Actual ZENER diodes are only up to a couple of volts. Above that you get avalanche diodes although everyone still calls them Zeners. What goes on inside the diode may be different, but how they look to the world is very similar.
Avalanche diodes have significant temperature coefficients that depend on their voltage. What's interesting is that it crosses over from a positive to a negative signed coefficient a little above 5v.
So 'Zeners' around 5v ara much more stable, as a percentage, over temperature
This is why you see designs all over the place using 5.1v zeners as references theat get multiplied up by resistor ratos (Nice stable resistors, Mmmmm
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Motorola Made a family of 'Voltage Reference Diodes' with very low tempco at several voltages. Inside each was the avalanche diode and a straightforward silicon diode in series. The avalanche part was engineered to have a controlled tempco that matched and cancelled that of the plain diode. The resulting thing was like a 'zener' but with a most unusually low tempco.
1N823A is the 6.2v part, probably the best one of the series. Like any other zener, you have to pay attention to specified current for accuracy and tempco best behaviour.
They were the best reference there was until the three terminal IC references using bandgap references came along. IC fabricators also sometimes use 'buried zeners' as references.
David