THE THERMAL LIMIT

A tail you cannot cut

Boltzmann Carriers have a spread of thermal energies, so some always climb the barrier

The number At best a 60 mV gate change is needed for every tenfold change in current at 300 K

The cost A five-decade on/off ratio needs a gate swing of at least 300 mV, which sets a floor on the supply voltage

Why it matters Switching energy goes as V², so a lower supply would cut power sharply

To go below 60 mV per decade, change the mechanism, not the geometry.

THREE SWITCHES

Compare the curves

A tunnel FET does not use thermal emission at all, so its swing can be far below 60 mV per decade for the first few decades of current. A negative-capacitance FET adds a ferroelectric layer to the gate stack that boosts the internal gate voltage by a factor A, so the swing becomes 60/A everywhere. Each has a catch: the tunnel FET runs out of current, and the negative-capacitance FET is hard to stabilise.

Steeper start, smaller current. Amplified gate, harder to make.

TUNNELING

Through the gap, not over the barrier

In a tunnel FET the source is p-type and the drain n-type. A gate voltage lowers the channel's conduction band until it sits below the source's valence band. Electrons can then tunnel straight across the gap, and the current turns on as fast as the energy window opens. Because the carriers are not thermal, the 60 mV limit does not apply, but the tunneling barrier is much thicker than the barrier of a MOSFET, so the on current is low.

Tunnel FETs are steep but weak: good for sensors and tiny currents.

THE FERROELECTRIC IDEA

Amplify the gate, store a bit

Negative capacitance In a ferroelectric, part of the polarization curve behaves like a negative capacitor, which can boost the voltage across the semiconductor

Hafnium oxide In 2011 hafnium oxide was found to be ferroelectric when doped, and it is already in CMOS fabs

FeFET memory Polarization sets the threshold voltage, so the transistor itself is a nonvolatile memory cell

Status Hysteresis and repeatability still make negative-capacitance logic a research topic, while ferroelectric memory is closer to products

Ferroelectric gates may reach memory before they reach logic.

CHECK THE ROUTES

How to beat the limit

Thermal limit, tunneling, or amplification: three stories about the same 60 mV.

UNIT 25 STUDY COMPLETE

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Ready for the Fab Challenge?

You've met the thermal limit on switching and the two research ideas that try to beat it.