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).
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:
A change ΔVOUT then occurs due to a change ΔRDark in the resistance of the detector when light strikes the active area:
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:
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
The characteristic curve for Signal vs. Chopping Frequency for each particular detector is provided in chapter 4 of the operating manuals.
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:
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:
Here, S/N is the Signal to Noise Ratio, Δf is the Noise Bandwidth, and Incident Energy has units of W/cm2.
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.
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.