The insulating layer in a CCD chip is the:

• A. Silicon dioxide layer
• B. Polysilicon layer
• C. Aluminum oxide layer
• D. Silicon nitride layer

Answer: (A)

The insulating layer is silicon dioxide. In a CCD, the patterned gate electrodes sit atop a thin dielectric that isolates them from the silicon substrate while still allowing the gates’ electric fields to modulate the potential in the channel. Silicon dioxide provides a clean, stable, high-quality interface with silicon, giving reliable insulation and the right capacitance for controlling charge transfer. The gates themselves are typically made of polysilicon, so polysilicon is not the insulating layer. Other materials like aluminum oxide or silicon nitride can serve other roles such as passivation or different dielectric schemes in some processes, but they are not the standard insulating layer between the substrate and the gates in a typical CCD.