MOSFET: Mass-Produced Mini-Machines

Innovation Jul 21, 2021

Wake up. Check your phone. Use the bathroom. Check your social media. Eat breakfast. Check your email. Go to college. Check your laptop.

Technology has become so intertwined with our daily lives that it's very easy to lose sight of just how much we've managed to integrate ourselves with it. We regularly interact with devices and AI, almost as much as real people. We see this with Alexa on our coffee table, Cortana on our desks, and Siri in our pockets. Electrical science and electronics have pervaded deep into every nook and cranny of our lifestyles. But if we stop and take a minute to self-assess, there's a much more profound truth that presents itself – we don't need to know the first thing about electronics to use any of these devices.

Therein lies the power of abstraction – one of the guiding principles of many areas of study, including electronics. Different people only need to concern themselves with the particular level of abstraction that is relevant to them. For instance, knowledge about a cell phone's inner components and workings only affects the manufacturer and sometimes the retailer but seldom concerns the customer. Be this as it may, we can gain valuable understanding of a subject if we deign to step outside our immediate level of abstraction. In this article, we'll be examining the functioning and applications of MOSFET, or the Metal Oxide Semiconductor Field Effect Transistor – a cornerstone of modern electronics, omnipresent in today's world. This is a field-effect transistor made of silicon that utilizes oxidation (controlled) to measure the chemical, physical and environmental variables.

The MOSFET, developed in Bell Labs in 1959, started nothing short of an electronics renaissance. Its small size, relatively simple design, and ability to be mass-produced and continuously miniaturized all lent to it becoming the most fundamental device for electronics, computers, telecom, sensors, power electronics and many other fields.

Alright, so what's the deal with that monster of a name? Let's try to break it down word-by-word, and see if we can gain a more intuitive understanding of these devices. Starting at the end of the acronym, we see the word 'transistor.' Here's a quick recap from 12th grade: a transistor is an electronic device used in two main ways: an amplifier and a switch. They are one of the most basic device components that we can construct using semiconductors. Simply put, when working as an amplifier, the transistor can convert a small input signal into a big output signal. When acting as a switch, the transistor is used to switch a circuit 'ON' or 'OFF.' Okay, that's the first of those words dealt with.

Let's now skip the 'FE' and head over to the 'MOS' part of MOSFET. This stands for 'Metal Oxide Semiconductor' - a metal gate, oxide insulation and a (silicon) semiconductor. The primary material used in the MOSFET is a semiconductor (i.e., silicon). That's our 'S.' The metal used in the early 1970s was aluminium, but now it has been replaced with highly efficient and doped material polysilicon. Polysilicon is resistant to high temperatures and is easily obtained via the chemical vapour deposition process. We then coat part of this material with a non-conducting oxide (usually a silicon oxide). That's the 'O'. The gate oxide, formed by thermal oxidation, separates the gate from the underlying source and drain terminals when the transistor is turned on. And finally, on top of this oxide layer, we also deposit a layer of some metal, like palladium. That's the 'M.'

Tackling our final two letters, 'FE,' is now a piece of cake. This transistor works by creating an electric field that affects how positive and negative charges move through the material. This is done by applying a voltage at one of the terminals. In other words, it creates a 'field effect.' Our MOSFET can be one of two types (depletion mode or enhancement mode) based on what this voltage does. And voilà, you have now grasped all the essentials of the workings of a MOSFET!

I mentioned before how MOSFETS are omnipresent, and I wasn't exaggerating. They are the central component in integrated circuits (ICs). Described as the "workhorse of the electronics industry," MOSFETs have various applications ranging from digital telecommunication systems to image sensing. They're also the origin of all microprocessors; they're used in various types of sensors, in calculators, vehicles, watches, air conditioning, LEDs… the list goes on and on. They have even shown use in studying quantum technology and quantum effects. One of the exciting applications is MOS sensors, widely used in the chemical and medical industry. MOS chemiresistors, ChemFET(chemical field-effect transistors), ISFET (ion-sensitive field-effect transistors) are interesting applications and a great example of how transistors play an essential role in chemical analysis.

A ChemFET is a chemically sensitive field-effect transistor used as a sensor to measure chemical concentrations in solutions. When the (target) analyte concentration changes, the current through the transistor will also change accordingly. The analyte solution separates the source and gate electrode. Due to a semi-permeable membrane on the FET surface containing receptor moieties that preferentially bind the target analyte, a concentration gradient between the solution and the gate electrode arises. This concentration gradient creates a chemical potential between the source and gate, which the FET measures. The design of a ChemFET is similar to the MOSFET, with its threshold voltage depending on the concentration gradient. The mobility of the analyte ion through the substrate to the receptor is improved by the use of ionophores; for example, the anion's mobility is facilitated by using quaternary ammonium salts like tetraoctylammonium bromide. The use of ChemFET varies from selective anion/cation sensing to a solid-state gas sensor. The ChemFET can be used in the liquid as well as a gas phase to detect the analyte.

An ISFET; ion-sensitive field-effect transistor is used for measuring the ion concentrations in the target solutions. The principle behind the sensor is that when the ion concentration changes, the current will change accordingly. Its build is the same as the MOSFET, but the metal gate is replaced with an ion-sensitive membrane, typically SiO2, Al2O3. The mechanism responsible for the oxide surface charge is explained by the site binding model, which explains the equilibrium between the Si-OH surface sites and the H+ ions in the solution. The hydroxyl groups that coat an oxide surface like SiO2 can donate or accept a proton and thus behave in an amphoteric way. ISFET forms the base for BioFET and is subsequently used in genetic engineering, glucose measurement and pH sensing.  

These were, of course, wildly simplified overviews of the whole picture. Still, it is a testament to how learning just a little bit of what goes on under the hood can be extremely enlightening. Hopefully, now you know just a bit more about these tiny, extremely useful devices that are everywhere, including right under our noses!




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