Many sources claim that when the base-emitter voltage becomes smaller than approximately 0.6V-0.7V, the bipolar junction transistor goes in the cutoff region and there is zero base and collector current.
Of course this is not completely true, since there will be some (exponentially decreasing) current for lower values of Vbe, and this exhibits the characteristics of diodes operated at low voltage (with an exponential I-V characteristic).
Does the Ebers-Moll model accurately predict the behavior of the BJT in cases where Vbe is so small that the base current becomes on the order of pA (10e-12 A)? In general there is a current gain parameter called beta ~ 5 - 100 which relates the collector current to the base current, but does this beta decrease if the base current becomes extremely small?
I am asking this because I have a particular application where I need extremely low currents, and I am not sure if the simulator (Cadence Spectre with foundry-provided npn models) is reliable in this unusual region of operation. I know that it uses the Gummel-Poon integral charge model, but this should be close to the Ebers-Moll model at low currents, I expect.