how to measure zener diode
Zener diodes have a tempco just like any other diode or bipolar. The difference is that the tempco can vary according to the zener rating. Reference diodes with a rating of 5.6V have a tempco close to zero. Below 5.6V the tempco is negative and above it is positive.
Zeners at 3.3V and 6.8V have tempco of similar magnitude than that of a transistor, namely 2-2.5mV/degC. This is handy if you need to cancel tempco's of zeners and transistors in circuits to obtain compensation.