detectivity

Detectivity, often called specific detectivity and denoted D*, is a figure of merit used to characterize the sensitivity of photodetectors and imaging systems. It combines the detector's area, noise performance, and readout bandwidth to enable comparison between devices of different sizes and speed.

Formulas and interpretation: D* = sqrt(A Δf) / NEP, where A is the active area, Δf is the electrical bandwidth, and NEP is the noise-equivalent power. An equivalent expression is D* = sqrt(A) R / i_n, with R the responsivity (A/W) and i_n the noise current spectral density (A/√Hz). In either form, a higher D* indicates better sensitivity for a given wavelength and bandwidth. The commonly used unit is the Jones: cm·√Hz/W, though SI-oriented presentations may use m·√Hz/W.

Noise and context: NEP represents the input optical power needed to produce a signal equal to the detector’s noise within the bandwidth; it is measured in W/√Hz and reflects the dominant noise sources, such as shot noise, thermal noise, and flicker noise. D* is often reported for a specified wavelength range and operating temperature, since both responsivity and noise depend on wavelength and conditions.

Practical considerations: D* increases with lower noise and higher responsivity, and it scales with sqrt(A Δf). However, enlarging the active area or bandwidth can introduce more noise or require more complex readout, so the gain in D* is not unlimited. D* is wavelength dependent and is most meaningful when comparing devices under the same operating conditions.

Applications: D* is widely used to compare photodetectors, including photodiodes, bolometers, quantum well infrared photodetectors, and imaging sensors, particularly in infrared and visible regimes. It provides a compact summary of sensitivity but should be interpreted alongside spectral response and noise characteristics.