|Background Current (IBG)|
The DC current produced by background radiation on a photovoltaic detector. Measured across a low impedance load.
|Bias Current (IB)|
The constant current applied across a photoconductive detector for proper operation.
An ideal radiator whose radiant emittance, W, follows the Stefan-Boltzmann law. The relationship is shown below.
A detector is referred to as BLIP (background limited) when its D* is limited by the noise associated with photons from the background radiation and not by intrinsic detector noise.
|Cutoff Frequency (fc)|
Related to time constant as follows: fc = 1 / 2pt
|Cutoff Wavelength (lco)|
The long wavelength point at which the detector responsivity has fallen to a specified percent of the peak responsivity. Cutoff wavelength is usually specified at 20% or 50% of peak.
A relative sensitivity parameter used to compare performance of different detector types. D* is the signal-to-noise ratio at a particular electrical frequency and in a 1 Hz bandwidth when 1 Watt of radiant power is incident on a 1 cm2 active area detector. The higher the D* value, the better the detector.
D*(cmHz1/2W-1) = [Active Area (cm2)]1/2 / NEP(W/Hz1/2)
|Dark Current (ID)|
The current through a photodiode when a specific reverse bias voltage is applied, with no incident radiant power. Also referred to as Reverse Current.
|Johnson (Thermal) Noise|
When operated in the PV mode at 0 volt bias, a photodiode will generate Johnson noise current due to its shunt resistance (RD) according to:
i2 = 4KTDf / RD
where K is the Boltzman constant, T is temperature in °K, Df is the noise measurement bandwidth and RD is the detector shunt resistance.
|Junction Capacitance (CD)|
The p-n junction of a photovoltaic detector has a capacitance proportional to the active size.
|Maximum Reverse Voltage (VR)|
A photovoltaic detector can be damaged by applying more than the maximum reverse voltage VR.
|NEP (Noise Equivalent Power)|
The radiant power that produces a signal-to-noise ratio of one at the output of the detector. Defined with respect to a particular chopping frequency, wavelength and effective noise bandwidth.
NEP @ lp = Noise (A/Hz1/2) / Responsivity @ lp (A/W)
|Open Circuit Voltage (Voc)|
The DC voltage generated by a photovoltaic detector when connected to a high impedance load.
|Photoconductive Detector (PC)|
A photon detector which exhibits an increase in conductivity as a function of radiant power.
|Photovoltaic Detector (PV)|
Any photon detector utilizing a p-n or p–i–n junction to convert radiant power directly into electric current. Also referred to as a photodiode.
The detector photocurrent (or voltage) output per unit incident radiant power at a particular wavelength. Expressed in units of amps/watt (or volts/watt).
Random generation of current in a diode results in Shot Noise according to:
i2 = 2qIDf
where I is the total current, q is the electric charge and Df is noise bandwidth.
|Shunt Resistance (RD)|
The resistance of a photodiode measured at zero-volt bias with no radiant power incident on the detector. RD is the slope, dV/dl, of the current-voltage curve at zero-volt bias. Also referred to as Dynamic Impedance.
|Time Constant (t)|
The time required for the detector signal to reach 63% of its final value after the onset of a fixed incident power. Related to cutoff frequency (fc) as follows:
t = 1 / 2pfc