  We propose a novel design of superconducting nanowire avalanche
photodetectors (SNAPs), which combines the advantages of multi-stage avalanche
SNAPs to lower the avalanche current I_AV and that of series-SNAPs to reduce
the reset time. As proof of principle, we fabricated 800 devices with large
detection area (15 um * 15 um) and five different designs on a single silicon
chip for comparison, which include standard SNSPDs, series-3-SNAPs and our
modified series-SNAPs with double-stage avalanche structure 2*2-SNAPs,
2*3-SNAPs, and 3*3-SNAPs. The former three types of the detectors demonstrate
fully-saturated device detection efficiencies of ~20% while the latter two
types are latching at larger bias currents. In addition, the I_AV of 2*2-SNAPs
is only 64% of the switching current I_SW that is lower than series-3-SNAPs
(74%) and well below that of 4-SNAPs (84%) reported elsewhere. We also measure
that the exponential decay times of the detectors are proportional to (1/n)^2
due to the lack of external choke inductors. In particular, the decay time of
3*3-SNAPs is only 0.89 ns compared to the standard SNSPDs' 63.2 ns, showing the
potential to attain GHz counting rates.
