Charge amplification by impact ionization in charge-coupled devices

Charge-coupled device (CCD) image sensors remain limited in their ability to detect low photon fluxes by the noise of the output charge detection stage, particularly at high output data rates, where the video bandwidths are large. This suggests that some form of predetection charge amplification would be desirable to overcome the output circuit noise. The authors have studied a method of charge and amplification using impact ionization in the high-field region between adjacent CCD gates and have measured avalanche-induced gain and noise on small charge packets. Of particular interest is that the noise data agrees well with theoretical expectations, which suggests that a CCD imager could be made to operate in a photon-counting mode. Neglecting hole ionization, the authors have derived an expression for the avalanche-generated noise which depends on the initial packet size, the noise on the packet before avalanching, and the total gain. The measured data agreed well with the theoretical expression. Using the theory, they have obtained an improvement in the postdetection signal-to-noise ratio for small packets that have undergone avalanche multiplication. The experimental data also revealed that electrons other than from impact ionization were being added to the packet during the gain procedure. These electrons were subsequently avalanched along with the initial packet of interest, giving rise to an enhanced dark-current effect