The gate layer in a CCD chip is typically made of polysilicon.

• A. Polysilicon
• B. Silicon
• C. Aluminum
• D. Copper

Answer: (A)

The gate electrode in a CCD is a conductor placed above a thin oxide that forms the potential wells guiding charge along the silicon channel. Polysilicon is chosen because it fits neatly into standard silicon processing: it can be deposited and patterned with the same tools used for the rest of the device, and it can be heavily doped to become a low-resistance conductor that can carry drive signals across many gate elements without adding excessive delay. Doping also lets engineers tune the gate’s work function to achieve consistent threshold voltages across the array. Additionally, polysilicon remains stable through the high-temperature steps used in CCD fabrication and forms a reliable, well-behaved interface with the silicon dioxide.

Metal gates like aluminum or copper tend to diffuse into silicon or require different processing schemes, which can create reliability issues and compatibility problems with the oxide interface. Pure silicon would need to be heavily doped to conduct well, but using polysilicon provides a convenient, dopable, process-compatible gate material that meets the needs of CCD technology.