The basic principle of a MOS structure as an injector is to use the electrons which can be accumulated in the potential "poket" below the oxide created by the positive oxide charge. By applying a negative pulse to the metal gate, the electrons can overcome the potential saddle point and be injected in the silicon bulk. Despite its basic simplicity, there are several design problems to realise a series of "point-like" injectors (placed across the whole sensitive area of the detector, in the direction parallel to the anodes) and to control them with the least number of external connections.
The figure below shows a detail of the MOS injector structure.
A p+ implant (black strips) runs below the metal and the oxide in order to prevent the formation of an electron accumulation layer; this is allowed only in certain areas, where the p+ implant is interrupted. These rectangular areas (MOS) constitute the real injectors. The metal gate is 100um long and 20um wide.