QWIP: A HIGHLY STABLE TECHNOLOGY
Quantum Well Infrared Photodetectors (QWIPs) are narrow band, photoconductive detectors based on a resonant electronic transition between the energy levels of an electron gas confined into a quantum well. They are made of a stack of semiconductor layers, a few nanometres thick, with different chemical compositions (e.g. GaAs / AlGaAs). The detector absorption (amplitude and spectral shape) can be controlled thanks to the implementation of an optical coupling structure (e.g. reflective diffraction gratings) on top of each pixel.
Use of GaAs based materials, which gives access to dual use, perennial industrial resources and well mastered manufacturing techniques. This in turn guarantees high yield, excellent uniformity over large surfaces (>150 mm diameter) and high operability for 2D array detector formats;
- majority carriers based device, that does not require surface passivation to master the dark current level;
- no low frequency noise, leading to very stable image characteristics and thus simplifying system design;
- tunable spectral sensitivity (4µm à 20µm) thanks to the band gap engineering: both narrow band and broadband detectors can be designed;
- high thermal stability;
- radiation hardiness, thanks to the high GaAs bandgap.
The concept of QWIPs has been devised in the early '80s. Substantial efforts have been done worldwide in the '90s, to understand the physical background of the QWIP operation. In France research has mainly been done at the Thomson Laboratoire Central de Recherches (now Thales Research and Technology). The first French LWIR QWIP demonstrators have been manufactured in 1997. Mass production of QVGA, 25µm pitch, LWIR QWIP IDDCAs started in 2006 at Sofradir, the QWIP chips being delivered by Thales. In 2008 the product line has been enriched with a TV format, 20µm pitch, LWIR QWIP FPA. In 2005 the QWIP production has been transferred to the III-V Lab (joint R&T venture between Thales and Alcatel).
In January 2013 the QWIP activity has been integrated to Sofradir.