Photodetectors fabricated from low-dimensional materials such as quantum dots, nanowires, and two-dimensional materials show tremendous promise based on reports of very high responsivities. However, it is not generally appreciated that maximizing the internal gain may compromise the detector performance at low light levels, reducing its sensitivity. Here, we show that for most low-dimensional photodetectors with internal gain the sensitivity is determined by the junction capacitance. Thanks to their extremely small junction capacitances and reduced charge screening, low-dimensional materials and devices provide clear advantages over bulk semiconductors in the pursuit of high-sensitivity photodetectors. This mini-review describes and validates a method to estimate the capacitance from external photoresponse measurements, providing a straightforward approach to extract the device sensitivity and benchmark against physical limits. This improved physical understanding can guide the design of low-dimensional photodetectors to effectively leverage their unique advantage and achieve sensitivities that can exceed that of the best existing photodetectors.
Keywords: Capacitance; Low-dimensional materials; Photodetectors; Quantum efficiency; Responsivity; Sensitivity.