Electron-multiplying CCD

From Academic Kids

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Emccd_readout_diag.png
Electrons are transferred serially through the gain stages making up the multiplication register of an EMCCD. The high voltages used in these serial transfers induce the creation of additional charge carriers through impact ionisation.
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Output_vs_input_electrons.png
There is a dispersion (variation) in the number of electrons output by the multiplication register for a given (fixed) number of input electrons (shown in the legend on the right). The probability distribution for the number of output electrons is plotted logarithmically on the vertical axis for a simulation of a multiplication register. Also shown are results from the empirical fit equation shown on this page.

An electron-multiplying CCD (EMCCD, also known as an L3Vision CCD or L3CCD) is a charge-coupled device in which a gain register is placed between the shift register and the output amplifier. In the gain register the electrons are multiplied by impact ionization similar to an avalanche diode. The gain probability at every stage of the register is small (P<2%) but as the number of elements is large (N>500), the overall gain can be very high (<math>g=(1+P)^N<math>), with single input electrons giving many thousands of output electrons. Reading a signal from a CCD gives a noise background, of typically a few electrons. In an EMCCD this noise is superimposed on many thousands of electrons rather than a single electron; the devices thus have negligible readout noise.

EMCCDs show a similar sensitivity to Intensified CCDs (ICCDs). However, as with ICCDs, the gain that is applied in the gain register is stochastic and the exact gain that has been applied to a pixel's charge is impossible to know. At high gains (>30), this uncertainty has the same effect on the signal-to-noise ratio as halving the quantum efficiency with respect to operation with a gain of unity. However, at very low light levels (where the quantum efficiency is most important) it can be assumed that a pixel either contains an electron - or not. This removes the noise associated with the stochastic multiplication at the cost of counting multiple electrons in the same pixel as a single electron. The dispersion in the gain is shown in the graph on the right. For multiplication registers with many elements and large gains it is well modelled by the equation:

<math>P\left (n \right ) = \frac{\left

  (n-m+1\right )^{m-1}}{\left (m-1 \right )!\left
  (g-1+\frac{1}{m}\right )^{m}}\exp \left ( -
  \frac{n-m+1}{g-1+\frac{1}{m}}\right )<math> if <math>n \ge m <math>

where P is the probability of getting n output electrons given m input electrons and a total multiplication register gain of g.

Because of the lower costs and the better resolution EMCCDs are capable of replacing ICCDs in many applications. ICCDs still have the advantage that they can be gated very fast and thus are useful in applications like range-gated imaging.

The low-light capabilities of L3CCDs are starting to find use in astronomy. In particular their low noise at high readout speeds makes them very useful for Lucky Imaging of faint stars, and high speed photon counting photometry.

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