How are voltage references established in low-power designs?
The typical 'bandgap' or 'Widlar' voltage reference consists of 

a) the Vbe of a transistor (which is temperature sensitive)
b) plus a temperature sensitive voltage, designed to track the error in (a)

Together these add up to the ubiquitous 1.22ish volts that you see in adjustable voltage regulators everywhere.

The thermometer for (b) above is made by running two transistors at different emitter current densities. At a 10:1 ratio, the difference in Vbe will be about 60mV. This is multiplied by 10 to 600mV, and added to the Vbe.

See this for a well written article on the topic.