Materials, modern diodes

Instruments based on the contact principle can further be divided into two classes mechanical thermometers and electrical thermometers. Mechanical thermometers are based on the thermal expansion of a gas, a liquid, or a solid material. They are simple, robust, and do not normally require power to operate. Electrical resistance thermometers utilize the connection between the electrical resistance and the sensor temperature. Thermocouples are based on the phenomenon, where a temperature-dependent voltage is created in a circuit of two different metals. Semiconductor thermometers have a diode or transistor probe, or a more advanced integrated circuit, where the voltage of the semiconductor junctions is temperature dependent. All electrical meters are easy to incorporate with modern data acquisition systems. A summary of contact thermometer properties is shown in Table 12.3. 

Many modern semiconductor devices comprise alternating layers of different materials forming superlattices and multiple quantum wells. One well-known example of such structures is the diode laser, a mass-produced device. This device depends on confinement of charges in the two-dimensional structures for enhanced laser output at lowered current thresholds. Such alternating semiconductor layers are usually manufactured either by chemical vapor deposition or by molecular beam epitaxy. The thickness of the layers can be closely controlled in both techniques. As mentioned earlier, electrodeposition also allows good control of thickness. 

Nanocomposites are multiphased materials with at least one phase in the nanometer range. The mechanical, electrical, thermal, optical and electrochemical properties of nanocomposites are different from that of component materials. Nanocomposites have potential applications in almost every field of the modern day scenario. They play a key role in tailoring properties to suit any technology and have potential applications like light-emitting diodes, photodiodes, photovoltaic cells, smart microelectronic devices, gas sensors, etc. 

One disadvantage of the glass electrode for measuring pH is its fragility. A modern development uses a field effect transistor where the gate surface is coated with a material, often aluminum oxide, whose surface is sensitive to pH. As the potential at the surface changes with pH, the current induced through the transistor varies. A temperature diode, incorporated in the electrode... 

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