|Sub-Electron Precision Charge Measurement with RNDR DEPFET Structures|
|The precise measurement of
charge down to one electron is required in High Time Resolution Astrophysics
(HTRA) for the observation of optical photons from rapidly and periodically
changing objects as are for example rotating neutron stars or compact
binary objects. Distant faint objects less than one electron in average
will be observed within one rotation period.
The double DEPFET structure(figure on the right) allows charge measurement with a precision much better than one electron. In a DEPFET the current is steered by the charge accumulated in the internal gate (IG). Measuring the charge does not destroy the charge.
Cell topology of the RNDR DEPFET pixel matrix (IG = internal gate, TG = transfer gate, CLG = clear gate).
|Shifting the charge
between the internal gates of the two DEPFETs allows repeated measurement
of the same charge (RNDR, Repeated Non Destructive Readout) and the
precision improves with the square root of the number of measurements. A
RNDR pixel matrix of four by four cells measuring 75 µm by
75 µm each
has been built and illuminated with faint laser pulses. Each light pulse
was injected shortly before performing 300 cycles of charge measurements.
The Graph on the left shows the measured charge distribution. The signal peaks from zero up to five and even more signal electrons are clearly distinguishable. The widths of the peaks correspond to a charge measurement precision of 0.25 electrons – a world record.
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