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IR Photoconductive Detectors


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IR Photoconductive Detectors

Mounted and Unmounted Detectors
Unmounted Photodiodes (150 - 2600 nm)
Mounted Photodiodes (150 - 1800 nm)
Calibrated Photodiodes (350 - 1800 nm)
Photoconductors (1.0 - 4.8 µm)

Lead Sulfide (PbS) and Lead Selenide (PbSe) photoconductors are widely used for the detection of infrared radiation from 1.0 to 4.8 µm. Photoconductors detect light in a broader wavelength range, offer higher detection capability, and provide better linear response in the IR than typical PIN junction photodiodes.

Photoconductors vs. Photodiodes
Unlike PIN junction photodiodes, which generate a photocurrent when light is absorbed in the depleted region of the junction semiconductor, the photoconductive material in these devices exhibits a decrease in electrical resistance when illuminated with IR radiation. Photoconductive detectors typically have a very linear response when illuminated with IR radiation.

Usage Notes
Photoconductors function differently than typical PIN junction photodiodes. We recommend that an optical chopper be employed when using these detectors with CW light, due to signal noise issues. PbS and PbSe detectors can be used at room temperature. However, temperature fluctuations will affect dark resistance, sensitivity, and response speeds (see the Temperature Considerations section in the Tutorial tab). See the tutorial tab for more details.

PbS and PbSe Photoconductive Detectors

Lead Sulfide (PbS) and Lead Selenide (PbSe) photoconductive detectors are widely used in detection of infrared radiation from 1000 to 4800 nm. Unlike standard photodiodes, which produce a current when exposed to light, the electrical resistance of the photoconductive material is reduced when illuminated with light. Although PbS and PbSe detectors can be used at room temperature, temperature flucturations will affect dark resistance, sensitivity, and response speeds (see Temperature Considerations below).

Photoconductor Basic Model
Photoconductor Basic Schematic
Click to Enlarge

Theory of Operation

For photoconductive materials, incident light will cause the number of charge carriers in the active area to increase, thus decreasing the resistance of the detector. This change in resistance leads to a change in measured voltage, and hence, photosensitivity is expressed in units of V/W. An example operating circuit is shown to the right. Please note that the circuit depicted is not recommended for practical purposes since low frequency noise will be present.

The detection mechanism is based upon the conductivity of the thin film of the active area. The output signal of the detector with no incident light is defined by the following equation:

Photoconductor Basic Model

A change ΔVOUT then occurs due to a change ΔRDark in the resistance of the detector when light strikes the active area:

Photoconductor Basic Model

Frequency Response
Photoconductors must be used with a pulsed signal to obtain AC signals. Hence, an optical chopper should be employed when using these detectors with CW light. The detector responsivity (Rf) when using a chopper can be calculated using the equation below:

Photoconductor Responsivity

Here, fc is the chopping frequency, R0 is the response at 0 Hz, and τr is the detector rise time.

Effects of Chopping Frequency
The photoconductor signal will remain constant up to the time constant response limit. PbS and PbSe detectors have a typical 1/f noise spectrum (i.e., the noise decreases as chopping frequency increases), which has a profound impact on the time constant at lower frequencies.

The detector will exhibit lower responsivity at lower chopping frequencies. Frequency response and detectivity are maximized for

Photoconductor Chopper Equation

The characteristic curve for Signal vs. Chopping Frequency for each particular detector is provided in chapter 4 of the operating manuals.

Temperature Considerations
These detectors consist of a thin film on a glass substrate. The effective shape and active area of the photoconductive surface varies considerably based upon the operating conditions, thus changing performance characteristics. Specifically, responsivity of the detector will change based upon the operating temperature.

Temperature characteristics of PbS and PbSe bandgaps have a negative coefficient, so cooling the detector shifts its spectral response range to longer wavelengths. For best results, operate the photodiode in a stable controlled environment. See the Operating Manuals for characteristic curves of Temperature vs. Sensitivity for a particular detector.

Typical Photoconductor Amplifier Circuit

Due to the noise characteristic of a photoconductor, it is generally suited for AC coupled operation. The DC noise present with the applied bias will be too great at high bias levels, thus limiting the practicality of the detector. For this reason, IR detectors are normally AC coupled to limit the noise. A pre-amplifier is required to help maintain the stability and provide a large gain for the generated current signal.

Based on the schematic below, the op-amp will try to maintain point A to the input at B via the use of feedback. The difference between the two input voltages is amplified and provided at the output. It is also important to note the high pass filter that AC couples the input of the amplifier blocks any DC signal. In addition, the resistance of the load resistor (RLOAD) should be equal to the dark resistance of the detector to ensure maximum signal can be acquired. The supply voltage (+V) should be at a level where the SNR is acceptable and near unity. Some applications require higher voltage levels; as a result the noise will increase. Provided in chapter 4 of the Operating Manual is a SNR vs. Supply Voltage characteristic curve to help determine best operating condition. The output voltage is derived as the following:

Photoconductor Amp Eq

Photoconductor Basic Amp Model
Amplifier Model

Signal to Noise Ratio
Since the detector noise is inversely proportional to the chopping frequency, the noise will be greater at low frequencies. The detector output signal is linear to increased bias voltage, but the noise shows little dependence on the bias at low levels. When a set bias voltage is reached, the detector noise will increase linearly with applied voltage. At high voltage levels, noise tends to increase exponentially, thus degrading the signal to noise ratio (SNR) further. To yield the best SNR, adjust the chopping frequency and bias voltage to an acceptable level. Provided in chapter 4 of the operating manuals are characteristic curves for SNR vs. Chopping Frequency and SNR vs. Supply Voltage for each particular detector.

Noise Equivalent Power
The noise equivalent power (NEP) is the generated RMS signal voltage generated when the signal to noise ratio is equal to one. This is useful, as the NEP determines the ability of the detector to detect low level light. In general, the NEP increases with the active area of the detector and is given by the following equation:

Photoconductor NEP

Here, S/N is the Signal to Noise Ratio, Δf is the Noise Bandwidth, and Incident Energy has units of W/cm2.

Dark Resistance
Dark Resistance is the resistance of the detector under no illumination. It is important to note that dark resistance will increase or decrease with temperature. Cooling the device will increase the dark resistance. Provided in chapter 4 of the operating manuals is a Dark Resistance vs. Temperature characteristic graph for each particular detector.

Detectivity (D) and Specific Detectivity (D*)
Detectivity (D) is another criteria used to evaluate the performance of the photodetector. Detectivity is a measure of sensitivity and is the reciprocal of NEP.

Photoconductor Detectivity

Higher values of detectivity indicate higher sensitivity, making the detector more suitable for detecting low light signals. Detectivity varies with the wavelength of the incident photon.

NEP of a detector depends upon the active area of the detector, which in essence will also affect detectivity. This makes it hard to compare the intrinsic properties of two detectors. To remove the dependence, Specific Detectivity (D*), which is not dependent on detector area, is used to evaluate the performance of the photodetector.

Photoconductor D*

The following table lists the photodiodes found on this page, along with the mounted photodiodes and packaged detectors which use the same internal photodiode.

 Photodetector Cross Reference
WavelengthMaterialUnmounted PhotodiodeUnmounted PhotoconductorMounted PhotodiodeBiased DetectorAmplified Detector
150 - 550 nmGaPFGAP71-SM05PD7ADET25KPDA25K
200 - 1100 nmSiFDS010-SM05PD2A
SM05PD2B
DET10APDA10A
200 - 1100 nmSi--SM1PD2A--
320 - 1100 nmSi----PDA8A
320 - 1100 nmSi----PDF10A
340 - 1100 nmSiFDS10X10----
350 - 1100 nmSiFDS100
FDS100-CAL a
-SM05PD1A
SM05PD1B
DET36APDA36A
400 - 1100 nmSiFDS025 b
FDS02 c
--
DET02AFC
DET025AFC
DET025A
DET025AL
-
400 - 1100 nmSiFDS1010
FDS1010-CAL a
-SM1PD1A
SM1PD1B
DET100APDA100A
400 - 1700 nmSi & InGaAsDSD2----
500 - 1700 nmInGaAs--DET10N--
700 - 1800 nmInGaAsFDGA05---PDA10CF
800 - 1700 nmInGaAs----PDF10C
800 - 1800 nmInGaAsFDGA05----
800 - 1800 nmInGaAsFGA10-SM05PD4ADET10CPDA10CS
800 - 1800 nmInGaAsFGA21
FGA21-CAL a
-SM05PD5ADET20CPDA20C
PDA20CS
800 - 1800 nmGeFDG03
FDG03-CAL a
-SM05PD6ADET30BPDA30B
800 - 1800 nmGeFDG50----
800 - 1800 nmGeFDG05
FDG05-CAL a
--DET50BPDA50B
800 - 1800 nmGeFDG1010-SM1PD5A--
850 - 1700 nmInGaAs---DET08CFC
DET08C
DET08CL
-
900 - 1700 nmInGaAsFGA01 b
FGA01FC c
--DET01CFC
SIR5-FC
-
1.0 - 2.9 µmPbS-FGPS3X3--PDA30G
1.2 - 2.6 µmInGaAsFGA20--DET10DPDA10D
1.5 - 4.8 µmPbSe-FGPSE2X2--PDA20H
  • Calibrated Unmounted Photodiode
  • Unmounted TO-46 Can Photodiode
  • Unmounted TO-46 Can Photodiode with FC/PC Bulkhead
  •  

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PbS Photoconductor: 1.0 - 2.9 µm
  • Good Performance for 1.0 - 2.9 µm Range
  • For Detection of CW Light We Recommend an Optical Chopper
  • Also Comes as a Packaged, Amplified Detector, the PDA30G
Item #InfoWavelength
Range
Active
Area
Package
Type
Rise
Timeb
Peak
Wavelength
Peak
Sensitivityc
Specific
Detectivityd
Dark
Resistance
Compatible
Sockets
FDPS3X3 info 1.0 - 2.9 µm 9 mm2 TO-5 200 µs 2.2 µm (Typ.) 2 x 104 V/W (Min)
5.0 x 104 V/W (Typ.)
1 x 1011
cm•Hz1/2/W
0.25 - 2.5 MΩ STO5S
STO5P
  • All measurements performed with a 50 Ω load unless stated otherwise
  • Rise time is measured from 0 - 63% of final value
  • Measured at peak wavelength, chopping frequency of 600 Hz, and bias voltage of 15 V, RDARK = RLOAD
  • Measured at peak wavelength and chopping frequency of 600 Hz
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal/Imperial Price Available / Ships
FDPS3X3 Support Documentation
FDPS3X3 PbS Photoconductor, 3 mm x 3 mm Active Area, 200 µs Rise Time, 1.0 - 2.9 µm
$177.50
Today
PbSe Photoconductor: 1.5 - 4.8 µm
  • Good Performance for 1.5 - 4.8 µm Range
  • For Detection of CW Light We Recommend an Optical Chopper
  • Also Comes as a Packaged, Amplified Detector, the PDA20H
Item #InfoWavelength
Range
Active
Area
Package
Type
Rise
Timeb
Peak
Wavelength
Peak
Sensitivityc
Specific
Detectivityd
Dark
Resistance
Compatible
Sockets
FDPSE2X2 info 1.5 - 4.8 µm 4 mm2 TO-5 10 µs 4 µm (Typ.) 3.0 x 103 V/W 2.5 x 109
cm•Hz1/2/W (Typ.)
0.1 - 3.0 MΩ STO5S
STO5P
  • All measurements performed with a 50 Ω load unless stated otherwise
  • Rise time is measured from 0 - 63% of final value
  • Measured at peak wavelength, chopping frequency of 600Hz, and bias voltage of 15 V, RDARK = RLOAD
  • Measured at peak wavelength and a chopping frequency of 600 Hz
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal/Imperial Price Available / Ships
FDPSE2X2 Support Documentation
FDPSE2X2 PbSe Photoconductor, 2 mm x 2 mm Active Area, 10 µs Rise Time, 1.5 - 4.8 µm
$210.00
Today
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