Sensor

A sensor is a device that receives and responds to a specific signal or stimulus. This stimulus is a property or condition that the sensor detects and then converts into an electrical signal. In a broad sense, a sensor is a module or subsystem that notices changes in its environment. It then sends this information to other electronics, such as a computer processor. Sensors are essential to modern life, appearing in everything from touch-sensitive elevator buttons to lamps that dim when you touch their base.

To understand how a sensor works, we must look at its sensitivity and its transfer function. Sensitivity describes how much the sensor's output changes when the input quantity changes. For example, if a mercury thermometer moves 1 cm for every 1 °C change in temperature, its sensitivity is 1 cm/°C. Most sensors use a linear transfer function, meaning the relationship between input and output follows a straight line. The sensitivity is essentially the slope of this line. To convert an electrical output, like voltage, back into a measurement like Kelvin, you must divide the output by this slope. Often, an offset is also added or subtracted to ensure the reading is correct. For instance, if a 0 V output represents -40 °C, you must add -40 to the result.

Chemical sensors are a specialized type of sensor that provide information about the chemical composition of a liquid or gas. These devices function through two main steps: recognition and transduction. In the recognition step, molecules of a specific substance, called an analyte, interact with receptor molecules on the sensor. This interaction causes a change in a physical parameter. In the transduction step, an integrated transducer reports this change by generating an output signal. If the recognition material is biological, such as a cell or a protein, the device is called a biosensor. Scientists also use biomimetic materials, like aptamers, to mimic biological functions in these sensors.

While sensors aim to be perfect, they often experience deviations that limit their accuracy. A sensor has a full scale range, which defines the maximum and minimum values it can measure. If the input exceeds these limits, the output signal will reach a plateau. Other errors include sensitivity error, where the slope of the transfer function is incorrect, and offset error, where the signal differs by a constant value. Nonlinearity occurs when the transfer function deviates from a straight line. There are also dynamic errors caused by rapid changes in the measured property over time. Long-term drift can happen over months or years due to physical changes in the sensor itself. Finally, noise represents random deviations in the signal that vary over time.

Technological progress has allowed sensors to move from large machines to microscopic scales. Using MEMS technology, which stands for microelectromechanical systems, engineers can manufacture microsensors in bulk. These microsensors are often much faster and more sensitive than macroscopic versions. There is also a growing demand for disposable sensors. These are used for short-term monitoring or single-shot measurements because they are inexpensive and simple to use. Using disposable sensors allows for data collection without the need for recalibration or the risk of contamination.

One of the most important histories in sensing involves MOSFET sensors. The MOSFET was invented at Bell Labs between 1955 and 1960. Since then, many specialized versions have been developed. In 1970, Johannessen introduced the open-gate field-effect transistor (OGFET), and Piet Bergveld invented the ion-sensitive field-effect transistor (ISFET). The ISFET is widely used in medicine for detecting DNA, glucose, and pH levels. Other developments include the adsorption FET (ADFET) in 1974 and various gas and pressure sensors by the mid-1980s. By the early 2000s, even more advanced versions like the DNA field-effect transistor (DNAFET) were developed.

Sensors are also the foundation of modern digital imaging. In 1969, Willard Boyle and George E. Smith developed the charge-coupled device (CCD). They discovered that electric charges could be stored on tiny MOS capacitors and moved along a row. This breakthrough allowed for the creation of the first digital video cameras for television. Later, Tsutomu Nakamura developed the MOS active-pixel sensor (APS) in 1985. This was followed by the development of the CMOS active-pixel sensor by Eric Fossum and his team in the early 1990s. These technologies allow our modern cameras to capture high-quality images.