  Shi and Xie [8] predicted the N-shape current-voltage characteristic $(CVC)$
for a 2DEG in a magnetic field where the zero-resistance state has been
observed in recent experiments. However it is known that in the absence of a
magnetic field the zero resistance state (zero differential resistance state)
is achieved in a system with the S-shape $CVC$. The difference in the behaviour
of systems with N- and S-shape $CVC$ was studied more than three decades ago
(see for example the review [9] and references therein) and is briefly
explained in this Comment. At present it is not clear whether the N-shape $CVC$
may lead to the zero resistance state in a 2DEG in a magnetic field.
