Glossary of Terms

Accuracy -

A measure of the deviation in the output of a device from the actual or expected value.

Active area

A

Defined as the diffused area where the photodiode junction is active and can convert photons can generate current.

Range: tens of mm sq. to a few cm sq.

Bandwidth

B

A measure of the information-carrying capacity of a medium. Specifically, it is the frequency of a sinusoidal input light at which the output current of a photodiode falls to 70% of the output in DC mode. B = 0.35 / tr, where tr is the rise time.

Range: a few kHz to over 1GHz

Breakdown Voltage

Vb

A reverse bias voltage applied to a photodiode where a small change in voltage causes a relatively large change in current flowing through the diode. A photodiode gets damaged passed this point. Usually defined as the reverse bias for a dark current at 10 µA.

Range: a few Volts to a few hundreds

Junction Capacitance

Cj

A measure of the ability of the device to store an electric charge. Depends on the substrate resistivity, the reverse voltage, and the active area.

Range : a few pF to a few tens of nF

Damage threshold

-

Point after which an excess of incoming optical power can damage irreversibly the photodiode. Occurs after the saturation point.

Dark Current

Id

A small value current that flows in a photodiode while reversed biased, due to thermal generation of carriers and surface leakage with no incident light (also called leakage current).

Range: a few pA to 10 m A

Degradation

-

A drift in output for a detector submitted to a constant input optical power for a long period of time.

Drift

-

A change in output over time due to temperature or bias variations.

Dynamic Range

-

Range of input light intensity over which the responsivity is within linearity specification.

Fall Time

tf

The time for the output current of a photodiode to fall from 90% to 10% of initial level in response to an instantaneous fall in input light power.

Range: Same as tr

Filter

-

An optical component which allows a range of certain wavelengths to be transmitted. The medium of transmission can be glass, epoxy, plastic, etc.

Gain Bandwidth Product

GBP

The GBP is the frequency bandwidth of an operational amplifier with a gain of 1. As the gain (feedback resistance) increases, the bandwidth of the op-amp decreases, as the GBP keeps the same value. The GBP limits the behavior of the op-amp, as the maximum frequency is defined by :

Gain Bandwidth Product

Where Ca is the amplifier input capacitance, Cj the photodiode junction capacitance and Cf is the feedback capacitance in a transimpedance amplification mode.

Range: from 0.5 Mhz to over 1 Ghz

Incident Light

-

Amount of light falling on a detector's active area measured in watts per square centimeter (also called Irradiance or flux).

Linearity

-

Specifically, a measure of deviation in responsivity of a device over a range of input light power. In general, a measure of deviation of a curve from a straight line within 1% linearity (also called accuracy).

Noise Equivalent Power

NEP

Optical power necessary to create a photocurrent equal to the noise, to obtain a signal-to-noise ratio of 1. It depends on the noise current and the responsivity.

Range: 10-15 to 10-11 W/ Sqrt (Hz)

Noise

-

Random fluctuation in a variable.

Noise bandwidth

-

Range of frequencies of noise that occur in a system.

Photoconductive mode

PC

Reverse biased mode of operation of a photodiode, in which lower capacitance and faster rise time are obtained. Generates more noise ( dark current ).

Photocurrent

Iph

Flow of current generated by a photodiode in response to an incident optical power. The absorbed photons generate free charge carriers that are collected within the device.

Photovoltaic Mode

PV

Unbiased mode of operation of a photodiode, preferred in low frequency (up to 350 MHz) as well as ultra low light level applications.

Quantum Efficiency

QE

Percentage of incident photons that generate electrons and hole that effectively participate in the photocurrent.

Range: A few % to over 100 %

Repeatability

-

The ability of a device to produce the same output reading when subject to same input applied consecutively over a short period of time (also called precision). Sometimes used to signify device to device variation in parameter.

Resistivity

r

Resistivity of material defines cincentration of carriers in the bulk substrate.

Range: between 1 and tens of thousands Ohms-cm

Resolution

-

The smallest incremental change in input that can be distinguished from the noise in the ouput.

Response Time

-

Time necessary for a device to begin changing its output in response to a variation in the input.

Responsivity

Rl

The ratio of output current (in Amperes) to input light power (in Watts). It is a constant in the dynamic range, at a given wavelength. (also known as sensitivity).

Range: from 0 and 1 A/W

Rise Time

tr

The time for the output current of a photodiode to rise from 10% to 90% of final level in response to an instantaneous rise in input light power.

Range: from a few ps to tens of ms

Saturation

-

The point after which a change in input optical power does not produce the same amount of change in the output. It marks the end of the dynamic range, and the beginning of the saturation region.

Selectivity

-

The ability of a device to distinguish the signal from the background.

Sensitivity

-

A proportionality constant that relates the output of a sensor to the input.

Shot noise

ish

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

Shunt Resistance

Rsh

In the photovoltaic mode, this resistance is a merit factor of the device. The higher it is, the lower the noise is. It is defined as the dark current at a reverse bias of 10 mV.

Range: a few M-Ohm to tens of G-Ohm

Span

-

Range of output over linearity range of input.

Stability

-

The ability of a device to produce the same output when subject to same input applied over a long period of time. Specific to photodetectors, a measure of the ability of the detector's responsivity not to drift over extended period of time (inverse of degradation).

Substrate

-

It is the material used to manufacture photodiodes. Usually slightly doped silicon wafers are used.

Thermal noise

-

All resistors have a thermal (or Johnson) noise associated with them, due to thermal generation of carriers