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I am familiar with Zener diodes and their parameters.

It's interesting to see the same temperature coefficient at approximately 6V across different manufacturers and different packages. See below image.

Would anyone happen to know why this is? I am assuming it is related to semiconductor chemistry but that is not my field of expertise.

enter image description here

enter image description here

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If I remember correctly from about 45 years ago it's because somewhere around that voltage there's a transition from avalanche diodes to true Zener diodes. They have different temperature coefficients and at the transition they tend to cancel out. In the magazine projects high stability power supplies chose "Zener" diodes of 5V6 for this reason (and availability).

It's been a while so someone will probably correct me!

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    \$\begingroup\$ That's pretty much correct to my understanding. The zener effect dominates at lower voltages, and avalanche breakdown at higher voltages. \$\endgroup\$
    – Hearth
    Commented Aug 17, 2022 at 0:12
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    \$\begingroup\$ Similar to what I have understood. You can make a temperature stable "poor man's 6.8V reference" by putting a 6V zener (tempco +2 mV/C) in series with a silicon diode (-2 mV/C). \$\endgroup\$
    – PStechPaul
    Commented Aug 17, 2022 at 3:43

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