It's not really linear anywhere, except in the sense that it's got a smooth VI graph, so you can pick a point on the curve and draw a tangent line if you want. That's not really linear, unless you have small enough signals or bad enough oscilloscopes.
It's much closer to exponential, which is Shockley diode equation / "diode law" which is the simplest model you would use besides the "diode is a one-way switch with a voltage threshold" model.
The diode law is I = Is[ exp(VD/nVT) - 1 ].
There are more sophisticated models of the diode that use multiple exponentials. Remember that these are all just approximations of reality based on some simplifying assumptions, the only real question is "how much do you want to simplify", and linear models are very rarely useful for modeling diodes.
It's much closer to exponential, which is Shockley diode equation / "diode law" which is the simplest model you would use besides the "diode is a one-way switch with a voltage threshold" model.
The diode law is I = Is[ exp(VD/nVT) - 1 ].
There are more sophisticated models of the diode that use multiple exponentials. Remember that these are all just approximations of reality based on some simplifying assumptions, the only real question is "how much do you want to simplify", and linear models are very rarely useful for modeling diodes.