Electronic – Controlling Transistors

It's simple, we test them before we sell them and throw the bad ones out.

There are lots of ways to do this - different people do different thing, often use a combination of:

• some tests are at speed to make sure they go fast enough.

• other tests involve a mode that turns some or all of the flipflops in the chip into giant serial shift registers, we clock known data into those chains, then run the chip for one clock and then scan the new results back out and check that they match our predicted results - automatic test tools generate a minimum set of "scan vectors" that will test every random gate or transistor on the chip - other vectors do special tests of ram blocks,

• others test that the external wires are all bonded correctly

• we make sure it's not pulling an unhealthy amount of current

Testing time costs money, we sometimes do some simple testing for obvious dead chips before they are packaged to discard the bad ones and then more testing after the packaging is done

Something very like this is already in use. All transistors generate noise, from a number of effects: http://www.nikhef.nl/~jds/vlsi/noise/sansen.pdf

Intel have a hardware random number generator that uses this: http://electronicdesign.com/learning-resources/understanding-intels-ivy-bridge-random-number-generator

The core of Ivy Bridge's ES is an RS-NOR latch with the set and reset inputs wired together (red). When the R/S input is de-asserted, the latch becomes metastable, and its output eventually settles to 0 or 1, depending on thermal noise.

It's still quite hard to eliminate various side-channels and effects of temperature and manufacturing variation, but that article gives a well-sourced discussion of why it's believed to be a good random number source.