A light switch clicks, and the light comes on. But imagine a switch that, with the same press, can either turn on the lamp or leave the room dark. It also remembers what it did last time, and sometimes it seems to flip a coin to decide whether to light up.

This kind of behavior was found in a crystal of black phosphorus — a semiconductor just a few atomic layers thick. Electric fields are applied from both sides, and they don't just add charge carriers; they reshape the energy landscape inside the crystal. Because of the giant Stark effect (the shifting of energy levels in an electric field), the band gap narrows, and two tunnel junctions, essentially 'punctures' through which electrons leak across the barrier, emerge in a single channel. Depending on the history of gate voltages, the current either flows as a calm diffusive stream or breaks into an avalanche breakdown (like lightning, but in miniature).

The most surprising part is that switching between states can be random. But this randomness isn't chaotic: within a narrow voltage window, it's statistically stable. The result is a programmable source of random numbers right in the transistor's 'body.' The authors emphasize that the same trick should work in other two-dimensional semiconductors. That means a single tiny element could combine nonlinear amplification, memory, and randomness generation, the three pillars of new computing architectures.