This chip was taped out back in 2019, which was sadly the last time I was in Korea! Led by colleagues Seungbum Baek and Prof. Jongphil Hong at Chungbuk National University.

Link to paper here.

A quick tutorial:

When you turn on SRAM (the chunky, but super fast memory on a CPU), each cell seemingly randomly jumps to a high or low voltage: a 1 or a 0. This randomness is determined by physical variations in each memory cell.

When building nanoscale transistors, it’s unsurprisingly challenging to make these cells perfectly identical. This is the stuff of nightmares when designing processors and analog ICs. But it’s perfect for physically unclonable functions.

Each SRAM cell can only generate a single bit as a response. But by cross-coupling them, we can generate multiple responses from a single SRAM cell. So we get the speed and power benefits of SRAM, with the added bonus of a huge challenge-response space.

This huge space of unique identifiers means that malicious attackers trying to crack a system will have a pretty bad time.