Electron tube having a photoelectron confining mechanism

An electron tube comprising:

• a photocathode provided so as to emit photoelectrons in correspondence with incident light;
• a semiconductor device having an electron incident surface for receiving the photoelectrons from said photocathode and an incident surface electrode having an opening which dees said electron incident surface, said semiconductor being arranged such that its electron incident surface faces said photocathode, and
• a confining mechanism provided between said photocathode and said electron incident surface of said semiconductor device so as to confine a spread of the photoelectrons from said photocathode, said confining mechanism having an opening for passing through the photoelectrons from said photocathode toward said electron incident surface,
• wherein said opening of said confining mechanism has a maximum inner diameter not greater than a minimum outer diameter of said incident surface electrode of said semiconductor device, said minimum outer diameter of said incident surface electrode being defined by a minimum distance between intersections where a line passing through a center of said electron incident surface intersects an outer edge of said incident surface electrode.