Glossary a To F

One of the terminals – called “drain” (D) – is connected to one of the n-type regions; “source” (S) terminal is connected to the p-type and the other n-type region; and “gate” (G) is placed across the p-type substrate, isolated with a very thin layer of glass; interestingly, this layer of glass is so thin that it is easily destroyed by electrostatic discharge – requiring all MOSFETs to be handled with care. Bipolar junction transistors: BJTs are a bit more messy predecessor to FETs, allowing the current between their two terminals – “collector” (C) and “emitter” (E) – to be controlled by a smaller current flowing from the third terminal – “base” (B) – to the emitter. Current-limiting resistors or constant current supplies are commonly employed to prevent trouble. Photodiodes work in a very similar way, but are used with an external voltage to achieve a current dependent on the amount of light shining on a reverse-biased junction. These principles are used in a number of piezoelectric devices, the most common of which are crystal oscillators – where the crystal vibrates at a resonant frequency when subjected to an external current, similar to some capacitor-inductor arrangements we will discuss later on.

Piezoelectric crystals: certain materials tend to generate voltages in response to mechanical strain, and vice versa – contract or expand in response to applied fields or the currents flowing through them; in some settings, this action may in turn affect the applied voltage or admitted current, resulting in oscillating action. Low-pass and high-pass filters can be cascaded to form band-pass or band-stop filters; identical filters can also be stacked to achieve a steeper response curve (n-th order filters, with gn frequency transmission function). You can use this arrangement to adjust the speed of a motor with a small potentiometer, or perform other non-critical tasks of this nature; but for anything that requires precision and repeatability, this is just poor engineering: people should be able to substitute transistors used in your circuit with comparable alternatives, or use a 5% accuracy input resistor, and still have it work. Potentiometers are usually not meant to conduct significant currents (and dissipate the resulting heat); therefore, using them requires some care.

These are always clearly labeled. Other common types of field-effect transistors are p-channel MOSFETs, which use p-n-p junctions, and switch on with a gate voltage is lower than the source voltage (and have slightly inferior electrical characteristics); four-terminal MOSFETs with no internal connection between the source and the middle semiconductor layer (the fourth terminal connected to this area is referred to as “body”), useful in some switching applications; less common depletion mode MOSFETs, which have a reversed operation, and are normally conductive until a field is applied to disrupt the channel; and somewhat simpler, depletion-only JFETs, which do not feature a glass insulator, and exhibit higher transconductance, but have the undeniable benefit of not having a gate-to-source threshold voltage to speak of. The most common variety today are electret microphones that utilize an internal, permanently charged membrane to form an interesting type of a capacitor; but even a regular speaker and some vanilla capacitors exhibit some audio sensitivity.

MOSFETs – metal-oxide-semiconductor field effect transistors – are perhaps the most intuitive, if recent, variety. This arrangement of connections – shown below on the left – results in a normal p-n diode that allows conduction from source to drain – but no conduction the other way round; MOSFET transistors are operated with this junction reverse-biased – i.e., drain more positive than source in case of n-p-n devices. This negative voltage will prevent that MOSFET from turning on until the charge is dissipated through the voltage divider (which takes time proportional to the capacitances and resistances involved) – and the gate is positive enough again. Photovoltaic cells rely on a p-n junction to generate free charge carriers in response to photon excitation, and also to separate charges – so that an electromotive force is created across their terminals, what are electric cables forming a voltage source. BJTs consist of a nominally non-conductive junction, most commonly n-p-n – with the outer layers connected to the collector and emitter terminals, and a very thin p-type layer sandwiched in between connected to the base terminal (see image below, left).