Noise in a Quatratran-based detector: a comparison with Superconducting Tunnel Junctions

The Quasiparticle Trapping Transistor (QuaTraTran, QTT) was proposed by Booth et al. [1, 2] as a new, multi-purpose superconducting electronic device for the amplification of low-level electrical signals at low temperatures. In this paper we consider its application as a cold 'pre-preamplifier' for signals coming from a superconducting tunnel junction (STJ) used as a soft X-ray detector. The amplification of signals in an STJ itself relies on multiple tunneling of quasiparticles, which has a large associated noise when compared to the Fano noise. In the soft X-ray domain (i.e for photon energies < 1 keV), tunnel noise is the limiting factor for the energy resolution. The amplification in a QTT, on the other hand, depends on the down-conversion of quasiparticles at the superconducting gap of a superconducting metal to the Fermi level when they enter a normal metal. The charge created in the absorber layer tunnels through an asymmetric SIS' junction with a high tunnel probability in one direction only. This SIS' junction is in direct contact with a NIS junction, together forming the QTT. We investigate the prospects of applying QTT read-out to a strip detector, and its possible advantages over a design based on STJs. The goal is to improve on the energy resolution of STJs, while retaining the advantages of the STJ approach, such as its application in imaging detectors and single photon detection. Estimates are made of influences on the noise of the SIS part of the QTT, the down-conversion process, thermal quasiparticles and recombination and thermal phonons.