Electrostatic focusing Spindt-type FEA for image sensor with HARP target

flat image sensor consisting of a Spindt-type field emitter array (FEA) and a high-gain avalanche rushing amorphous photoconductor (HARP) target has been studied with the aim of developing ultrahigh-sensitivity compact television cameras . This image sensor is called an FEA-HARP sensor. The output signal current of the FEA-HARP sensor is obtained by reading out holes that have accumulated on the HARP target depending on the incident light with scanning electrons emitted sequentially from Spindt-type field emitters (FEs) placed on each pixel. Resolution of the FEA-HARP sensor is determined by the spatial spread of the electrons on the HARP target, and a spread that is wider than the pixel area deteriorates the resolution. While a previous FEA-HARP sensor with a magnetic focusing system that suppresses the spread of the electrons had sufficient resolution, this focusing system needs large cylindrical permanent magnets. Therefore, constructing a compact camera with the previous FEA-HARP sensor is difficult. To overcome this problem, we investigated the feasibility of using an electrostatic focusing system, which has a focusing electrode on a gate electrode instead of magnets.