Photodiode

A photodiode is a semiconductor device engineered to convert incident photon radiation into electrical current. It is widely employed across the electromagnetic spectrum, from visible and infrared detection to ultraviolet, X-ray, and gamma-ray sensing. Its versatility enables applications in light measurement, optical communication, spectroscopy, and energy conversion technologies such as solar cells.

Principle of Operation

A photodiode commonly takes the form of a pn junction or PIN diode, designed so that photon absorption can efficiently generate charge carriers. When a photon of adequate energy strikes the semiconductor, it creates an electron–hole pair through the inner photoelectric effect. If this absorption occurs within the depletion region or within one diffusion length of it, the built-in electric field of the junction sweeps electrons towards the cathode and holes towards the anode, producing a photocurrent.
The total current flowing through the device is the sum of the photocurrent and the dark current, the latter representing current present in the absence of illumination. Low dark current is crucial for enhancing sensitivity. For a given spectral distribution, the photocurrent is approximately proportional to the incident irradiance. Photodiodes therefore operate most effectively in reverse-bias, which increases the depletion width, lowers capacitance, and improves collection efficiency.

Photovoltaic Mode

In photovoltaic mode, the photodiode operates at zero external bias. The photocurrent flows through a short circuit connecting the anode to the cathode. If the circuit is opened or presents significant impedance, a voltage is generated that forward biases the junction. This is the fundamental operating mode of solar cells, which may be regarded as large-area photodiodes.
The useful operating point for power generation is reached when the voltage is high enough to minimise the forward current relative to the photocurrent. Under short-circuit conditions, any forward current generated is negligible compared with the photocurrent, ensuring maximum current output.

Photoconductive Mode

In photoconductive mode, the diode is reverse-biased so that the cathode is driven positive relative to the anode. This widens the depletion layer, reducing junction capacitance and enabling faster response times because charge carriers are collected more rapidly.
The trade-off is the introduction of increased dark current, which contributes to electronic noise. Avalanche effects may further amplify noise under high reverse bias. Nevertheless, for high-speed applications such as optical communication, the improved temporal response outweighs the noise penalty. Good PIN diodes exhibit leakage currents below 1 nA, making thermal noise from the load resistance a dominant factor in many circuits.

Related Devices

Avalanche photodiodes (APDs) are specialist devices designed to operate near the reverse breakdown voltage, where impact ionisation enables internal signal gain. Each photogenerated carrier triggers an avalanche, thereby enhancing responsivity for low-light detection.
Phototransistors operate by amplifying the photocurrent generated at the base–collector junction. The bipolar phototransistor, enclosed in a transparent package, was developed during the mid-twentieth century. The incoming light generates carriers that act as a base current, which the transistor amplifies by its current gain. Though highly responsive, phototransistors exhibit slower response times compared with photodiodes and do not offer superior low-light detection.
Other variants include field-effect phototransistors (photoFETs), which use light to induce a gate-like potential controlling the drain–source current. A solaristor, a more recent two-terminal device, integrates photovoltaic action with memristive behaviour to function as both a phototransistor and energy harvester.

Materials

The semiconductor material determines a photodiode’s spectral response, noise characteristics, and efficiency. Only photons with energies exceeding the bandgap contribute to photocurrent generation. Common materials include silicon, germanium, and compound semiconductors such as gallium arsenide. Silicon devices, owing to their larger bandgap, generally produce lower noise compared with germanium.
Emerging low-dimensional materials such as graphene and molybdenum disulphide (MoS₂) offer tunable bandgaps, high carrier mobility, and potential for ultrathin, flexible photodiodes. These developments have broadened the design space for next-generation photodetectors.

Unwanted and Wanted Photodiode Effects

Any pn junction may act as a photodiode when illuminated. Consequently, electronic components such as integrated circuits, transistors, and diodes may malfunction if stray light reaches internal junctions. Encapsulation in opaque housings usually prevents this, but high-energy radiation such as ultraviolet light, X-rays, or gamma rays may still penetrate enclosures and induce photocurrents. Radiation-hardened designs mitigate these effects, an important consideration in space and nuclear applications.
In contrast, deliberate use of these effects enables innovations such as light-emitting and light-absorbing diodes (LEADs), in which an LED functions as both a light source and light detector. LEDs may also be employed for energy-harvesting and sensing applications.

Features and Performance Parameters

Several critical characteristics determine the performance and suitability of a photodiode in a given application.
ResponsivitySpectral responsivity expresses the relationship between generated photocurrent and incident light power, typically in amperes per watt. Quantum efficiency represents the fraction of photons that successfully generate electron–hole pairs.
Dark CurrentDark current arises from background radiation and the intrinsic saturation current of the semiconductor junction. Accurate measurement techniques require calibration to subtract this background. Dark current is a major source of noise in communication systems using photodiodes.
Response TimeThe response time is influenced by both carrier transit-time spread and the RC time constant of the diode and external circuitry. Carrier movement under the electric field of the depletion region determines the transit-time contribution. The Shockley–Ramo theorem shows that the total charge collected is equal to the elementary charge, despite the presence of two carriers. The RC time constant acts as a low-pass filter, broadening the impulse response and limiting modulation bandwidth in communication systems.
Noise-Equivalent Power (NEP)NEP defines the minimum detectable optical power that produces a photocurrent equal to the noise current in a 1 Hz bandwidth. The inverse of NEP is the detectivity, and when scaled by the square root of detector area yields the specific detectivity, which allows for comparison of detectors independent of size or bandwidth. Devices with higher detectivity are capable of detecting weaker signals.