Transistors may stop shrinking because the off switch could leak
The off state is an electrical barrier, not a mechanical break, and thinning it could let electrons sneak across.
Veritasium · How Does a Transistor Work?A transistor stays off because of an invisible electrical wall. If transistors keep getting smaller, engineers might struggle to keep that wall strong enough.
A transistor is a sandwich with ends rich in loose electrons and a middle rich in empty slots for electrons, called holes. It has a metal contact at either tip, the source and the drain, plus a third contact, the gate, that is kept separate from the sandwich by a thin oxide coating.
When the sandwich is made, electrons spill from the ends into the middle to fill holes. That movement empties a zone of anything that can carry current. Engineers call it the depletion layer, meaning a layer depleted of mobile charges.
That empty zone becomes the off switch. Loaded with extra electrons, the middle turns negative and repels any electrons trying to cross from one end to the other. No current flows, and the transistor holds a zero.
A small positive voltage on the gate turns it to a one. The voltage attracts electrons, overcomes that repulsion, and shrinks the depletion layer until electrons can move through in a conducting channel. There are no moving parts, and the switch can flip very quickly on voltage alone.
The switch can also be made tiny. Today's transistors are only about 22 nanometers wide, or about 50 atoms across, and Moore's law, the expectation that chipmakers will double the count of transistors packed onto silicon every two years, calls for them to keep getting smaller.
There may be a limit. As the two ends are placed very close, quantum effects matter more and electrons can slip straight across. It might become impossible to build the electrical wall tall enough to hold them back. That would mean current could keep flowing even when the transistor is meant to be off.
Questions people ask
What exactly is quantum tunneling?
Quantum tunneling means a particle crosses a barrier it does not have enough energy to climb over. At very small scales, electrons behave like smeared-out waves, so there is a small chance of finding one on the far side. In a tiny transistor, that chance becomes large enough to matter.
How small is 22 nanometers?
A nanometer is one-billionth of a meter. Twenty-two nanometers is thousands of times thinner than a human hair, and only about 50 silicon atoms laid side by side. At that scale, moving the parts even slightly closer changes the electrical behavior.
Can engineers just accept a little leakage and keep shrinking?
No. Leakage wastes power and creates heat, and it can blur the difference between zero and one. Chipmakers have slowed the old pace of shrinking and are improving design, materials, and three-dimensional structures, while researchers explore very different devices.