What's the Photodetector？

A photodetector is a device manual that functions to convert light into electricity in optical communication systems. This transformation primarily relies on the photovoltaic effect of semiconductor materials. The photovoltaic effect refers to the phenomenon where light exposure causes a potential difference between different parts of a non-uniform semiconductor or at the junctions between semiconductors and metals.

Additionally, there is the photoconductive effect, which occurs when electrons absorb photon energy under illumination, transitioning from a bound state to a free state, thereby changing the material's conductivity. When light is incident upon a photoconductive material, if it is an intrinsic semiconductor, and the photon energy is sufficiently strong, electrons in the valence band can be excited to the conduction band. This process increases the photoconductor's conductivity, caused by radiation altering the conductivity of the irradiated material. Photons interacting with the photoconductive material result in intrinsic or impurity absorption, generating additional photogenerated carriers, thus changing the semiconductor's conductivity and producing the photoconductive effect.

The Working Principle of a Photodetector

The working principle of a mid-infrared photodetector involves three fundamental processes: (1) generation of photogenerated carriers under illumination; (2) diffusion or drift of these carriers to form an electric current; (3) amplification of the photocurrent in a circuit, converting it into a voltage signal. When light illuminates the surface of the series detector, if the material's band gap is smaller than the energy of the incident photons (Eg < hv), electrons in the valence band can transition to the conduction band, creating a photocurrent.

As light travels through a semiconductor, its energy gradually decreases due to absorption by photons within the semiconductor. The primary type of photon absorption in semiconductors is intrinsic absorption, which includes direct and indirect transitions. By examining the intrinsic absorption spectrum of a semiconductor, one can not only determine the semiconductor's band gap but also distinguish between direct and indirect bandgap semiconductors. Intrinsic absorption typically results in high absorption coefficients due to the semiconductor's band structure, producing a continuous absorption spectrum. This spectrum reveals a distinct absorption edge at the onset of intrinsic absorption. However, for silicon, which is an indirect bandgap material, the probability of transitions is lower compared to group III-V materials, resulting in a much smaller absorption coefficient. Consequently, light penetrates deeper into silicon under the same photon energy. Direct bandgap materials exhibit a much steeper absorption edge than indirect bandgap materials. Graphs illustrating the relationship between the operating wavelength of incident light and optical absorption coefficients and penetration depths for several common semiconductor materials, such as InGaAs photodiode, biased Si detector, HgCdTe photodetector，highlight these differences.

Types of Photodetectors

● Photodiodes: These are semiconductor devices with a p-n or p-i-n structure (where 'i' stands for intrinsic layer). Light is absorbed in the depletion region, generating photocurrent. Despite their small size, these devices can exhibit frequency response times, high linearity, and very high quantum efficiency (meaning nearly every incident photon generates an electron) when paired with appropriate electronic equipment. A particularly sensitive type is the avalanche photodiode, which is sometimes capable of photon counting.

● Metal-Semiconductor-Metal Photodetectors: These include two Schottky contacts. They provide faster response than photodiode material, with bandwidths reaching several hundred GHz.

● Phototransistors: Similar to photodiodes, they utilize internal amplification of the photocurrent. However, they are less commonly used compared to photodiodes.

● Photoresistors: Employing specific semiconductor materials like cadmium sulfide (CdS), these devices are more affordable than photodiodes but have slower response speeds and are less sensitive, with a highly non-linear or saturated response.

● Photomultiplier Tubes: These vacuum tube devices offer extremely high sensitivity (even suitable for photon counting) and rapid response. However, they are expensive, bulky, and require max voltages.

● Pyroelectric Detectors: Using nonlinear crystals such as LiTaO₃, these detector element generate pyroelectric voltage pulses upon absorbing heat from light pulses at the absorbing coating. They are typically used to measure millijoule pulse energies in Q-switched lasers.

● Thermal Detectors (Power Meters): Measure temperature increases caused by absorbed light. These robust detectors can handle very high external power supply but possess lower sensitivity, moderate linear response range, and a relatively limited dynamic range.

Research is ongoing in the development of photodetectors using carbon nanotubes and graphene, which have the potential to provide broad wavelength ranges and rapid response speeds. Methods for integrating these devices into optoelectronic chips are also under exploration.

Important Characteristics of Photodetectors

Photodetectors must meet specific requirements based on practical needs:

● Sensitivity Across Wavelengths: Detectors need to exhibit high sensitivity within a given wavelength range. In some cases, it's essential that the sensitivity remains constant or at least stable within this range. Sometimes, detectors should have no response outside peak wavelengths; for example, solar-blind detectors are sensitive only in short UV ranges and not to sunlight.

● Operational Power Range: Detectors must function properly within a defined power range. The maximum power limit is often dictated by nonlinear response or potential damage, while the minimum is determined by noise. The dynamic range (expressed in decibels as the ratio between maximum and minimum detectable peak power) is crucial; some detectors maintain high linearity even beyond a 70 dB dynamic range.

● Responsivity and Quantum Efficiency: High responsivity and quantum efficiency are necessary to avoid additional quantum noise, which is critical for detecting squeezed light states and affects photon detection probability in single-photon detectors.

● Size of Active Area: When detecting strongly divergent beams from laser diodes, the active area size of the detector becomes important. For sources with large, variable laser beam divergence angles, an integrating sphere can be used to measure total light power.

● Detection Bandwidth: This may start from 0 Hz or a finite frequency, with peak frequencies determined by internal processes like carrier speed in semiconductor materials or associated electronics (such as RC time constants). Certain resonant detectors operate within narrow frequency ranges, making them suitable for lock-in detection.

● Suitability for Pulses vs. Continuous Light: Some detectors, like pyroelectric detectors, are better suited for pulse detection rather than continuous light. Timing precision is crucial when detecting pulses at single-photon levels, as some detectors experience "dead time" after pulse detection where sensitivity is reduced.

● Complexity of Required Electronics: Different detectors require varying levels of electronic complexity. High voltage applications or the detection of very small voltages can increase device size and cost.

● Special Requirements for Cooling: Some mid-infrared detectors need cooling to very low temperatures, limiting their applicability in many situations.

● Arrays of Photodetectors: One-dimensional or two-dimensional photodetector arrays introduce additional considerations, such as crosstalk and readout techniques.

● Device Size, Robustness, and Cost: Many applications impose constraints on these factors.

The diverse properties of various types of detector bandwidth mean that in traditional fields, fundamental requirements can eliminate certain detector types, narrowing the options. It's essential to balance different aspects during application to optimize performance and suitability.

Photonic Applications of Photodetectors

When we think of photodetectors, their essential roles in industrial production, scientific research, and energy management often come to mind. However, they also play significant yet subtle roles in our everyday lives, often unnoticed.

Mobile Phones

In smartphones, a proximity sensor composed of an infrared LED, photodiode, and comparator activates when you use your phone. When you bring the phone close to your face or put it in your pocket, the reduced light intensity is detected, prompting the screen to turn off, conserving battery life and preventing accidental touches. Additionally, as you move from a dim room to a bright outdoor setting, the light IC in your phone senses changes in ambient light and automatically adjusts the screen brightness. Moreover, every photo taken with your phone involves converting light into electrical signals via image sensors before data processing transforms these signals into cherished photographic memories.

Lighting

Street lamps that line city streets rely on photodetectors for proper functioning. Acting as the "eyes" for streetlights, light ICs monitor ambient light levels, signaling the lighting system to switch on at dusk and off at dawn. In smart lighting systems, whether in homes or public buildings, photodetectors automatically adjust light brightness based on environmental conditions, making lighting more intelligent and energy-efficient while reducing unnecessary energy consumption.

Automatic Doors

Photodetectors are integral to automatic door control systems found in malls, hotels, and office buildings. As you approach an automatic door, your body interrupts the light source, causing a change in the signal received by the light IC. This change is converted into an electrical signal that commands the door's motor to open, facilitating smooth entry and exit. The same principle applies to automatic faucets and hand dryers, enhancing user convenience while saving energy.

Fire Alarms

Common in kitchens and public places, fire and smoke detector NEP utilize photodetectors to detect ultraviolet or infrared light from flames, or changes in light scattering and absorption caused by smoke particles, to trigger alarms. These devices warn of potential fire hazards, protecting lives and property. For everyone's safety, it's important to practice fire and electrical safety and refrain from smoking in public areas.

Robotic Vacuum Cleaners

As smart appliances become prevalent, robotic vacuum cleaners are often people's first choice. Their key technologies, such as map building, obstacle avoidance, and stain recognition, depend heavily on photodetectors. APD and MPPC can be used for laser intensity, helping build cleaning maps and plan routes while identifying and avoiding obstacles. By detecting changes in reflected light, these robots assess stain severity and type, enabling targeted cleaning strategies. For models with mopping functions, photodiodes can monitor water tank levels, prompting users to refill and clean wastewater tanks as needed, ensuring continuous and efficient cleaning.

Beyond these examples, photodetectors are cleverly embedded in smart cars, fitness trackers, barcode scanners, and various other devices and facilities. These devices continuously sense light variations, translating them into meaningful signals that enhance our convenience, comfort, and safety. They exemplify the seamless integration of technology into daily life and the remarkable achievements of human ingenuity in practical light-tight applications. As technology advances, the performance and application scope of photodetectors will continue to expand and improve, promising even more exciting developments in our lives.