Photoelectric Effect.
The effect is fundamental to the operation of a CCD.
Atoms in a silicon crystal have
electrons arranged in
discrete energy bands. The lower energy band is called
the Valence Band, the upper band is the Conduction
Band. Most of the electrons occupy the Valence band but
can be excited into the conduction band by heating
or by the absorption of a photon. The energy required
for this transition is 1.26 electron volts. Once in this
conduction band the electron is free to move about in
the lattice of the silicon crystal. It leaves behind a ‘hole’
in the valence band which acts like a positively charged
carrier. In the absence of an external electric field
the hole and electron will quickly re-combine and be
lost. In a CCD an electric field is introduced to sweep
these charge carriers apart and prevent recombination.
Valence
Band
Conduction
Band
1.26eV
Thermally generated electrons are indistinguishable from
photo-generated electrons . They constitute a
noise source known as ‘Dark Current’ and it is important
that CCDs are kept cold to reduce their number.
1.26eV corresponds to the energy of light with a
wavelength of 1mm. Beyond this wavelength silicon becomes
transparent and CCDs constructed from silicon become
insensitive.
Hole Electron