Nanohole-assisted SiGe/Si phototransistor with high absorption efficiency

Silicon-based heterojunction phototransistors (HPTs) demonstrate advantages of high internal gain and compatibility with CMOS technology. A novel, to the best of our knowledge, nanohole-assisted SiGe/Si HPT was designed and fabricated to achieve high absorption and alleviate the contradiction between responsivity and speed of detectors. A 2D nanohole array with a 1000 nm period to support a lateral transmission mode was embedded into the absorption region through the optical window located on the base mesa. The diameter and depth of the nanohole were optimized to reach a higher optical absorption efficiency of 0.80. The nanohole-assisted SiGe/Si HPT was fabricated using a BiCMOS-compatible mesa process. Under 850 nm incident light, the novel device with a window radius of 20 µm exhibited a responsivity of 13.02 A/W and a specific detectivity of 2.56 × 10¹² cm·Hz^0.5/W, representing an enhancement of 525.9% and 498.1% compared with the standard SiGe/Si HPT with the same structure but without a nanohole array. The nanohole-assisted Si-HPT demonstrates highly promising potential for applications in low-light detection.