A classical MOSFET turns on by thermionic emission: raising the gate voltage lowers a potential barrier until electrons in the source's thermal tail spill over it into the channel. Because that tail follows a Boltzmann distribution, the current can never rise faster than roughly a decade per 60 mV of gate swing at room temperature — the "thermal limit" that caps how efficiently a MOSFET can switch off, no matter how the device is engineered.
A Tunnel FET replaces the MOSFET's n-i-n (or p-i-p) structure with a p-i-n junction. Instead of pushing electrons over a barrier, the gate aligns the valence band of the p-type source with the conduction band of the intrinsic channel, letting electrons tunnel directly through the forbidden gap — band-to-band tunneling, a genuinely quantum process with no classical analogue. Because tunneling turns on abruptly once the bands line up, the current can rise with a subthreshold swing below 60 mV/decade, in principle allowing correct operation at a much lower supply voltage — the central promise of "beyond-CMOS" low-power logic.
MOSFET: I ∝ exp(qV_GS / (n·kT)) → SS ≥ (kT/q)·ln(10) ≈ 60 mV/dec
TFET: I ∝ T_WKB(V_GS), tunneling prob. → SS can drop below 60 mV/dec
- Transistor type — switches the device model between a classical MOSFET (thermionic barrier) and a TFET (band-to-band tunneling through a p-i-n junction).
- Gate voltage — sweeps VGS; watch the electron markers cross the channel by hopping over the barrier (MOSFET) or tunneling straight through it (TFET).
- I-V chart — plots drain current vs gate voltage on a log scale for both device types simultaneously, so the shallower MOSFET slope and the steeper TFET slope can be compared directly.
Real-world relevance: TFETs and other steep-slope switches are studied as one of the leading "beyond-CMOS" nanoelectronic device concepts, aimed at the fundamental energy-efficiency wall silicon MOSFETs hit as supply voltages stop scaling down — a wall that limits battery life in every mobile and IoT chip built today.