Understanding optronic sensors and related sensor systems is essential to understanding the design and operation of a large number of defence systems. Hence this six-part article series starts with an overview of these sensors.
Optronic sensors constitute the heart of a variety of systems ranging from simple gadgets like light meters to the most complex of military systems like precision-guided munitions, laser rangefinders, target trackers, remote sensing systems, navigation sensors, sniper and explosive detectors, fibre-optic and laser-based communication systems, night vision devices, Lidar and spectroscopic sensors. While individual photosensors such as PIN photodiodes and avalanche photodiodes find applications in military optronic systems including laser rangefinders and target designators, Lidar sensors and navigation sensors, sensor arrays such as complementary metal-oxide semiconductor (CMOS), charge-coupled device and avalanche photodiodes are at the core of imaging sensor systems including Ladar sensors, night vision devices, laser and imaging infrared seekers.
Types of photosensors
Photosensors are classified into two major categories: Photoelectric and thermal. Photoelectric sensors are further of two types: Devices that depend on the external photo effect for their operation and devices that make use of some kind of internal photo effect.
Photo-emissive sensors are based on external photo effect. Common photo-emissive sensors include non-imaging sensors such as vacuum photo cells and photomultiplier tubes, and imaging sensors such as image intensifier tubes.
Photoconductors and junction-type photosensors use internal photo effect. Photoconductors are bulk semiconductor devices whose resistance decreases with increase in incident light intensity. These are also known by the name of photoresistors, light-dependent resistors and photocells.
Junction-type photosensors are further amplifying and non-amplifying type. Amplifying-type junction photosensors include phototransistors, photothyristors and photo-field-effect transistors (FETs). Non-amplifying types of junction photosensors include photodiodes, solar cells, CMOS and charge-coupled devices. CMOS and charge-coupled device are imaging sensors. The category of thermal sensors includes thermocouple (or thermopile) type sensors, bolometric sensors and pyroelectric sensors.
Thermal sensors absorb incident radiation and operate on the resulting temperature rise, whereas photoelectric sensors are based on quantum effect. Compared to photoelectric sensors, thermal sensors are sluggish in their response to the incident radiation. However, these offer a much wider operational wavelength band than photoelectric sensors.
Major parameters used to characterise the performance of photosensors include responsivity, noise equivalent power (NEP), sensitivity (usually measured as detectivity or D*), quantum efficiency, response time and noise.
Responsivity is the ratio of electrical output to radiant light input determined in the linear region of response. It is measured in amperes per watt (A/W) or V/W if the photosensor produces a voltage output rather than a current output. Responsivity is a function of the incident radiation’s wavelength and band-gap energy. Spectral response is a related parameter. It is a curve that shows variation of responsivity as a function of wavelength.
Most photoelectric sensors have a narrow spectral response, whereas most thermal sensors have a wide spectral response. As an example, spectral responses of silicon, germanium, indium-gallium-arsenide photodiodes are in the range of 200-1100nm, 500-1900nm and 700-1700nm, respectively, in comparison to thermistors’ 0.5-10µm.
Silicon photodiodes exhibit a response from ultraviolet through visible and into near infrared part of the spectrum. With silicon having band-gap energy of 1.12eV at room temperature, its spectral response peaks in the near infrared region between 800nm and 950nm. Peak responsivity figures for silicon PIN photodiodes are in the range of 0.4-0.6A/W in comparison to avalanche photodiodes’ 40-80A/W. Thermal sensors have poorer responsivity than photoelectric sensors. For instance, the responsivity of pyroelectric sensors is in the range of 0.5-5µA/W.
The shape of the spectral response curve of silicon photosensors, particularly in blue and UV parts of the spectrum, can be altered by choosing an appropriate manufacturing process. Fig. 1 shows typical spectral response curves of normal, blue-enhanced and UV-enhanced, and low-noise silicon photodiodes. Low-noise spectral response corresponds to photodiodes’ photovoltaic mode of operation wherein no external bias is applied to the photodiode. Since dark current is a function of bias voltage magnitude, photovoltaic mode of operation eliminates dark current as a source of noise. In this case, NEP will be lower, thereby allowing greater sensitivity at lower wavelengths but at the cost of slightly lower responsivity at higher wavelengths.
Silicon becomes transparent to radiations of longer than 1100nm wavelength. On the contrary, wavelengths in ultraviolet region are absorbed in the first 100nm thickness of silicon. Even the most careful surface preparation leaves some surface damage, which reduces collection efficiency for this wavelength. High absorption coefficient of silicon in blue- and UV-enhanced spectral region leads to carrier generation within heavily doped p+ (or n+) contact surface of P-N and PIN photodiodes. This causes rapid decrease in quantum efficiency of this region due to shorter lifetime and surface recombination.
Spectral response of photodiodes in blue- and UV-enhanced region is enhanced by minimising near-surface recombination. This is achieved by using techniques such as thin and highly graded p+ (or n+ or metal Schottky) contacts, lateral collection to minimise percentage of heavily-doped surface area and passivation layer. A passivation layer such as silicon nitride, silicon dioxide or titanium dioxide can reduce surface recombination.