What actually happens when a transistor turns on

You already know a transistor is a voltage-controlled switch (Track 1). Now we zoom in on how that switch flips. The answer involves a beautiful piece of solid-state physics: building a temporary conducting layer of electrons on demand, by waving a tiny voltage at it.

Key Concept

Depletion Region

When a small positive voltage is applied to the gate, it repels positive charges (holes) from the silicon directly under the oxide, leaving behind a 'depleted' zone with almost no free carriers. Still no current flows — but the stage is being set.

Key Concept

Inversion Layer

Push the gate voltage past the threshold and the surface 'inverts' — it now contains so many electrons attracted up from the bulk that the semiconductor surface behaves like a conductor. This thin sheet of electrons is the channel.

Key Concept

Channel Formation

The moment the inversion layer appears, source and drain are connected by a conductive bridge of electrons. The transistor is ON. Increase Vgs further and you pull more electrons in, lowering channel resistance and increasing drain current.

Diagram · Channel formation, live

interactive

Gate Voltage (Vgs)Id = 1.0 nA

0 VVth = 0.5 V1.5 V

Drag Vgs from 0 toward 1.5 V. Watch the depletion region appear, then the inversion layer light up, then electrons fill the channel. The mini I-V graph plots Id vs Vgs in real time.

Key Concept

Subthreshold Swing (SS)

How many millivolts of gate voltage you need to change drain current by a factor of 10 below threshold. Lower = sharper switch. The thermodynamic minimum is **60 mV/decade at room temperature** (the 'Boltzmann limit'). A real silicon MOSFET hits ~70–90 mV/dec; advanced devices try to break the 60 limit using quantum effects or piezo gating.

Checkpoint · +5 XP

What is the theoretical minimum subthreshold swing at 300 K?

What makes a great transistor switch?

1. Fast ON — quickly form the inversion layer. 2. Fast OFF — collapse the channel cleanly. 3. Sharp transition — low subthreshold swing. 4. Low OFF leakage — almost zero current with the gate at 0 V.

These four metrics are the entire scoreboard. Every architectural choice — fin shape, gate-all-around, crystal substrate, piezo gating — is just trying to win on one of these axes.

Try It Yourself

Compare two architectures

In Transistor Sim, set architecture to Planar FET with Silicon. Note the Subthreshold Swing output. Switch to Crystal-EM Hybrid with GaN. Did SS improve? By how much?

Open Transistor Sim

Lesson Summary

Apply gate voltage → form a depletion region → flip an inversion layer → electrons flood the channel.
Threshold voltage (Vth) is where the inversion layer appears.
Subthreshold swing (SS) measures how sharply current rises around Vth — minimum 60 mV/decade at 300 K.
A great switch turns on fast, off completely, and transitions sharply with low leakage.

Test Your Knowledge · +50 XP

What forms first as gate voltage rises?

The inversion layer is:

Subthreshold swing measures:

Why does low subthreshold swing matter?

Which is NOT a goal of a good switch?