DEPFET Active Pixel Sensors

TL;DR

DEPFET active pixel sensors are fully depleted, back-illuminated silicon detectors with high quantum efficiency, fast readout, low noise, and versatile pixel sizes, suitable for X-ray spectroscopy from 0.2 keV to 20 keV.

Contribution

This paper details the design, operation, and performance of DEPFET active pixel sensors, highlighting their high efficiency, low noise, and fast readout capabilities for X-ray detection.

Findings

Achieved ~130 eV FWHM energy resolution at 6 keV

Full frame readout time of a few milliseconds for 512x512 pixels

Read noise typically three electrons RMS

Abstract

An array of DEPFET pixels is one of several concepts to implement an active pixel sensor. Similar to PNCCD and SDD detectors, the typically 0.45 mm thick silicon sensor is fully depleted by the principle of sideward depletion. They have furthermore in common to be back-illuminated detectors, which allows for ultra-thin and homogeneous photon entrance windows. This enables relatively high quantum efficiencies at low energies and close to 100% for photon energies between 1 keV and 10 keV. Steering of the DEPFET sensor is enabled by a so-called Switcher ASIC and readout is performed by e.g. a VERITAS ASIC. The configuration enables a readout time of a few microseconds per row. This results in full frame readout times of a few milliseconds for a 512 x 512 pixel array in a rolling shutter mode. The read noise is then typically three electrons equivalent noise charge RMS. DEPFET detectors can be applied in particular for spectroscopy in the energy band from 0.2 keV to 20 keV. For example, an energy resolution of about 130 eV FWHM is achieved at an energy of 6 keV which is close to the theoretical limit given by Fano noise. Pixel sizes of a few tens of microns up to a centimetre are feasible by the DEPFET concept.