Active Area 
The defined area within a photodiode that is diffused to create a PN or NP junction that can convert photons into current. Active areas can be any shape and can vary in size from less than a mm² to over a cm².

Avalanche Photodiode (APD)
An avalanche photodiode (APD) is a light sensitive semiconductor device that exploits the photoelectric effect to convert light to electricity. APDs are photodetectors that provide a built-in first stage of gain through avalanche multiplication. By applying a high reverse bias voltage, typically 100 – 200V for silicon based APD’s, an APD will produce an internal current gain effect (usually specified at 100X) due to impact ionization (avalanche effect).

Bandwidth (Frequency Response)
The frequency of a square wave input light signal at which the output current of a photodiode, having been converted to an output voltage by a load resistor or amplifier, is reduced to -3dB (70%) of its output in a low frequency mode. Bandwidth (B) is related to rise time (t_r) by the formula: B = 0.35 / t_r

Breakdown Voltage
The level of reverse bias voltage at which the photodiode’s dark current transitions from small incremental increases with increased voltage to a large increase. Commonly specified as the reverse bias voltage required to cause a dark current greater than 10 μA. Substantial operation above this point can cause damage to the photodiode.

Damage Threshold
The level of incoming optical power that can irreversibly damage the photodiode. This level occurs after the saturation point.

Dark Current
The reverse current that flows in a photodiode in the dark (total absence of light), when a reverse bias voltage is applied. Dark current increases with increasing temperature. Also referred to as leakage current.

Dynamic Range
The range of input light intensities (specified in watts) over which the photodiode’s responsivity remains linear within a specified linearity value (specified in %). (See Linearity below). Dynamic Range values vary based on a photodiode’s substrate resistivity (higher resistivity = reduced Dynamic Range) and reverse bias voltage (higher bias voltage = improved Dynamic Range).

Fall Time
The time required for the output current of an illuminated photodiode to fall from 90% to 10% of its initial “on” level in response to an “off” level in input light power. Accurate measurement requires the light source turning on and off be faster than the photodiode.

Filter
An optical component which passes or blocks certain wavelengths. Filters can be glass, epoxy, plastic, thin film coatings and may be applied directly to the photodiode’s surface or placed above it.

Gain Bandwidth Product
The GBP is the frequency bandwidth of an operational amplifier with a gain of 1. As the gain (equal to the feedback resistance in most circuits) increases from 1, the bandwidth of the op-amp decreases, as the GBP remains a constant.

Incident Light
Amount of light falling on the detector’s active area, measured in watts per square centimeter (also called Irradiance or flux).

Indium Gallium Arsenide Photodiode
A photodiode built on InGaAs which is produced on an InP substrate. Polarity is P on N. It can respond to wavelengths from 800nm to 2600nm depending of fabrication enhancements.

Junction Capacitance
The measure of the ability of the photodiode to store an electric charge. Junction capacitance varies with the photodiode’s substrate resistivity (higher resistivity = lower capacitance), reverse bias voltage (higher bias voltage = lower capacitance), and active area size (larger active area = higher capacitance).

Linearity
A measure of the deviation from linear of a photodiode’s responsivity to a range of input light power levels. Like Dynamic Range, Linearity values vary based on a photodiode’s substrate resistivity (higher resistivity = reduced linearity) and reverse bias voltage (higher bias voltage = improved linearity). It is usually shown by plotting photodiode response vs input light power and determining the plot’s percentage deviation from a straight line.

Mesa Construction
Mesa construction is a manufacturing process used in the semiconductor industry where the substrate / material is etched producing elevated areas with a flat top. These pillars / device structures stand up above the substrate. This process is usually done to stop parasitic capacitance and increase switching speeds.

Noise Equivalent Power (NEP)
The optical power (specified in watts) necessary to create a photocurrent in the photodiode equal to the noise of the photodiode, thereby obtaining a signal-to-noise ratio of 1. NEP is a function of the photodiode’s noise current and responsivity.

Photoconductive Mode
The mode of operation of a photodiode in which a reverse bias voltage is applied. Applying a reverse bias voltage will lower the photodiode’s capacitance, provide faster rise and fall times, improve its linearity range and slightly improve responsivity at wavelengths above 950nm for silicon based devices. It also creates dark current which is a factor in noise generation.

Phototransistor
A device that is able to sense light levels and alter the current flowing between emitter and collector according to the level of light it receives. Polarity can be PNP or NPN.

Photovoltaic Mode
The mode of operation of a photodiode in which no reverse bias voltage is applied. With no bias voltage applied, the photodiode’s shunt resistance becomes the dominating factor in noise generation.

Photo Current
The flow of current generated by the photovoltaic effect occurring within a photodiode’s PN junction (active area) in response to incident optical power. Basically, absorbed photons generate free charge carriers that are collected within the device.

PIN Photodiode
A photodiode with a wide, undoped intrinsic region sandwiched between heavily doped p+ and n+ regions. These are built to be used with a reverse bias voltage (photoconductive mode). Marktech’s position for silicon based devices is any photodiode built on N-type silicon with a resistivity greater than 400 ohm-cm is considered a PIN photodiode.

P-N Photodiode

A photodiode without a wide, undoped intrinsic region sandwiched between heavily doped p+ and n+ regions. These are built to be used without a reverse bias voltage (photovoltaic mode). Marktech’s position for silicon based devices is any photodiode built on N-type silicon with a resistivity of 10 to 100 ohm-cm is considered a P-N photodiode. (Also referred to as P on N).

Planar Construction
Planar construction is a manufacturing process used in the semiconductor industry which starts with an oxide layer on the semiconductor wafer surface followed by “layers” of doped regions, oxides and metalizations to produce a semiconductor device. The process involves the basic procedures of silicon dioxide (SiO₂) oxidation, SiO₂ etching and heat diffusion or implanting. The final steps involve oxidizing the entire wafer with a SiO₂ layer, etching contact vias down to the active regions, and depositing metalizations.

Quantum Efficiency (QE)
The percentage of incident photons that generate electrons and holes that result in the photodiode producing a photocurrent. QE varies with wavelength and to a slight degree improves at some wavelengths with reverse bias voltage.

Resistivity
The measure of how strongly a semiconductor material opposes electrical flow. It is expressed in ohm-cm and is proportional to the concentration of carriers inherent in the silicon (or other starting material) wafers from which the photodiodes are fabricated.

Responsivity
The ratio of the photdiode’s output current (in amperes) to the input light power (in watts). Responsivity is directly related to Quantum Efficiency and like it, varies with wavelength and to a slight degree improves at some wavelengths with reverse bias voltage. (Also referred to as Sensitivity).

Rise Time
The time required for the output current of an illuminated photodiode to rise from 10% to 90% of its fully “on” level in response to an “on” level in input light power. Accurate measurement requires that the light source turn on faster than the photodiode.

Saturation
The input optical power level at which an increase in input optical power no longer produces an increase in the photodiode’s current output. This can occur when the input optical power becomes too high for the entire photodiode or when the input optical power’s concentration is too high for the region of the photodiode being illuminated to convert it. There is a range of input optical power between the end of the dynamic range and the beginning of saturation that is known as the non-linear range.

Shot Noise
Noise related to the statistical fluctuation in both the photocurrent and the dark current. It is the dominant noise factor in the photoconductive mode.

Shunt Resistance
The shunt resistance is the voltage-to-current ratio in the vicinity of 0V. Since current cannot be measured at 0V, shunt resistance is determined by measuring the dark current at +10mV and -10mV, and using Ohm’s law, calculating the resistance.

Silicon Photodiode
A photodiode built on silicon. Polarity can be P on N or N on P. It can respond to wavelengths from 200nm to 1100nm depending of fabrication enhancements.