  Avalanche photodiode (APD) has been intensively investigated as a promising
candidate to replace photomultiplier tubes (PMT) for weak light detection.
However, in conventional APDs, a large portion of carrier energy drawn from the
electric field is thermalized, and the multiplication efficiencies of electron
and hole are low and close. In order to achieve high gain, the device should
work under breakdown bias, where carrier multiplication proceeds
bi-directionally to form a positive feedback multiplication circle. However,
breakdown is hard to control, in practice, APDs should work under Geiger mode
as a compromise between sustainable detection and high gain. The complexity of
system seriously restricts the application. Here, we demonstrate an avalanche
photodiode holding high gain without breakdown, which means no quenching
circuit is needed for sustainable detection. The device is based on a GaN/AlN
periodically-stacked-structure (PSS), wherein electron holds much higher
efficiency than hole to draw energy from the electric field, and avalanche
happens uni-directionally with high efficiency. and a recorded high gain (10^4)
tested under constant bias is obtained in a prototype device, wherein the
stable gain can be determined by the periodicity of the GaN/AlN PSS. This work
not only brings a new light into avalanche multiplication mechanism, but also
paves a technological path with high commercial value to realize highly
sensitive avalanche devices working under constant bias like PMT.
