I took two of the little green VDR's off of a Tempest board, and put
them on a curve tracer. It appears they act like:
I = V^3
Here is the plot
V I
0 0
1 5uA
2 30uA
3 90uA
4 250uA
5 500uA
6 800uA
7 1.3mA
8 2mA
9 3mA
10 4mA
12 8mA