Last revised: January 29, 2025

Photodiodes are a two-electrode, radiation-sensitive junction formed in a semiconductor material in which the reverse current varies with illumination. Photodiodes are used for the detection of optical power and for the conversion of optical power to electrical power.

Photodiodes can be PN, PIN, or avalanche. PN photodiodes feature a two-electrode, radiation-sensitive PN junction formed in a semiconductor material in which the reverse current varies with illumination.

PIN photodiodes are diodes with a large intrinsic region sandwiched between P-doped and N-doped semiconducting regions. Photons absorbed in this region create electron-hole pairs that are then separated by an electric field, thus generating an electric current in a load circuit.

Avalanche photodiodes are devices that utilize avalanche multiplication of photocurrent by means of hole-electrons created by absorbed photons. When the device's reverse-bias voltage nears breakdown level, the hole-electron pairs collide with ions to create additional hole-electron pairs, thus achieving a signal gain.

Spectral Response

The photodiode spectral response can be measured in:

• X-ray
• UV
• Visible
• IR

X-ray photodiodes are optimized for X-ray, gamma ray, and beta radiation detection.

UV enhanced photodiodes are optimized for the UV and blue spectral regions, which requires special fabrication processes. Visible photodiodes operate in the visible range without enhancement for operation in UV or IR.

IR enhanced photodiodes are optimized for the near IR spectral region, which requires special fabrication processes.

The spectral response range of incident light the photodiode detects is also called wavelength range. Important active area specifications to consider include active area diameter or length and active area height. If the active area is circle shaped, specify the diameter, otherwise specify the length of the active area. The active area height is applicable to photodiodes that are not circular.

Photodiode arrays are packaged as multiples. Photodiode arrays will contain a certain number of elements (Photodiodes). Some photodiodes can be a position sensitive detector.

Important photodiode performance specifications to consider include:

• Sensitivity
• Rise time
• Quantum efficiency
• Operating temperature

Sensitivity is a measure of the effectiveness of a detector in producing an electrical signal at the peak sensitivity wavelength. Rise time is the time necessary for a detector's output to go from 10% to 90% of its final value. A photodiode's capability to convert light energy to electrical energy, expressed as a percentage, is its quantum efficiency.

Noise Factor

Photodiodes can have a noise factor. This is measured as the dark current and noise equivalent power (NEP).

Dark current is the current associated with a detector during operation in the dark with an applied reverse bias.

Increased temperature and reverse bias will result in increased dark current. Also, larger active areas will generally have a higher dark current. Noise equivalent power is the power of incident light, at a specific wavelength, required to produce a signal on the detector that is equal to the noise.

Common materials of construction for photodiodes include:

• Silicon
• Indium gallium arsenide
• Germanium
• Gallium nitride
• Silicon carbide

Standards

BS EN 120005 — Specification for harmonized system of quality assessment for electronic components - blank detail specification — photodiodes, photodiode-arrays (not intended for fiber optic applications).

IEC 62088 — Nuclear instrumentation — photodiodes for scintillation detectors — test procedures. 

JIS C 5990 — General rules of photodiodes for fiber optic transmission.

JIS C 5991 — Measuring methods of photodiodes for fiber optic transmission.

Photodiodes FAQs

How do different types of photodiodes vary in terms of sensitivity and response time?

Photodiodes vary in terms of sensitivity and response time based on several factors, including their design and the materials used.

Sensitivity

Sensitivity in photodiodes is a measure of how effectively the device converts light into an electrical signal. It is often quantified by responsivity, which is the ratio of the output current to the incident light power. A photodiode with high responsivity can detect lower light levels more effectively.

The sensitivity is also influenced by the dark current, which is the small current that flows through the photodiode even when no light is present. Lower dark currents are preferable in low-light applications as they indicate better sensitivity and less noise.

Response Time

The response time of a photodiode is the time it takes for the device to respond to a change in light intensity. Photodiodes with fast response times are ideal for high-speed applications, such as optical communication and safety systems.

The response time is influenced by the carrier transit time and the capacitance of the p-n junction. The transit time is the time taken for charge carriers to move through the high-field region of the junction, which can be much shorter than the carrier lifetime.

The capacitance of the photodiode affects the RC time constant, which is a measure of how quickly the output responds to a changing input. A smaller capacitance leads to a faster time response and larger bandwidth.

Trade-offs

There is often a trade-off between sensitivity and speed. For instance, reducing the junction area can decrease capacitance and increase speed, but it may also reduce sensitivity if not all the available light can be directed onto the smaller area.

Similarly, reducing the load resistance can increase speed but decrease sensitivity, resulting in another sensitivity-speed trade-off.

How are the factors affecting photodiode sensitivity?

Photodiode sensitivity is influenced by several key factors, which determine how effectively the device converts light into an electrical signal. Here are the main factors affecting photodiode sensitivity:

Responsivity

Responsivity is a measure of how efficiently a photodiode converts incident light into an electrical signal. It is calculated as the ratio of the output current to the incident light power. A higher responsivity indicates that the photodiode can detect lower light levels more effectively.

Dark Current

Dark current is the small current that flows through the photodiode even when no light is present. Lower dark currents are preferable, especially in low-light applications, as they indicate better sensitivity and less noise.

Junction Capacitance

The capacitance of the p-n junction affects the photodiode's response time and sensitivity. A smaller capacitance can lead to a faster response time and larger bandwidth, which is usually desirable. However, reducing capacitance by decreasing the junction area may also reduce sensitivity if not all the available light can be directed onto the smaller area.

Load Resistance

The load resistance (RL) also plays a role in sensitivity. A larger RL is best for high sensitivity, while a smaller RL is best for high speed. This results in a trade-off between sensitivity and speed.

Material and Design

The materials used in the photodiode and its design can also impact sensitivity. Different materials have varying levels of responsivity and dark current characteristics, which can affect overall sensitivity.

These factors often involve trade-offs, such as between sensitivity and speed, which must be optimized based on the specific application requirements.

What are trade-offs between sensitivity and response time in photodiodes?

The trade-offs between sensitivity and response time in photodiodes are influenced by several factors related to their design and operation.

Sensitivity vs. Response Time

Sensitivity is primarily determined by the photodiode's responsivity and dark current. High responsivity means the photodiode can detect lower light levels more effectively, while low dark current indicates better sensitivity with less noise.

Response Time is the time it takes for the photodiode to respond to changes in light intensity. Fast response times are crucial for high-speed applications, such as optical communication.

Capacitance and RC Time Constant

The capacitance of the p-n junction affects both sensitivity and response time. A smaller capacitance leads to a faster response time and larger bandwidth, which is desirable for high-speed applications.

However, reducing capacitance by decreasing the junction area can reduce sensitivity if not all the available light can be directed onto the smaller area. This creates a trade-off between sensitivity and speed.

Load Resistance

The load resistance (RL) also plays a role in this trade-off. A larger RL is best for high sensitivity, while a smaller RL is best for high speed. This results in another sensitivity-speed trade-off.

Design Considerations

Reducing the junction area or increasing the reverse-bias voltage can improve response time but may impact sensitivity. For instance, smaller detector areas give a faster time response but may not capture all the light, affecting sensitivity.

These trade-offs must be optimized based on the specific application requirements, balancing the need for sensitivity with the need for speed.

What is the impact of junction capacitance on photodiode performance?

The impact of junction capacitance on photodiode performance is significant, particularly in terms of response time and bandwidth.

Junction Capacitance and Response Time

The capacitance of the p-n junction affects the photodiode's response time. A smaller capacitance leads to a faster response time, which is desirable for high-speed applications.

The response time is characterized by the RC time constant, where ( R ) is the load resistance and ( C ) is the junction capacitance. The time constant measures how quickly the output responds to a changing input. A smaller capacitance results in a smaller RC time constant, leading to a quicker response.

Capacitance and Bandwidth

A smaller junction capacitance also results in a larger bandwidth. The bandwidth is the range of frequencies over which the photodiode can effectively respond to changes in light intensity. A larger bandwidth is beneficial for applications requiring rapid signal processing.

Factors Affecting Junction Capacitance

The junction capacitance is not constant; it decreases with increasing reverse-bias voltage. This is because the junction width increases with the reverse-bias voltage, reducing capacitance.

Reducing the junction area can also decrease capacitance, leading to a faster response. However, this may reduce sensitivity if not all the available light can be directed onto the smaller area, creating a trade-off between sensitivity and speed.

The density of donors on the weakly doped side of the junction can be adjusted to reduce capacitance. A lower donor density increases the junction width, thereby reducing capacitance.

Trade-offs

There is a trade-off between reducing capacitance for faster response and maintaining sensitivity. Smaller detector areas can lead to faster response times but may not capture all the light, affecting sensitivity.

Similarly, reducing the load resistance can decrease the RC time constant, improving speed but reducing sensitivity.

These factors must be carefully balanced based on the specific requirements of the application to optimize photodiode performance.

How does reverse-bias voltage affect photodiode performance?

Reverse-bias voltage significantly affects the performance of a photodiode, particularly in terms of its capacitance, response time, and bandwidth.

Junction Capacitance

The capacitance of a p-n junction decreases with increasing reverse-bias voltage. This is because the junction width increases with the reverse-bias voltage, reducing the capacitance.

The reduced capacitance is beneficial as it leads to a faster response time and larger bandwidth, which are desirable for high-speed applications.

Response Time

A smaller junction capacitance results in a smaller RC time constant, which measures how quickly the output responds to a changing input. This leads to a quicker response time.

The reverse-bias voltage makes the photoconductive mode (where ( V_B > 0 )) inherently faster than the photovoltaic mode (where ( V_B = 0 )).

Bandwidth

Increasing the reverse-bias voltage enhances the bandwidth of the photodiode. The bandwidth is the range of frequencies over which the photodiode can effectively respond to changes in light intensity.

Trade-offs

While increasing the reverse-bias voltage improves speed and bandwidth, there is a practical limit due to the risk of electrical breakdown in the junction. Typical reverse-bias voltages are in the range of 5-10 V.

There is also a sensitivity-speed trade-off. Reducing the junction area to decrease capacitance and increase speed may reduce sensitivity if not all the available light can be directed onto the smaller area.

These factors must be carefully balanced based on the specific requirements of the application to optimize photodiode performance.

How does the load resistance affect photodiode performance?

The load resistance (( R_L )) plays a crucial role in the performance of a photodiode, particularly affecting its sensitivity and response time.

Sensitivity

A larger load resistance is beneficial for high sensitivity. This is because a higher ( R_L ) allows for a greater voltage drop across the photodiode for a given photocurrent, enhancing the output signal's amplitude. This makes it easier to detect lower light levels, improving the photodiode's sensitivity.

Response Time

Conversely, a smaller load resistance is preferable for achieving high-speed performance. A lower ( R_L ) reduces the RC time constant, which is the product of the load resistance and the junction capacitance. This results in a faster response time, allowing the photodiode to quickly react to changes in light intensity.

Trade-offs

There is a trade-off between sensitivity and speed when adjusting the load resistance. While a larger ( R_L ) enhances sensitivity, it also increases the RC time constant, slowing down the response time. On the other hand, reducing ( R_L ) improves speed but decreases sensitivity, as the output signal amplitude is reduced.

These trade-offs must be carefully balanced based on the specific requirements of the application to optimize photodiode performance.

Photodiodes Media Gallery

References

GlobalSpec—A brief intro to photodiode light sensors

GlobalSpec—Photonics and Lasers

Image credits:

Digi-Key Corporation | First Sensor AG | OSI Optoelectronics