ABSTRACT We have demonstrated the operation of composite superconducting tungsten and aluminum transition‐edge sensors which take advantage of quasiparticle trapping and electrothermal feedback. We call these devices W/Al QETs (quasiparticle‐trap‐assisted electrothermal feedback transition‐edge sensors). The quasiparticle trapping mechanism makes it possible to instrument large surface areas without increasing sensor heat capacity, thus allowing larger absorbers and reducing phonon collection times. The sensor consists of a 30‐nm‐thick superconducting tungsten thin film with Tc∼80 mK deposited on a high‐purity silicon substrate. The W film is patterned into 200 parallel lines segments, each 2 μm wide and 800 μm long. Eight superconducting aluminum thin film pads are electrically connected to each segment, and cover a much larger surface area than the W. When phonons from particle interactions in the silicon crystal impinge on an aluminum pad, Cooper pairs are broken, forming quasiparticles which diffuse to the tungsten lines where they are rapidly thermalized. The W film is voltage biased, and self‐regulates in temperature within its superconducting transition region by electrothermal feedback. Heat deposited in the film causes a current pulse of ∼100 μs duration, which is measured with a series array of dc superconducting quantum interference devices. We have demonstrated an energy resolution of half‐maximum for 6 keV x rays incident on the backside of a 1 cm×1 cm×1 mm (0.25 g) silicon absorber, the highest resolution that has been reported for a fast (duration) calorimetric detector with an absorber mass≳0.1 g. Applications of this technology include dark matter searches and neutrino detection.