EMCCD Tutorial
Q2 What happens during the electron multiplying process?
EMCCDs use a conventional CCD structure, except that the shift register extended
with an additional section – the Gain Register. The gain register is essentially
a chain of pixels across which the signal charge is transferred. Within each pixel
unit of the gain register, one of the electrodes (phases) responsible for charge
transfer is replaced with two electrodes. The first is held at a fixed potential
and the second is clocked as normal, except that much higher voltages (between 40V
and 60V amplitude) are used than are necessary for charge transfer alone. The large
electric field generated between the fixed voltage electrode and the clocked electrode
is sufficiently high for the electrons to cause ‘impact ionization’ as they transfer.
The impact ionization phenomenon results in generation of new electrons in a probabilistic
fashion, i.e. multiplication or EM gain. The multiplication per transfer is actually
quite small, only around X1.01 to X1.015 times maximum, but when done over a large
number of transfers substantial gain is achieved.