This is the simplest, least-exciting stage, so I'll just add a couple of supplementary details.

The device needs a variable 10-15VDC supply. This is so you can adjust the frequency and amplitude of tunneling events -- too low a voltage and you'll get only a few, weak events. Too high, and the timer on your microcontroller won't run fast enough to easily measure the time between events.

Overall, the exact semiconductor you use has a bigger effect on tunneling frequency, but it's still nice to be able to tune it a little. Also the first hex inverter responsible for amplification can run a little hot at higher voltages. It's a pretty rugged part though.

I found around 10VDC worked fine for me -- that put the average time distance between events as a few microseconds (say 4), and I'm running the microcontroller at 4Mhz. However in another similar device, I used a 20Mhz microcontroller and pushed the voltage up to about 15V to improve performance somewhat.

The 5V linear regulator is just there to power the microcontroller and signal buffer. We buffer the signal with a second hex inverter to give it better defined rising and falling edges, and also so that the signal cannot ever exceed 5V. I chose a linear regulator instead of a switched one because this is not a battery powered device, so the extra power cost is not really relevant, it is cheaper, and it produces less noise.