Photoconductors—also referred to as photoresistors, light-dependent resistors (LDRs) and photocells—are semiconductor photosensors whose resistance decreases with increase in incident light intensity. These are bulk semiconductor devices with no p-n junction. The resistance change in a photoconductor is of the order of 6 decades, ranging from few tens of mega-ohms under dark conditions to few tens or hundreds of ohms under bright light conditions. Other features include wide dynamic response, spectral coverage from ultraviolet to far infrared, and low cost. However, photoconductors have response time of the order of hundreds of milliseconds.
Commonly used materials for photoconductors are cadmium-sulphide (CdS), lead-sulphide (PbS), lead-selenide (PbSe), mercury-cadmium-telluride (HgCdTe) and germanium-copper (Ge:Cu). Inexpensive CdS photoconductors are used in many consumer items like camera light meters, clock radios, security alarms, street lights and so on. On the other hand, Ge:Cu cells are used for infrared astronomy and infrared spectroscopy. Fig. 2 shows some representative photoconductors.
Photoconductors are further classified as intrinsic or extrinsic depending upon whether an external dopant has been added or not to the semiconductor material. Intrinsic photoconductors operate at shorter wavelengths as electrons have to jump from valence to conduction band. Extrinsic photoconductors have a spectral response covering longer wavelengths.
A simple application circuit using a photoconductor would be a potential divider arrangement comprising a fixed resistor and photoconductor. A fixed DC voltage is applied to the potential divider and the voltage appearing across either the fixed resistor or the photoconductor is taken as a measure of light intensity. A better option would be to use the photoconductor in conjunction with an operational amplifier wired either as a trans-impedance amplifier or voltage-mode amplifier. This gives better responsivity and higher output voltage.
Photodiodes are junction-type semiconductor light sensors that generate current or voltage when p-n junction in the semiconductor is illuminated by light of sufficient energy. Spectral response of the photodiode is a function of the band-gap energy of the material used in its construction. Photodiodes are mostly constructed using silicon, germanium, indium-gallium-arsenide (InGaAs), lead-sulphide (PbS) and mercury-cadmium-telluride (HgCdTe). Fig. 3 shows spectral characteristics of these photodiodes.
Types of photodiodes
Depending upon construction, there are several types of photodiodes. These include p-n photodiodes, PIN photodiodes, Schottky-type photodiodes and avalanche photodiodes.
p-n photodiodes comprise a p-n junction. When light with sufficient energy strikes the photodiode, its electrons are pulled up into conduction band, leaving behind holes in valence band. These electron-hole pairs occur throughout p-layer, depletion layer and n-layer materials. When the photodiode is reverse biased, photo-induced electrons move down the potential hill from p-side to n-side. Similarly, photo-induced holes add to the current flow by moving across the junction to p-side. Shorter wavelengths are absorbed at the surface, while longer wavelengths penetrate deep into the diode. p-n photodiodes are used for precision photometry applications like medical instrumentation, analytical instruments, semiconductor tools and industrial measurement systems.
In PIN photodiodes, an extra high-resistance intrinsic layer is added between p and n layers (Fig. 4). This reduces the transit or diffusion time of photo-induced electron-hole pairs, which, in turn, improves the response time. PIN photodiodes feature low capacitance and therefore high bandwidth, which makes them suitable for high-speed photometry as well as optical communication applications.
Single-element PIN photodiodes, quadrant photodiodes, and one- and two-dimensional arrays of photodiodes find extensive application in a variety of sensor systems for military applications. Prominent among these include laser warning sensor suites on armoured fighting vehicles and airborne platforms, laser seekers in laser-guided munitions, laser receivers in laser communication systems and focal-plane arrays in Ladar sensors.
Photodiodes and photodiode arrays integrated with peripheral components such as low-noise pre-amplifier circuits are also available in a single package.
Fig. 5 shows single, quad and two-dimensional arrays of PIN photodiodes in some of the more common package configurations. Photodiodes are available in many more packages including customised ones. In Schottky type photodiodes, a thin gold coating is sputtered onto the n-material to form a Schottky-effect p-n junction. Schottky photodiodes have enhanced UV response.
Avalanche photodiodes are high-speed, high-sensitivity photodiodes utilising an internal gain mechanism that functions by applying a relatively higher reverse bias voltage than PIN photodiodes. These have fast response times similar to that of PIN photodiodes. Also, responsivity of avalanche photodiodes (40-80A/W) is around 100 times more than silicon PIN photodiodes (0.4-0.6A/W). Moreover, avalanche photodiodes offer an excellent signal-to-noise ratio that is comparable to the value offered by photomultiplier tubes. Hence these are used in a variety of applications requiring high sensitivity, such as long-distance optical communication and optical distance measurement. Military laser rangefinders based on time-of-flight principle have their receivers’ front end invariably employing a silicon or indium-gallium-arsenide avalanche photodiode depending upon whether it is an Nd:YAG laser rangefinder or an eye-safe laser rangefinder. Just like PIN photodiodes, avalanche photodiodes are available as single detectors as well as linear or two-dimensional arrays in similar package configurations.