  We study theoretically the current-voltage characteristics (IVCs) of the
Josephson field effect transistor - a ballistic SNINS junction with
superconducting (S) electrodes confining a planar normal metal region (N),
which is controlled by the gate induced potential barrier (I). The calculations
were performed using the computation technique developed earlier for long
single-channel junctions in the coherent multiple Andreev reflections (MAR)
regime. We find significant difference of the subgap current structure in these
junctions compared to the subharmonic gap structure in tunnel junctions and
atomic-size point contacts. For long junctions, whose length significantly
exceeds the coherence length, the IVC exhibits current peaks at multiples
(harmonics) of the difference $\delta_m$ between the static Andreev levels,
$eV_n = n\delta_m$. Moreover, the averaged IVC follows the power rather than
exponential behavior, and has a universal scaling with the junction
transparency. This result is qualitatively understood using an analytical
approach based on the concept of resonant MAR trajectories. In shorter
junctions whose length is comparable to the coherence length, the IVC has an
exponential form common for point contacts, however the current structures
appear at the subharmonics of the Andreev interlevel distance, $eV_n =
\delta_m/n$ rather than the gap subharmonics $2\Delta/n$.
