  InGaAs/InP single-photon detectors (SPDs) are the key devices for
applications requiring near-infrared single-photon detection. Gating mode is an
effective approach to synchronous single-photon detection. Increasing gating
frequency and reducing module size are important challenges for the design of
such detector system. Here we present for the first time an InGaAs/InP SPD with
1.25 GHz sine wave gating using a monolithically integrated readout circuit
(MIRC). The MIRC has a size of 15 mm * 15 mm and implements the miniaturization
of avalanche extraction for high-frequency sine wave gating. In the MIRC,
low-pass filters and a low-noise radio frequency amplifier are integrated based
on the technique of low temperature co-fired ceramic, which can effectively
reduce the parasitic capacitance and extract weak avalanche signals. We then
characterize the InGaAs/InP SPD to verify the functionality and reliability of
MIRC, and the SPD exhibits excellent performance with 27.5 % photon detection
efficiency, 1.2 kcps dark count rate, and 9.1 % afterpulse probability at 223 K
and 100 ns hold-off time. With this MIRC, one can further design miniaturized
high-frequency SPD modules that are highly required for practical applications.
