved dramatically:
- Planar MOSFET (until ~28nm): The classic flat transistor on the silicon surface
- FinFET (14nm–3nm): The channel rises up as a thin "fin," with the gate wrapping around three sides for better control. Invented by UC Berkeley professor Chenming Hu.
- Gate-All-Around (GAA) / Nanosheet (2nm and beyond): Horizontal nanosheets or nanowires with the gate completely surrounding the channel from all four sides.
Analogy: Gripping a Hose
Controlling current through a transistor is like squeezing a water hose. A planar MOSFET squeezes from one side (weak control). A FinFET squeezes from three sides (better). GAA squeezes from all sides (best control) — like wrapping your whole hand around the hose.
Knowledge Check
Knowledge Check
1 / 2What does the gate do in a MOSFET?