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Photodiode
Photodiode
Photodiode
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Photodiode
One Ge (top) and three Si (bottom) photodiodes
Component typePassive, diode
Working principle‍Converts light into current
Pin namesanode and cathode
Electronic symbol

A photodiode is a semiconductor diode sensitive to photon radiation, such as visible light, infrared or ultraviolet radiation, X-rays and gamma rays.[1] It produces an electrical current when it absorbs photons. This can be used for detection and measurement applications, or for the generation of electrical power in solar cells. Photodiodes are used in a wide range of applications throughout the electromagnetic spectrum from visible light photocells to gamma ray spectrometers.

Principle of operation

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A photodiode is a PIN structure or p–n junction. When a photon of sufficient energy strikes the diode, it creates an electron–hole pair. This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in electric field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. The total current through the photodiode is the sum of the dark current (current that is passed in the absence of light) and the photocurrent, so the dark current must be minimized to maximize the sensitivity of the device.[2] Therefore, photodiodes operate most ideally in reverse bias.

To first order, for a given spectral distribution, the photocurrent is linearly proportional to the irradiance.[3]

Photovoltaic mode

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I-V characteristic of a photodiode. The linear load lines represent the response of the external circuit: I=(Applied bias voltage-Diode voltage)/Total resistance. The points of intersection with the curves represent the actual current and voltage for a given bias, resistance and illumination.

In photovoltaic mode (zero bias), photocurrent flows into the anode through a short circuit to the cathode. If the circuit is opened or has a load impedance, restricting the photocurrent out of the device, a voltage builds up in the direction that forward biases the diode, that is, anode positive with respect to cathode. If the circuit is shorted or the impedance is low, a forward current will consume all or some of the photocurrent. This mode exploits the photovoltaic effect, which is the basis for solar cells – a traditional solar cell is just a large area photodiode. For optimum power output, the photovoltaic cell will be operated at a voltage that causes only a small forward current compared to the photocurrent.[3]

Photoconductive mode

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In photoconductive mode the diode is reverse biased, that is, with the cathode driven positive with respect to the anode. This reduces the response time because the additional reverse bias increases the width of the depletion layer, which decreases the junction's capacitance and increases the region with an electric field that will cause electrons to be quickly collected. The reverse bias also creates dark current without much change in the photocurrent.

Although this mode is faster, the photoconductive mode can exhibit more electronic noise due to dark current or avalanche effects.[4] The leakage current of a good PIN diode is so low (<1 nA) that the Johnson–Nyquist noise of the load resistance in a typical circuit often dominates.

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Avalanche photodiodes are photodiodes with structure optimized for operating with high reverse bias, approaching the reverse breakdown voltage. This allows each photo-generated carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device.[5]

Electronic symbol for a phototransistor
S604B PhotoTransistor

A phototransistor is a light-sensitive transistor. A common type of phototransistor, the bipolar phototransistor, is in essence a bipolar transistor encased in a transparent case so that light can reach the base–collector junction. It was invented by John N. Shive at Bell Labs in 1948[6]: 205  but it was not announced until 1950.[7] The electrons that are generated by photons in the base–collector junction are injected into the base, and this photodiode current is amplified by the transistor's current gain β (or hfe). If the base and collector leads are used and the emitter is left unconnected, the phototransistor becomes a photodiode. While phototransistors have a higher responsivity for light they are not able to detect low levels of light any better than photodiodes.[citation needed] Phototransistors also have significantly longer response times. Another type of phototransistor, the field-effect phototransistor (also known as photoFET), is a light-sensitive field-effect transistor. Unlike photobipolar transistors, photoFETs control drain-source current by creating a gate voltage.

A solaristor is a two-terminal gate-less phototransistor. A compact class of such solaristors was demonstrated in 2018 by ICN2 researchers. The novel concept is a two-in-one power source plus transistor device that runs on solar energy by exploiting a memresistive effect in the flow of photogenerated carriers.[8]

Materials

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The material used to make a photodiode is critical to defining its properties, because only photons with sufficient energy to excite electrons across the material's bandgap will produce significant photocurrents.

Materials commonly used to produce photodiodes are listed in the table below.[9]

Material Electromagnetic spectrum
wavelength range (nm)
Silicon 190–1100
Germanium 400–1700
Indium gallium arsenide 800–2600
Lead(II) sulfide <1000–3500
Mercury cadmium telluride 400–14000

Because of their greater bandgap, silicon-based photodiodes generate less noise than germanium-based photodiodes.

Binary materials, such as MoS2, and graphene emerged as new materials for the production of photodiodes.[10]

Unwanted and wanted photodiode effects

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Any p–n junction, if illuminated, is potentially a photodiode. Semiconductor devices such as diodes, transistors and ICs contain p–n junctions, and will not function correctly if they are illuminated by unwanted light.[11][12] This is avoided by encapsulating devices in opaque housings. If these housings are not completely opaque to high-energy radiation (ultraviolet, X-rays, gamma rays), diodes, transistors and ICs can malfunction[13] due to induced photo-currents. Background radiation from the packaging is also significant.[14] Radiation hardening mitigates these effects.

In some cases, the effect is actually wanted, for example to use LEDs as light-sensitive devices (see LED as light sensor) or even for energy harvesting, then sometimes called light-emitting and light-absorbing diodes (LEADs).[15]

Features

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Response of a silicon photo diode vs wavelength of the incident light

Critical performance parameters of a photodiode include spectral responsivity, dark current, response time and noise-equivalent power.

Spectral responsivity
The spectral responsivity is a ratio of the generated photocurrent to incident light power, expressed in A/W when used in photoconductive mode. The wavelength-dependence may also be expressed as a quantum efficiency or the ratio of the number of photogenerated carriers to incident photons which is a unitless quantity.
Dark current
The dark current is the current through the photodiode in the absence of light, when it is operated in photoconductive mode. The dark current includes photocurrent generated by background radiation and the saturation current of the semiconductor junction. Dark current must be accounted for by calibration if a photodiode is used to make an accurate optical power measurement, and it is also a source of noise when a photodiode is used in an optical communication system.
Response time
The response time is the time required for the detector to respond to an optical input. A photon absorbed by the semiconducting material will generate an electron–hole pair which will in turn start moving in the material under the effect of the electric field and thus generate a current. The finite duration of this current is known as the transit-time spread and can be evaluated by using Ramo's theorem. One can also show with this theorem that the total charge generated in the external circuit is e and not 2e as one might expect by the presence of the two carriers. Indeed, the integral of the current due to both electron and hole over time must be equal to e. The resistance and capacitance of the photodiode and the external circuitry give rise to another response time known as RC time constant (). This combination of R and C integrates the photoresponse over time and thus lengthens the impulse response of the photodiode. When used in an optical communication system, the response time determines the bandwidth available for signal modulation and thus data transmission.
Noise-equivalent power
Noise-equivalent power (NEP) is the minimum input optical power to generate photocurrent, equal to the rms noise current in a 1 hertz bandwidth. NEP is essentially the minimum detectable power. The related characteristic detectivity () is the inverse of NEP (1/NEP) and the specific detectivity () is the detectivity multiplied by the square root of the area () of the photodetector () for a 1 Hz bandwidth. The specific detectivity allows different systems to be compared independent of sensor area and system bandwidth; a higher detectivity value indicates a low-noise device or system.[16] Although it is traditional to give () in many catalogues as a measure of the diode's quality, in practice, it is hardly ever the key parameter.

When a photodiode is used in an optical communication system, all these parameters contribute to the sensitivity of the optical receiver which is the minimum input power required for the receiver to achieve a specified bit error rate.

Applications

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P–n photodiodes are used in similar applications to other photodetectors, such as photoconductors, charge-coupled devices (CCD), and photomultiplier tubes. They may be used to generate an output which is dependent upon the illumination (analog for measurement), or to change the state of circuitry (digital, either for control and switching or for digital signal processing).

Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, medical devices[17] and the receivers for infrared remote control devices used to control equipment from televisions to air conditioners. For many applications either photodiodes or photoconductors may be used. Either type of photosensor may be used for light measurement, as in camera light meters, or to respond to light levels, as in switching on street lighting after dark.

Photosensors of all types may be used to respond to incident light or to a source of light which is part of the same circuit or system. A photodiode is often combined into a single component with an emitter of light, usually a light-emitting diode (LED), either to detect the presence of a mechanical obstruction to the beam (slotted optical switch) or to couple two digital or analog circuits while maintaining extremely high electrical isolation between them, often for safety (optocoupler). The combination of LED and photodiode is also used in many sensor systems to characterize different types of products based on their optical absorbance.

Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a more linear response than photoconductors.

They are also widely used in various medical applications, such as detectors for computed tomography (coupled with scintillators), instruments to analyze samples (immunoassay), and pulse oximeters.

PIN diodes are much faster and more sensitive than p–n junction diodes, and hence are often used for optical communications and in lighting regulation.

P–n photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, avalanche photodiodes, intensified charge-coupled devices or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night vision equipment and laser rangefinding.

Comparison with photomultipliers

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Advantages compared to photomultipliers:[18]

  1. Excellent linearity of output current as a function of incident light
  2. Spectral response from 190 nm to 1100 nm (silicon), longer wavelengths with other semiconductor materials
  3. Low noise
  4. Ruggedized to mechanical stress
  5. Low cost
  6. Compact and light weight
  7. Long lifetime
  8. High quantum efficiency, typically 60–80%[19]
  9. No high voltage required

Disadvantages compared to photomultipliers:

  1. Small area
  2. No internal gain (except avalanche photodiodes, but their gain is typically 102–103 compared to 105-108 for the photomultiplier)
  3. Much lower overall sensitivity
  4. Photon counting only possible with specially designed, usually cooled photodiodes, with special electronic circuits
  5. Response time for many designs is slower
  6. Latent effect

Pinned photodiode

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Not to be confused with PIN photodiode.

The pinned photodiode (PPD) has a shallow implant (P+ or N+) in N-type or P-type diffusion layer, respectively, over a P-type or N-type (respectively) substrate layer, such that the intermediate diffusion layer can be fully depleted of majority carriers, like the base region of a bipolar junction transistor. The PPD (usually PNP) is used in CMOS active-pixel sensors; a precursor NPNP triple junction variant with the MOS buffer capacitor and the back-light illumination scheme with complete charge transfer and no image lag was invented by Sony in 1975. This scheme was widely used in many applications of charge transfer devices.

Early charge-coupled device image sensors suffered from shutter lag. This was largely explained with the re-invention of the pinned photodiode.[20] It was developed by Nobukazu Teranishi, Hiromitsu Shiraki and Yasuo Ishihara at NEC in 1980.[20][21] Sony in 1975 recognized that lag can be eliminated if the signal carriers could be transferred from the photodiode to the CCD. This led to their invention of the pinned photodiode, a photodetector structure with low lag, low noise, high quantum efficiency and low dark current.[20] It was first publicly reported by Teranishi and Ishihara with A. Kohono, E. Oda and K. Arai in 1982, with the addition of an anti-blooming structure.[20][22] The new photodetector structure invented by Sony in 1975, developed by NEC in 1982 by Kodak in 1984 was given the name "pinned photodiode" (PPD) by B.C. Burkey at Kodak in 1984. In 1987, the PPD began to be incorporated into most CCD sensors, becoming a fixture in consumer electronic video cameras and then digital still cameras.[20]

A CMOS image sensor with a low-voltage-PPD technology was first fabricated in 1995 by a joint JPL and Kodak team. The CMOS sensor with PPD technology was further advanced and refined by R.M. Guidash in 1997, K. Yonemoto and H. Sumi in 2000, and I. Inoue in 2003. This led to CMOS sensors achieve imaging performance on par with CCD sensors, and later exceeding CCD sensors.

Photodiode array

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A one-dimensional photodiode array chip with more than 200 diodes in the line across the center
A two-dimensional photodiode array of only 4 × 4 pixels occupies the left side of the first optical mouse sensor chip, c. 1982.

A one-dimensional array of hundreds or thousands of photodiodes can be used as a position sensor, for example as part of an angle sensor.[23] A two-dimensional array is used in image sensors and optical mice.

In some applications, photodiode arrays allow for high-speed parallel readout, as opposed to integrating scanning electronics as in a charge-coupled device (CCD) or CMOS sensor. The optical mouse chip shown in the photo has parallel (not multiplexed) access to all 16 photodiodes in its 4 × 4 array.

Passive-pixel image sensor

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The passive-pixel sensor (PPS) is a type of photodiode array. It was the precursor to the active-pixel sensor (APS).[20] A passive-pixel sensor consists of passive pixels which are read out without amplification, with each pixel consisting of a photodiode and a MOSFET switch.[24] In a photodiode array, pixels contain a p–n junction, integrated capacitor, and MOSFETs as selection transistors. A photodiode array was proposed by G. Weckler in 1968, predating the CCD.[25] This was the basis for the PPS.[20]

The noise of photodiode arrays is sometimes a limitation to performance. It was not possible to fabricate active pixel sensors with a practical pixel size in the 1970s, due to limited microlithography technology at the time.[25]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Photodiode

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from Grokipedia
A photodiode is a consisting of a p-n junction that converts incident light into electrical current by generating electron-hole pairs through absorption in the . This process relies on the inner , where with energy greater than the semiconductor's bandgap excite electrons from the valence band to the conduction band, producing a proportional to the light intensity. Photodiodes operate in two primary modes: photovoltaic mode, which functions without external bias and generates a voltage across the junction similar to a , and photoconductive mode, where reverse bias is applied to increase sensitivity, speed, and linearity by widening the and reducing capacitance. Key characteristics include high quantum efficiency (often exceeding 80% in devices), low noise, fast response times (down to picoseconds), and spectral sensitivity depending on the material— for visible and near-infrared (400–1100 nm), while materials like InGaAs extend to longer wavelengths. Common types include PN photodiodes for basic detection, PIN photodiodes with an intrinsic layer for reduced capacitance and higher bandwidth, and avalanche photodiodes (APDs) that amplify the signal through internal gain via impact ionization under high reverse bias, enabling detection of weak signals in low-light applications. Photodiodes find widespread use in optical communication systems for fiber-optic receivers, light measurement in spectrophotometers, imaging arrays in cameras, and environmental sensing for smoke detectors and barcode scanners, owing to their compact size, reliability, and cost-effectiveness.

Fundamentals

Definition and Basic Operation

A photodiode is a p-n junction that converts incident light into electrical current by generating charge carriers through absorption, with the resulting being proportional to the light intensity. The core structure consists of a p-type region doped with acceptors, an n-type region doped with donors, forming the p-n junction, along with a at the interface where mobile charges are scarce, and terminals designated as the (connected to the p-side) and (connected to the n-side). In its basic operation, light photons with energy greater than the semiconductor's bandgap are absorbed, primarily in or near the , exciting electrons from the valence band to the conduction band and creating electron-hole pairs. The built-in across the separates these carriers, with electrons drifting toward the n-side and holes toward the p-side, producing a measurable in an external circuit. This process relies on the inherent to the p-n junction, enabling the device to function as an optical detector without external amplification in simple configurations. Under zero , where no external voltage is applied across the terminals, the photodiode's current-voltage (I-V) characteristic shifts due to illumination, generating an proportional to the logarithm of the light intensity as the flows through the device's . This voltage buildup occurs because the generated is restricted by the forward-biased junction, creating a potential difference that can power low-current loads directly. A key performance metric is the quantum efficiency (η), defined as the ratio of the number of charge carriers collected at the electrodes to the number of incident photons: η=number of charge carriers collectednumber of incident photons\eta = \frac{\text{number of charge carriers collected}}{\text{number of incident photons}} This parameter quantifies the device's efficiency in converting photons to electrical signal, typically expressed as a percentage, and depends on factors like absorption coefficient and carrier collection probability within the active region.

Historical Development

The , foundational to photodiode operation, was first observed in 1839 by French physicist , who noted that certain materials exposed to light in an electrolytic solution generated a voltage. This discovery laid the groundwork for light-sensitive devices, though practical applications remained elusive for decades. In 1873, English engineer Willoughby Smith reported the of , demonstrating that the material's electrical resistance decreased under illumination, enabling the creation of early selenium-based photodetectors used in and light measurement. The modern era of photodiodes began in the mid-20th century with advancements in semiconductor technology at Bell Laboratories. In 1941, Russell Ohl accidentally discovered a p-n junction in a crystal that produced a photovoltaic response to , patenting the concept and paving the way for the first practical p-n junction photodiodes by the early 1950s. This breakthrough shifted focus from brittle cells to more robust devices, improving sensitivity and reliability for applications like and optical sensing. Concurrently, Japanese researcher Jun-ichi Nishizawa invented the structure in 1950 and extended it to the PIN photodiode in 1952, introducing an intrinsic layer between p- and n-regions to enhance absorption and reduce capacitance. Key milestones in the 1960s and 1970s advanced photodiode performance for specialized uses. PIN photodiodes gained prominence in the 1960s for , supporting early fiber-optic systems with their low noise and high-speed response. Avalanche photodiodes, also pioneered by Nishizawa in 1952, saw practical development in the 1970s for low-light detection, leveraging internal gain mechanisms to amplify signals in applications like and scientific instrumentation. In 1975, Sony's Yoshiaki Hagiwara invented the pinned photodiode, which facilitated integration into charge-coupled devices (CCDs) and later CMOS sensors in the , revolutionizing imaging in such as cameras. Entering the , photodiodes evolved toward advanced materials for broader spectral coverage and efficiency. Post-2000 developments emphasized III-V compound semiconductors like InGaAs for infrared detection, enabling high-performance devices in telecom and sensing. Recent advances up to 2025 include perovskite-based photodiodes, with significant progress since the 2010s yielding fast, stable detectors for imaging and through solution-processable fabrication. Similarly, two-dimensional materials such as and transition metal dichalcogenides have driven innovations in flexible, high-efficiency photodiodes, addressing limitations in traditional for wearable and applications. These shifts have transformed photodiodes from discrete components into integral parts of integrated circuits, powering modern and optical systems.

Operating Principles

Photovoltaic Mode

In photovoltaic mode, a photodiode operates without any external bias voltage, relying on the built-in potential difference across the p-n junction to separate photogenerated charge carriers. When photons with energy greater than the bandgap are absorbed, they create electron-hole pairs primarily in or near the . These carriers are separated by the internal : electrons drift toward the n-side and holes toward the p-side, generating a that can produce a measurable voltage across the device terminals. This mode leverages the , similar to that in solar cells, but is tailored for light detection rather than efficient power conversion. The carrier dynamics in this mode involve both and drift processes. Generated electron-hole pairs in the neutral regions diffuse randomly until reaching the , where the strong built-in field sweeps them apart efficiently, minimizing recombination. Under short-circuit conditions (zero voltage across the device), the resulting current flows freely, while in open-circuit conditions, carrier accumulation builds up a voltage opposing further separation. The short-circuit current IscI_{sc} is expressed as Isc=qηPAhν,I_{sc} = q \eta \frac{P A}{h \nu}, where qq is the , η\eta is the quantum efficiency, PP is the incident , AA is the active area, and hνh \nu is the . The open-circuit voltage VocV_{oc} is approximated by the diode equation Voc≈kTqln⁡(IscI0+1),V_{oc} \approx \frac{kT}{q} \ln \left( \frac{I_{sc}}{I_0} + 1 \right), where kk is Boltzmann's constant, TT is the absolute temperature, and I0I_0 is the dark saturation current. This operating mode offers distinct advantages, including very low due to negligible dark current and the ability to function in a self-powered manner without external circuitry. However, it has limitations such as slower response times compared to biased modes, as the absence of an external field reduces carrier collection and increases transit times. Photodiodes in photovoltaic mode are optimized for precise detection with linear response to intensity, differing from solar cells which prioritize maximizing power output through larger areas and specific choices.

Photoconductive Mode

In photoconductive mode, a reverse voltage is applied across the photodiode, widening the compared to zero-bias operation and thereby improving the separation and collection efficiency of photogenerated electron-hole pairs while reducing junction capacitance. This configuration causes the device to function as a light-dependent , where the generated current varies directly with the intensity of incident light, enabling precise measurement of . The photocurrent in this mode is expressed as Iph=R⋅PI_{ph} = R \cdot P, where RR is the (typically in A/W) and PP is the incident ; the total current is then I=Iph+IdarkI = I_{ph} + I_{dark}, with the dark current IdarkI_{dark} increasing under the applied reverse bias voltage VrV_r. The 3 dB bandwidth, which determines the , is influenced by the junction CjC_j and is commonly limited by the , approximated as f3dB=12πRLCjf_{3dB} = \frac{1}{2\pi R_L C_j}, where RLR_L is the load resistance; higher reverse bias reduces CjC_j, extending the bandwidth for faster operation. Linearity in photoconductive mode is a key feature, with the output current maintaining a proportional relationship to light intensity across several orders of magnitude until saturation occurs, and the applied voltage enhances this by minimizing carrier recombination and diffusion effects that could introduce nonlinearity. This mode offers advantages such as superior speed and reduced relative to unbiased operation, rendering it ideal for high-frequency applications like fiber-optic communications and ranging systems.

Materials and Fabrication

Semiconductor Materials

Silicon is the most widely used semiconductor material for photodiodes operating in the visible and near-infrared (NIR) spectrum, with a bandgap energy of 1.12 eV that enables efficient absorption of photons up to approximately 1100 nm. Germanium, featuring a narrower bandgap of 0.67 eV, extends sensitivity into the infrared region up to about 1700 nm, making it suitable for mid-IR detection. Gallium arsenide (GaAs) photodiodes, with a bandgap of 1.43 eV, target NIR applications around 870 nm, offering higher electron mobility compared to silicon for faster response times. Indium gallium arsenide (InGaAs), tunable with a bandgap around 0.75 eV, provides extended NIR coverage up to 1.7 μm, ideal for telecommunications wavelengths. Key material properties influencing photodiode performance include the wavelength-dependent absorption coefficient α(λ), which quantifies how strongly light is absorbed; the , given by 1/α, determines the optimal placement of the p-n junction to maximize carrier collection efficiency. For instance, exhibits α values on the order of 10^4 cm⁻¹ at 800 nm, leading to shallow penetration depths of about 1 μm, while germanium's lower α in the IR necessitates thicker absorption layers. , measuring charge transport speed, and minority , affecting recombination rates, directly impact response time; high-mobility materials like GaAs (electron mobility ~8500 cm²/V·s) enable bandwidths exceeding 10 GHz. Material selection for photodiodes hinges on the target wavelength range from to , with dominating UV-visible applications due to its broad absorption and stability up to 150°C. stability is critical, as bandgap energies decrease with rising , shifting absorption edges; III-V compounds like InGaAs maintain performance better in harsh environments than . Cost-performance trade-offs favor for low-cost, high-volume visible detectors, while III-V materials such as GaAs and InGaAs are preferred for high-speed telecom despite higher fabrication expenses. Emerging materials as of 2025 include halide perovskites, which offer broadband absorption and high quantum efficiencies, often approaching 90% in optimized visible-NIR devices, though stability issues under humidity and heat limit commercial adoption. Two-dimensional materials like enable ultrafast detection with response times on the order of picoseconds to nanoseconds, benefiting from high carrier mobilities exceeding 10,000 cm²/V·s at , but challenges in bandgap engineering and integration persist. Doping levels in these materials form the p-n essential for carrier separation; in , p-type doping typically uses at concentrations of 10^{15}-10^{18} cm^{-3} to create acceptor sites, while n-type doping employs at similar levels to provide donor electrons.
MaterialBandgap (eV)Wavelength Range (nm)Key Application
(Si)1.12300-1100Visible-NIR detection
Germanium (Ge)0.67400-1700Mid-IR sensing
(GaAs)1.43200-870High-speed NIR
(InGaAs)~0.75900-1700Telecom wavelengths

Fabrication Techniques

The fabrication of photodiodes begins with wafer preparation, where high-purity substrates are produced to ensure minimal defects and uniform electrical properties. For silicon-based photodiodes, the Czochralski process is widely employed to grow single-crystal ingots from a molten source, followed by slicing into wafers that serve as the foundation for device layers. Doping of these wafers to create n-type or p-type regions is achieved through , where dopant atoms like or are introduced via thermal processes, or , which accelerates dopant ions into the lattice for precise control over concentration profiles. These steps are critical for establishing the base conductivity and junction characteristics essential to photodiode functionality. Junction formation follows wafer preparation, particularly for PIN structures that require an intrinsic region to minimize and enhance speed. Epitaxial growth techniques such as metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) are used to deposit the intrinsic layer with atomic-level precision on the doped substrate, enabling low-defect interfaces and tailored bandgap properties. For instance, MOCVD facilitates uniform deposition of III-V materials like InGaAs for near-infrared detection, while MBE offers superior control for lattice-matched heterostructures in advanced devices. These methods ensure the p-i-n junction's abrupt doping transitions, which are vital for efficient carrier collection. Metallization and passivation steps protect the device and optimize optical coupling. Ohmic contacts, typically formed using titanium-aluminum (Ti/Al) stacks for n-type regions, are evaporated or sputtered onto the surface followed by annealing to achieve low-resistance interfaces without rectification. Passivation layers, such as (SiO₂), are then deposited via to encapsulate the junction and prevent surface recombination, while anti-reflective coatings like SiO₂ or multilayer stacks reduce reflection losses at the light-entry surface, improving quantum efficiency by up to 20-30% in photodiodes. Packaging completes the fabrication by ensuring environmental stability and optical performance. Hermetic sealing, often in TO-can or ceramic packages, uses metal lids welded or soldered to the base to exclude moisture and contaminants, thereby enhancing long-term reliability in harsh conditions. Integration with , such as aspheric lenses or windows, is incorporated during assembly to focus incident onto the active area, minimizing and maximizing for applications like fiber-optic receivers. Yield considerations in photodiode fabrication emphasize defect control to achieve high uniformity across wafers. Techniques like gettering during epitaxial growth remove impurities, targeting defect densities below 10⁸ cm⁻², while CMOS-compatible processes enable scaling to large arrays by leveraging standard and backend steps for monolithic integration. This compatibility supports fabrication yields exceeding 90% for focal plane arrays in systems. Modern advances as of 2025 include of hybrid III-V on platforms, where heterogeneous bonding transfers epitaxial III-V layers onto CMOS-processed wafers, enabling compact, high-performance photodiodes for datacom with bandwidths over 100 GHz. Additionally, techniques have emerged for custom structures, such as facet-attached microlenses, allowing and precise optical alignment in photonic integrated circuits.

Device Structures and Types

PIN Photodiode

The PIN photodiode features a layered structure consisting of a p-type region, an intrinsic (undoped or lightly doped) region, and an n-type region, denoted as p-i-n. The intrinsic region, typically several micrometers thick, separates the heavily doped p and n layers and expands significantly under reverse bias to form a wide depletion zone. This design contrasts with standard p-n photodiodes by minimizing carrier diffusion and enhancing the uniformity across the absorption area. A primary advantage of the PIN structure is the reduced junction capacitance due to the extended depletion width, which lowers the RC time constant and improves high-frequency performance. The capacitance is approximated by the formula C=ϵAWi+Wd,C = \frac{\epsilon A}{W_i + W_d}, where ϵ\epsilon is the permittivity of the semiconductor material, AA is the active area, WiW_i is the intrinsic region width, and WdW_d represents the depletion widths in the p and n regions (often negligible in heavily doped layers). The wider depletion region also enables higher quantum efficiency across a broad spectral range from ultraviolet to infrared wavelengths, as more photogenerated carriers are collected before recombination. Compared to p-n diodes, PIN photodiodes exhibit lower noise levels, primarily from reduced thermal noise associated with the lower capacitance and minimized dark current. In operation, particularly in photoconductive mode under reverse bias, the strong in the intrinsic region sweeps photogenerated electron-hole pairs toward the respective contacts with minimal recombination, enabling efficient carrier collection. This configuration supports high-speed applications, with typical bandwidths ranging from 10 to 100 GHz depending on the intrinsic layer thickness and material. PIN photodiodes are briefly referenced here for their enhanced performance in reverse-biased photoconductive operation, as detailed in broader mode discussions. Commonly employed in telecommunications receivers for optical signal detection, PIN photodiodes benefit from their balance of speed and sensitivity in fiber-optic systems. However, they require higher reverse bias voltages—often tens of volts—to fully deplete the intrinsic region and achieve optimal performance, which can increase power consumption and . Additionally, the multi-layer introduces greater fabrication challenges, including precise control of the intrinsic region's doping and thickness uniformity during epitaxial growth or diffusion processes.

Avalanche Photodiode

Avalanche photodiodes (APDs) are specialized photodiodes that achieve internal current gain through carrier multiplication, enabling enhanced sensitivity for low-light detection in optical systems. Unlike standard photodiodes, APDs operate under high reverse bias to trigger impact ionization, amplifying the photocurrent while introducing specific noise characteristics. This gain mechanism makes APDs particularly valuable for applications requiring high signal-to-noise ratios, such as fiber-optic communications and photon counting. The structure of an APD typically incorporates a high-field multiplication region within a p-i-n configuration or utilizes separate absorption and multiplication layers to separate photon absorption from carrier multiplication, optimizing quantum efficiency and reducing noise. In the p-i-n based design, the intrinsic region is divided such that photogeneration occurs in a lower-field absorption zone, while multiplication happens in a narrower, high-field avalanche region under reverse bias exceeding 100 V. Separate absorption-multiplication structures, often denoted as SAM or SACM (separate absorption, charge, and multiplication), further enhance performance by tailoring material properties for specific wavelengths, such as InGaAs absorption layers paired with InP multiplication regions for near-infrared detection. The gain in APDs arises from , where photogenerated carriers gain sufficient in the high to ionize additional atoms, creating secondary electron-hole pairs that further multiply. This process yields a multiplication gain M=IoutIphM = \frac{I_\text{out}}{I_\text{ph}}, where IoutI_\text{out} is the output current and IphI_\text{ph} is the primary , with typical values ranging from 100 to 1000 depending on and . The total output current can be expressed as I=M⋅(Iph+Idark),I = M \cdot (I_\text{ph} + I_\text{dark}), where IdarkI_\text{dark} accounts for thermally generated carriers. However, the stochastic nature of ionization leads to excess noise, quantified by the noise factor F(M)≈MxF(M) \approx M^x, with xx as the excess noise index (typically 0.2–0.8 for optimized designs), which degrades the at high gains. APDs are classified by the initiating carrier and multiplication dynamics: electron-initiated types, common in InP-based devices, leverage higher coefficients for lower , while hole-initiated variants in exploit hole multiplication for visible-light applications. Reach-through APDs extend the to fully deplete the absorption layer, ensuring uniform field penetration and higher efficiency, whereas electron-hole APDs allow both carriers to contribute to , though this often increases due to mixed ionization rates. APDs favor electron initiation through doping profiles that prioritize electron , achieving gains up to 1000 with moderate . Despite their advantages, APDs face limitations from excess , which scales with gain and limits usable MM to avoid signal degradation, as well as the risk of premature breakdown from field nonuniformities or defects. High operating voltages also necessitate precise control and often thermoelectric cooling to suppress thermal generation of dark current and maintain gain stability, particularly in arrays or high-temperature environments. Advances through 2025 have focused on low-noise APDs, incorporating type-II superlattices like InGaAs/GaAsSb for absorption and AlGaAsSb for multiplication, achieving gains over 100 with excess noise factors below 2 and gain-quantum efficiency products exceeding 3500% at 2 μm wavelengths, ideal for quantum sensing in mid-infrared regimes. In 2025, further advancements include digital alloy AlAsSb/GaAsSb APDs demonstrating low dark current and noise for optical communications, and thin absorber AlInAsSb SACM APDs with suppressed dark currents at 2 μm. These structures mitigate noise via engineered band alignments that favor single-carrier multiplication, enabling single-photon-level detection with reduced cooling requirements.

Performance Characteristics

Responsivity and Sensitivity

Responsivity is a fundamental performance metric for photodiodes, defined as the ratio of the generated photocurrent IphI_{ph} to the incident optical power PP, expressed as R(λ)=IphPR(\lambda) = \frac{I_{ph}}{P}. This yields units of amperes per watt (A/W), quantifying the device's efficiency in converting light to electrical signal. The responsivity is wavelength-dependent, R(λ)R(\lambda), and follows the relation R(λ)=qληhcR(\lambda) = \frac{q \lambda \eta}{h c}, where qq is the elementary charge, λ\lambda is the wavelength, η\eta is the quantum efficiency, hh is Planck's constant, and cc is the speed of light; this equation highlights the linear scaling with photon energy and efficiency. Spectral response curves, plotting R(λ)R(\lambda) versus λ\lambda, typically peak near the material's bandgap and drop sharply beyond the cutoff wavelength, such as for silicon photodiodes where maximum responsivity occurs around 900 nm. Quantum efficiency η(λ)\eta(\lambda) is a key factor in , representing the ratio of charge carriers collected to incident photons, often reaching 70-90% in optimized devices. External quantum efficiency accounts for losses like surface reflection, while internal quantum efficiency excludes these, focusing on absorption and collection within the . Material selection influences η(λ)\eta(\lambda), with offering high values in the visible to near-infrared due to its 1.12 eV bandgap, though broader spectra require materials like InGaAs for extended response. Sensitivity metrics extend beyond to characterize minimum detectable signals. The (NEP) is the incident power yielding a of 1 in a 1 Hz bandwidth, typically in W/√Hz. Specific detectivity D∗D^*, a normalized , is given by D∗=AΔfNEPD^* = \frac{\sqrt{A \Delta f}}{NEP}
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