> As a rule, you want your resistors made of metal. Semiconductor resistors have a lot of really surprising behavior.

So much so that probably the most common semiconductor resistors where linearity is needed are actually transistors. Stick on external reference resistor in, use current mirrors inside to distribute the reference current around the circuit, and use transistors in the active region (linear region for FETs). It's also cheaper since transistors take less area than resistors.

Follow up dumb question - I thought transistors take time to charge/discharge/do-physics-things, how do they avoid resistance changing over time.

They change a bit, but primarily at startup. After that they're controlled by the stability of the biasing.

I'd recommend "Designing Analog Chips" by Hans Camenzind[0], the designer of the 555 timer. Particularly chapter 3, "Current Mirrors".

[0] http://www.designinganalogchips.com/ has a free pdf