In the case of a thick light-absorbing layer 2, a phenomenon of a decrease
in the time resolution occurs. However, when the thickness of the light-absorbing
layer 2 is limited, a portion of low electron concentration in one electron
group is cut out, and hence overlap regions of adjacent electron concentration
distributions decrease. Therefore, by shortening the transit time necessary for
the passage of electrons, regions of overlapping electron distributions due to
diffusion can also be suppressed. Furthermore, the strength of an electric field
within a light-absorbing layer can be increased by thinning the light-absorbing
layer. Therefore, the time resolution of infrared rays can be remarkably improved
by a synergistic action of these effects. If it is assumed that the time resolution
is 40 ps (picoseconds), for example, when the thickness of a light-absorbing layer
is 1.3 m which is nearly equal to the wavelength of infrared, then a possible
time resolution is 7.5 ps when this thickness is 0.19 m.
