Photodiode Terminology - by Samir Miah
Dark Current – is the current flowing through the photodiode in the absence of light. Dark current includes photocurrent generated by background radiation and saturation current. Dark current increases as the temperature of the photodiode increases[4]. Dark current is linearly proportional to the increase in temperature[1]. Dark current is a major source of noise in photodiodes with reverse bias[2].
Responsivity/sensitivity[1] – the ratio of photo-current produced (in Amperes) to the incident radiant energy (also known as incident light power – measured in Watts). The unit of responsivity is A/W, Amperes/Watts.
Quantum Efficiency[1] – the photodiode’s capability of converting light energy into electrical energy. It is expressed as a percentage and is related to responsivity through following formula
QE = (1.24 x 10^5 * R) / 𝜆
where R is responsivity and 𝜆 is the wavelength.
(note: QE is affected by temperature changes, may be relevant to our design during later stages).
Rise time – rise times of less than 2ns are obtainable below 900nm[1].
“This is the measure of the time response of a photodiode to a stepped light input, and is defined as the time required for the output to change from 10 % to 90 % of the steady output level. The rise time depends on the incident light wavelength and load resistance.”[2]
Spectral Response[1,2] – a photodiodes ability to detect and absorb photons with varying wavelengths. Silicon diodes are not very efficient above 900nm.
Spectral response of silicon diode [1]
Junction Capacitance – the junction capacitance depends on the depletion layer depth and reverse bias voltage[1].
“Any variation of the charge within a p-n diode with an applied voltage variation yields a capacitance, which must be added to the circuit model of a p-n diode. This capacitance related to the depletion layer charge in a p-n diode is called the junction capacitance.”[5]
Negative Bias[6] – P-type material is connected to negative terminal of a battery and N-type is connected to the positive terminal. This results in cathode having higher voltage than anode. Depletion range increases as the holes and electrons are pulled away from the walls of P-type and N-type regions respectively.
[3] “Typical photodiode materials are:
- silicon (Si): low dark current, high speed, good sensitivity between roughly 400 and 1000 nm (best around 800–900 nm)
- germanium (Ge): high dark current, slow speed due to large parasitic capacity, good sensitivity between roughly 900 and 1600 nm (best around 1400–1500 nm)
- indium gallium arsenide phosphide (InGaAsP): expensive, low dark current, high speed, good sensitivity roughly between 1000 and 1350 nm (best around 1100–1300 nm)
- indium gallium arsenide (InGaAs): expensive, low dark current, high speed, good sensitivity roughly between 900 and 1700 nm (best around 1300–1600 nm)”
References
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