Why is EPROM programming voltage critical?

[Circlotron]

An EPROM might be specified as having a programming voltage of 12.75V +/- 0.25V. What is going on inside an EPROM to necessitate such a narrow range of acceptable programming voltage?

[moffy]

Normally there is an oxide layer on the gate that charge has to be injected through by electron tunneling so the voltage needs to be high enough for enough electrons to tunnel through without breaking down the very thin oxide layer. Also consider variations in thickness of the layer due to process variations and you end up with a moderately precise voltage range.

P.S. One would also like to keep programming time down, so that means a higher voltage but shorter programming pulse.

[CaptDon]

Some of the old 25 volt and later 21 volt VPP stuff had feeble transient clamp diodes on the programming voltage rail and they would fail dead short especially if you hit a 21 volt part with 25 volts. Some of the 12.75 volt stuff seems to have undervoltage lockout where they fail to program at all if the 12.75 is low. The manufacturer guarantees a 'data retention time' and that is hugely effected by programming voltage. You can surely go outside the limits but at your own risk.

[David Hess]

Intel's Flash memory had a warning about ringing on the programming supply which would likely destroy the part.  Just a couple of volts high is a real risk even if it is transient.