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Sub-Electron Precision Charge Measurement with RNDR DEPFET Structures

 

 

 
  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.
  Schematic of the RNDR DEPFET pixel matrix cell topology

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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Graph of the distribution of photons at the time when the detector is irradiated with a weak optical light source.