The voltage of a Zener diode in the Zener breakdown region depends on temperature. Suppose we have 3 Zeners with the following specs:
Which one will have the least thermal drift?
I know the answer, but if you are doing this for a school assignment you should be able to prove it and show your work.
If not, a perusal of detailed datasheets (many are not very detailed) and perhaps some testing, will indicate the 5.1V zener is the best of that lot at 5mA, with low tempco and low dynamic resistance. Lower voltage zeners have a negative temperature coefficient and higher voltage ones have a positive temperature coefficient. Note that in the range between 3 and 8V zener current affects tempco but does not very much affect it above or below that range, so zener current can be used to tune the tempco.
These days it's usually better to use a cheap band gap reference for modest- performance applications. There are specialized ovenized buried zener references for higher performance requirements, but they need in excess of 6V so are inconvenient if you have only low voltage available.
