Quantum dot photodetectors represent a significant advancement in optoelectronic technology due to their tunable bandgap, high absorption coefficients, and compatibility with various device architectures. These detectors are categorized into three primary configurations: photoconductive, photovoltaic, and phototransistor-based, each offering distinct advantages for specific applications. The spectral selectivity, responsivity, and noise characteristics of quantum dot photodetectors are critical parameters that determine their performance, while hybrid systems and infrared detection capabilities further expand their utility.

Photoconductive quantum dot detectors operate on the principle of photoconductivity, where incident light generates electron-hole pairs, increasing the conductivity of the active layer. The photoconductive gain in these devices arises from the prolonged carrier lifetime due to charge trapping, enabling high responsivity. For instance, PbS quantum dot photoconductors have demonstrated responsivities exceeding 10^7 A/W under optimized conditions. However, this high gain often comes at the cost of increased noise, primarily due to generation-recombination and dark current contributions. Noise reduction techniques include surface passivation to minimize trap states and the use of heterostructures to suppress dark current. Spectral selectivity is achieved by tuning the quantum dot size, allowing detection from visible to short-wave infrared regions.

Photovoltaic quantum dot detectors, typically structured as p-n or p-i-n junctions, rely on the built-in electric field to separate photogenerated carriers. These devices exhibit lower noise compared to photoconductive detectors due to reduced persistent photoconductivity effects. Colloidal quantum dots, such as those made from HgTe, have been integrated into photovoltaic configurations for mid-wave and long-wave infrared detection, achieving detectivities above 10^11 Jones at room temperature. The spectral response is determined by the quantum dot bandgap and can be extended through ligand engineering or hybridization with other materials. Charge extraction efficiency is a key factor in photovoltaic detectors, and interfacial engineering using materials like ZnO or TiO2 has been shown to improve performance.

Phototransistor-based quantum dot detectors leverage the gate-tunable conductivity of field-effect transistors to achieve high gain and low noise. In these devices, quantum dots are integrated into the channel or as a floating gate, modulating the transistor output in response to light. PbSe quantum dot phototransistors have exhibited responsivities of 10^5 A/W with fast response times, making them suitable for high-speed applications. The spectral response can be tailored by selecting quantum dots with appropriate bandgaps, while noise is mitigated through optimized gate biasing and dielectric engineering. Hybrid configurations, such as combining quantum dots with graphene, exploit the high carrier mobility of graphene to enhance photoresponse speed and sensitivity.

Spectral selectivity is a defining feature of quantum dot photodetectors, enabled by quantum confinement effects. By adjusting the quantum dot size, the absorption edge can be precisely tuned across a broad spectral range. For example, CdSe quantum dots with diameters of 2-6 nm cover the visible spectrum, while PbS quantum dots can be tuned from 800 nm to over 2 µm. Multispectral detection is achieved by stacking quantum dot layers with different sizes or by employing energy-filtering contacts. Narrowband detection, essential for applications like spectral imaging, is realized through resonant structures or charge collection narrowing techniques.

Responsivity, a measure of the detector's sensitivity to light, depends on the quantum efficiency and gain mechanism. Photoconductive detectors exhibit high responsivity due to internal gain, whereas photovoltaic detectors prioritize high quantum efficiency. Noise equivalent power (NEP) and specific detectivity (D*) are critical metrics for comparing detector performance. For instance, InAs quantum dot photodetectors in the short-wave infrared range have reported D* values exceeding 10^12 Jones at 77 K. Noise reduction strategies include cooling, surface passivation, and the use of low-noise readout circuits.

Hybrid quantum dot systems enhance detector performance by combining the strengths of multiple materials. Quantum dot-graphene hybrid photodetectors leverage graphene's high mobility and broadband absorption alongside the quantum dots' tunable bandgap. These devices achieve high responsivity and fast response times, with reported values of 10^8 A/W and sub-millisecond recovery times. Similarly, quantum dot-perovskite hybrids exploit the perovskite's excellent charge transport properties to improve extraction efficiency. Hybrid systems are particularly promising for infrared detection, where traditional semiconductors face limitations.

Infrared detection with quantum dots is a rapidly advancing field, driven by applications in night vision, medical imaging, and telecommunications. HgTe quantum dots, for example, cover the mid-wave to long-wave infrared spectrum (3-12 µm), with detectivities competitive with bulk semiconductors. Noise in infrared quantum dot detectors is dominated by thermal contributions, necessitating cooling or advanced filtering techniques. Recent developments in type-II quantum dots, such as CdSe/CdTe core/shell structures, have enabled efficient long-wave infrared detection at higher operating temperatures.

In summary, quantum dot photodetectors offer versatile and high-performance solutions for a wide range of applications. Photoconductive, photovoltaic, and phototransistor configurations each provide unique benefits, while spectral tunability and hybrid integration further enhance their capabilities. Advances in noise reduction and infrared detection continue to push the boundaries of what is achievable, positioning quantum dot photodetectors as a key technology in modern optoelectronics.