FOXFET structure — device modelling and analysis

A FOXFET structure for biasing AC coupled detector structures has been analyzed by a two-dimensional device simulation. For this purpose, a floating strip junction with zero current boundary condition has been applied. The floating strip voltage increase is analyzed from depletion layer spreading through three charge regions: electron accumulation surface region, hole current flow region and depleted bulk region. As a result, the floating strip potential increases approximately as the square root of the drain/backside reverse voltage. Strip potential saturation is observed for oxide charge densities larger than 5 × 1011 cm−2 and results in a weaker gate control and oxide thickness influence. Current conduction mechanisms are critically discussed and drift-diffusion injection from the floating strip junction is proposed instead of the thermionic emission model. Strip potential increase by an additionally injected strip current is due to the effect of space-charge-limited-current (SCLC) for an injected strip current larger than approximately 10−9 A/μm. The dynamic resistance calculated from numerically obtained strip current/voltage curves has a slope of 0.85 for a lower injected strip current (Is<10−10 A/μm) and decreases to 0.6 for a larger one.