The working principle of a medium wave infrared detector (MWIR) is based on the physical effects of the interaction between infrared radiation and matter. The core mechanism includes two categories: photoelectric effect and thermoelectric effect. By capturing the 3-5 μ m band infrared radiation emitted by the target object, it is converted into measurable electrical signals. The following is a layered analysis of the specific principles:
1、 Optoelectronic effect: direct conversion of electrons excited by photons
The photoelectric effect is the mainstream working mechanism of MWIR detectors, especially suitable for scenarios that require high response speed and sensitivity, such as military reconnaissance and autonomous driving. The core process is as follows:
Photon absorption
When MWIR photons (wavelength 3-5 μ m) emitted by the target object are irradiated onto the detector material (such as indium antimonide InSb, mercury cadmium telluride HgCdTe), the photon energy is absorbed by the material.
Key parameter: Photon energy needs to be greater than the material bandgap width (Eg), for example, the bandgap width of HgCdTe can be adjusted by adjusting the Cd composition (0.2)
Electronic Transition and Carrier Generation
After absorbing photons, electrons in the material transition from the valence band to the conduction band, forming electron hole pairs (carriers).
Quantum efficiency: The quantum efficiency of high-quality MWIR detectors can reach over 80%, which means that 80 electron hole pairs can be excited for every 100 incident photons.
Electrical signal output
Carriers move directionally under the action of the electric field inside the detector, forming a photocurrent. By measuring the magnitude of photocurrent, the number of emitted photons can be pushed back to obtain the infrared radiation intensity of the target object.
Typical structure:
Photovoltaic (PV) type: utilizing the built-in electric field of p-n junction to separate charge carriers, without the need for external bias voltage, suitable for low-power applications such as portable thermal imaging devices.
Optical guide type (PC): enhances carrier drift speed through external bias voltage, with faster response speed, commonly used in high-speed detection scenarios.
2、 Thermoelectric effect: indirect conversion driven by temperature changes
Medium wave infrared detectorThe thermoelectric effect indirectly generates an electrical signal through the temperature change of the detector material after absorbing infrared radiation, which is suitable for non cooled MWIR detectors (such as industrial detection and smart homes). The core process is as follows:
Infrared radiation absorption and thermalization
After absorbing MWIR radiation, the molecular vibrations of detector materials (such as vanadium oxide VO ₓ, amorphous silicon) intensify and the material temperature increases.
Heat capacity and response time: The smaller the heat capacity of the material, the faster the temperature change, and the shorter the response time (typical value<10ms).
Thermistor resistance variation
The increase in temperature causes a change in the resistance of the thermistor material (such as a decrease in VO ₓ resistance with increasing temperature).
Temperature Coefficient of Resistance (TCR): The TCR of high-quality thermosensitive materials can reach -2% to -5%/℃, which means that for every 1 ℃ increase in temperature, the resistance changes by 2% to 5%.
Bridge Circuit and Signal Amplification
The thermistor is connected to the Wheatstone bridge circuit, and temperature changes cause the bridge to become unbalanced, resulting in a weak voltage signal output.
Amplify the signal to a processable range (such as millivolts) through a low noise amplifier (LNA), and ultimately generate a thermal image.