Characterization of very large silicon avalanche photodiodes

In this paper, we discuss recent advances in the high gain, very large area silicon avalanche photodiode technology. Very large area APDs (square and octagonal design, up to 45 cm² area) have been built using the planar process. These APDs have been successfully operated as scintillation spectrometers. When cooled to -40 °C, the energy resolution of 5.9 keV X-rays (⁵⁵Fe source) upon direct interaction with the 45 cm² APD was measured to be 2.4 keV (FWHM). The APD was coupled to a CsI(Tl) scintillator and irradiated with 662 keV gamma rays from ¹³⁷Cs source and the energy resolution was measured to be ∼10% (FWHM) at the temperature of -40 °C. A rise time of < 2 ns was measured with the 40 cm² APD upon irradiation with 5.5 MeV alpha particles. A new packaging design has been developed to allow cooling of these large APDs to liquid nitrogen temperature. Evaluation of the 45 cm² APD has been conducted at LN₂ temperatures. Its gain was found to be as high as 10⁴, while the noise was measured to be ∼0.8 electrons (rms). Detection of low intensity optical signal (<8 photons/pulse) with the 45 cm² APD has also been accomplished.