  Weakly coupled semiconductor superlattices under dc voltage bias are
nonlinear systems with many degrees of freedom whose nonlinearity is due to
sequential tunneling of electrons. They may exhibit spontaneous chaos at room
temperature and act as fast physical random number generator devices. Here we
present a general sequential transport model with different voltage drops at
quantum wells and barriers that includes noise and fluctuations due to the
superlattice epitaxial growth. Excitability and oscillations of the current in
superlattices with identical periods are due to nucleation and motion of charge
dipole waves that form at the emitter contact when the current drops below a
critical value. Insertion of wider wells increases superlattice excitability by
allowing wave nucleation at the modified wells and more complex dynamics. Then
hyperchaos and different types of intermittent chaos are possible on extended
dc voltage ranges. Intrinsic shot and thermal noises and external noises
produce minor effects on chaotic attractors. However, random disorder due to
growth fluctuations may suppress any regular or chaotic current oscillations.
Numerical simulations show that more than 70\% of samples remain chaotic when
the standard deviation of their fluctuations due to epitaxial growth is below
0.024 nm (10\% of a single monolayer) whereas for 0.015 nm disorder suppresses
chaos.
