  At present, ultraviolet sensors are utilized in numerous fields ranging from
various spectroscopy applications via biotechnical innovations to industrial
process control. Despite of this, the performance of current UV sensors is
surprisingly poor. Here, we break the theoretical one photon - one electron
barrier and demonstrate a device with a certified external quantum efficiency
(EQE) above 130% in UV range without external amplification. The record high
performance is obtained using a nanostructured silicon photodiode with
self-induced junction. We show that the high efficiency is based on effective
utilization of multiple carrier generation by impact ionization taking place in
the nanostructures. While the results can readily have a significant impact on
the UV-sensor industry, the underlying technological concept can be applied to
other semiconductor materials, thereby extending above unity response to longer
wavelengths and offering new perspectives for improving efficiencies beyond the
Shockley-Queisser limit.
