Formerly referred to as "on-chip multiplication
gain", this technology enables multiplication
of charge (i.e., electrons) collected in each pixel
of the CCD's active array. Secondary electrons are
generated via an impact-ionization process that
is initiated and sustained when higher-than-typical
clock voltages (up to 50 V) are applied to a special
extended portion of the CCD's serial register.
The level of EM gain can be controlled by either
increasing or decreasing the voltage; the gain is
exponentially proportional to the voltage. Multiplying
the signal above the read noise of the output amplifier
enables ultra-low-light detection at high operation
speeds. EM gain can exceed 1000x.
It is possible to adapt this technology to all
current CCD architectures. The illustration below
depicts a frame-transfer device.
Some cameras with EM gain utilize two output
amplifiers: (1) an EM gain amplifier that enables
the camera to be used for low-light, high-speed
applications and (2) a traditional amplifier for