Floating-diffusion electrometer with adjustable sensitivity

Claim 1

A floating-diffusion electrometer of the type including an electrometer field effect transistor, a floating diffusion disposed in a semiconductor substrate and electrically connected to the gate of said electrometer field effect transistor, and a charge transfer path in said semiconductor substrate for transferring charge to said floating diffusion, the effective area of the floating diffusion parallel to the top surface of said semiconductor substrate controlling the sensitivity of said floating-diffusion electrometer, improved to further include: a gate electrode insulated from aid semiconductor substrate and located proximate to said floating diffusion; means for applying to said gate electrode, when relatively high electrometer sensitivity is required, a potential which confines the effective area of said floating diffusion; and means for applying to said gate electrode, when relatively low electrometer sensitivity is required, a potential which expands the effective area of said floating diffusion..