{"id":3785,"date":"2019-11-14T17:45:40","date_gmt":"2019-11-14T22:45:40","guid":{"rendered":"https:\/\/www.circuitcrush.com\/?p=3785"},"modified":"2021-06-30T14:38:25","modified_gmt":"2021-06-30T18:38:25","slug":"what-is-a-transistor","status":"publish","type":"post","link":"https:\/\/www.circuitcrush.com\/what-is-a-transistor\/","title":{"rendered":"An Introduction to Transistors"},"content":{"rendered":"<p>When someone poses the question <em>what is a transistor<\/em> or <em>how does a transistor work<\/em> they can literally find tons of material that answers those questions and gives more detail than perhaps one would care for.<\/p>\n<p>Indeed, many books exist on the subject of transistors, semiconductor physics, transistor characteristics, and their operation. They range from the beginner level to the engineering level and even on to cutting edge research and new breakthroughs in the design of transistors and semiconductors.<\/p>\n<p>Because of this, the answers you will find to the questions above (and similar ones) range from the simplistic to the extremely complicated.<\/p>\n<p><!--more--><\/p>\n<p>Even while ignoring engineering-level concepts concerning transistors it\u2019s absurd to think that all the things a hobbyist should know about them can appear in one article.<\/p>\n<p>Toward that end, this will be the first post on the ubiquitous transistor and the first of many others that will show up in the future.<\/p>\n<p>If you\u2019re new to electronics or need to brush up on semiconductor basics I strongly suggest you read <a href=\"https:\/\/www.circuitcrush.com\/how-diodes-work\/\" target=\"_blank\" rel=\"noopener noreferrer\">How Diodes Work \u2013 An Introduction<\/a>. This post will cover some basic semiconductor principles if you\u2019re unfamiliar with the topic.<\/p>\n<p>I\u2019m going to take three things as a given before we dive into the meat of the post.<\/p>\n<ul>\n<li>People of all levels of experience &#8212; from complete novice to electronics guru &#8212; will read this.<\/li>\n<li>There is a crap-load of things to say and learn about transistors.<\/li>\n<li>I\u2019m going to assume you know nothing about transistors and start with very simple, basic information in this post.<\/li>\n<\/ul>\n<p>In that vein, this introductory post on how transistors work will start with some history of the transistor (and what was in use before it), the main types of transistors a hobbyist will see, and a mile-high overview of what transistors are used for and how they work. In the future we\u2019ll go into a lot more detail on the individual types of transistors and their uses.<\/p>\n<h1><strong>What is a Transistor?<br \/>\n<\/strong><\/h1>\n<h2><strong>The Land Before Transistors<br \/>\n<\/strong><\/h2>\n<p>Before the transistor the device of choice (perhaps because it was the only choice) was the vacuum tube.<\/p>\n<p>These days vacuum tubes are pretty much obsolete, though they still do find their way into some niche applications. In fact, certain audiophiles swear vacuum tubes give sound a warmer, richer quality than those omnipresent transistors.<\/p>\n<p>The structure of a basic tube is as follows\u2026<\/p>\n<p>A glass container encloses the vacuum tube, sealing it. As you may guess the tube contains no air, hence the name vacuum tube. The parts of a basic tube are the heater, cathode and anode. Electrical connections to these parts are made through the base of the container.<\/p>\n<p>Figure 1 depicts a variety of vacuum tubes.<\/p>\n<p>&nbsp;<\/p>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-3787 \" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/Vacuum-Tubes-300x200.jpg\" alt=\"Before the transistor there were vacuum tubes\" width=\"407\" height=\"271\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Vacuum-Tubes-300x200.jpg 300w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Vacuum-Tubes-150x100.jpg 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Vacuum-Tubes-768x512.jpg 768w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Vacuum-Tubes-600x400.jpg 600w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Vacuum-Tubes.jpg 1000w\" sizes=\"(max-width: 407px) 100vw, 407px\" \/><\/p>\n<p style=\"text-align: left;\"><strong><em>Figure 1: an assortment of vacuum tubes.<\/em><\/strong><\/p>\n<p>Unfortunately, vacuum tubes have a few drawbacks.<\/p>\n<p>First, they are fragile and break easily. Shock or vibration can kill a tube, so you need to handle them with care.<\/p>\n<p>Second, they slurp up watts like a ravenous man coming off a hunger strike. The heater requires a great deal of power which makes them unsuitable for applications requiring battery power.<\/p>\n<p>Their size is another disadvantage. Early computers, composed of vacuum tubes, would fill a large room (in addition to consuming more than a hundred kilowatts of power). Today we can cram all that computing power on a single, inexpensive chip. Transistors are the basic building blocks of such chips. This is possible because transistors can be made much smaller than tubes.<\/p>\n<p>And finally, the tube heater has a finite life span. This means the tubes needed to be replaced regularly. Modern transistors are much more robust.<\/p>\n<h2><strong>History of the Transistor<br \/>\n<\/strong><\/h2>\n<p>In 1948 William Shockley, Walter Brattain, and John Bardeen were able to produce the first working point-contact transistor at Bell Telephone Laboratories.<\/p>\n<p>Unfortunately, the original point-contact transistors made by Shockley and company were extremely unreliable. It would take another twelve years &#8212; until 1960 &#8212; to make transistors more rugged and viable for use in commercial products. A transistor many of us are still familiar with today \u2013 the bipolar junction transistor or BJT \u2013 had arrived.<\/p>\n<p>In the early 1960s, many electronics manufactures started replacing vacuum tubes with transistors, especially in low-power and low-frequency applications.<\/p>\n<p>Around the same time Robert Noyce found that more than one transistor could be constructed on a single piece of semiconductor material. The integrated circuit, or IC, was born.<\/p>\n<p>Research and development into semiconductors later yielded another type of transistor many of us are familiar with \u2013 the field effect transistor or FET. These (especially the MOSFET) became viable in the mid-1960s and started showing up in products late in that decade.<\/p>\n<p>Since then, electronics has never been the same. Many of the electronic devices we take for granted today are only possible because of the transistor. Everything from your smartphone, laptop, TV, kitchen appliances, IoT devices, audio equipment and more exist in the form they do today because of transistors.<\/p>\n<h2><strong>What is a Transistor? Two Main Transistor Types<br \/>\n<\/strong><\/h2>\n<h3><strong>The Bipolar Junction Transistor (or BJT)<br \/>\n<\/strong><\/h3>\n<p>For our purposes, we\u2019re going to talk about the two most common types of transistors that you\u2019ll see.<\/p>\n<p>The first type is the <em>bipolar junction transistor<\/em>, which we\u2019ll refer to as the <em>BJT<\/em> from here on.<\/p>\n<p>BJTs were the original large-scale production transistor and are robust.<\/p>\n<p>They sport three terminals: the <em>base<\/em>, <em>collector<\/em>, and <em>emitter<\/em>.<\/p>\n<p>Refer to figures 2 and 3 for the following discussion. Current flowing into the base controls the amount of current flowing from the collector to the emitter. If the base current is below a certain threshold, the transistor is in <em>cutoff mode <\/em>and no current flows in the collector. <strong>Think of this as an open switch<\/strong>.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-3788 size-full\" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/What-is-a-Transistor-BJT.png\" alt=\"What is a transistor? Here's an NPN BJT\" width=\"408\" height=\"416\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/What-is-a-Transistor-BJT.png 408w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/What-is-a-Transistor-BJT-147x150.png 147w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/What-is-a-Transistor-BJT-294x300.png 294w\" sizes=\"(max-width: 408px) 100vw, 408px\" \/><\/p>\n<p style=\"text-align: left;\"><strong><em>Figure 2: an NPN BJT along with its schematic symbol. When no current flows into the base, the transistor acts like an open switch. With the proper amount of current flowing into the base, current flows from the collector to emitter, essentially shorting them out, like a switch with closed contacts.<\/em><\/strong><\/p>\n<p>When the base current increases, the BJT enters the <em>active region <\/em>where the collector current is proportional to the base current by a <em>gain<\/em> factor (a.k.a. <em>beta<\/em>). For example, if the base current is 10 mA the collector current may be 10 times that (or some other gain factor). If the base current continues to increase, the collector current reaches a maximum value and stops increasing. The BJT is now in <em>saturation mode. <\/em><strong>Think of saturation mode like a closed switch<\/strong>.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-3789 size-large\" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/BJT-Operation-1024x576.jpg\" alt=\"How a BJT works\" width=\"1024\" height=\"576\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/BJT-Operation-1024x576.jpg 1024w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/BJT-Operation-150x84.jpg 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/BJT-Operation-300x169.jpg 300w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/BJT-Operation-768x432.jpg 768w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/BJT-Operation-600x338.jpg 600w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/BJT-Operation.jpg 1280w\" sizes=\"(max-width: 1024px) 100vw, 1024px\" \/><\/p>\n<p style=\"text-align: left;\"><strong><em>Figure 3: the current through the BJT is proportional by some gain factor to the current flowing into the base. So, a little current flowing into base yields a greater current flowing from the collector, through the transistor to the emitter.<\/em><\/strong><\/p>\n<p>For now, that\u2019s all we\u2019re going to say about the operation of the BJT.<\/p>\n<p>There are two types of BJTs: <em>NPN<\/em> and <em>PNP<\/em>. The names have to do with their semiconductor junctions (this is a good time to stop and read the <a href=\"https:\/\/www.circuitcrush.com\/how-diodes-work\/\" target=\"_blank\" rel=\"noopener noreferrer\">post on diodes<\/a> if you need to).<\/p>\n<p>An NPN BJT sandwiches P-type semiconductor material between two layers of N-type semiconductor material. Take a gander at figure 4 if you need help visualizing this.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-3790 size-full\" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-NPN-BJT-Works.png\" alt=\"Structure of a BJT\" width=\"593\" height=\"334\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-NPN-BJT-Works.png 593w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-NPN-BJT-Works-150x84.png 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-NPN-BJT-Works-300x169.png 300w\" sizes=\"(max-width: 593px) 100vw, 593px\" \/><\/p>\n<p style=\"text-align: left;\"><strong><em>Figure 4: an NPN BJT has P-type material between two layers of N-type material.<\/em><\/strong><\/p>\n<h3 style=\"text-align: center;\">Become the Maker you were born to be. Try <a href=\"https:\/\/learnarduinonow.com\">Arduino Academy<\/a> for FREE!<\/h3>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-4238\" src=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/FB_Cover2.png\" alt=\"\" width=\"389\" height=\"148\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/FB_Cover2.png 828w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/FB_Cover2-300x114.png 300w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/FB_Cover2-150x57.png 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/FB_Cover2-768x292.png 768w\" sizes=\"(max-width: 389px) 100vw, 389px\" \/><\/p>\n<p>The schematic symbol for a PNP transistor is slightly different. Figure 5 depicts the schematic symbol for both NPN and PNP BJTs. You may be guessing that a PNP BJT sandwiches N-type material between two layers of P-type material, sort of the opposite of an NPN transistor. If so, you\u2019re right. Think of figure 4 and swap the N-regions with the P-regions to visualize a PNP BJT.<\/p>\n<p>You\u2019ll use NPN transistors more often than the PNP version, but it\u2019s important to be aware of both types.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-3791 size-full\" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/BJT-Schematic-Symbol.jpg\" alt=\"BJT-Schematic-Symbols\" width=\"363\" height=\"264\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/BJT-Schematic-Symbol.jpg 363w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/BJT-Schematic-Symbol-150x109.jpg 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/BJT-Schematic-Symbol-300x218.jpg 300w\" sizes=\"(max-width: 363px) 100vw, 363px\" \/><\/p>\n<p style=\"text-align: left;\"><strong><em>Figure 5: schematic symbols for the 2 types of BJTs.<\/em><\/strong><\/p>\n<h3><strong>The MOSFET<br \/>\n<\/strong><\/h3>\n<p>The second common transistor type likely to cross your bench is the <em>MOSFET<\/em>.<\/p>\n<p>Like the BJT, the MOSFET is a three terminal device. However, the terminals on a MOSFET have different names than those of the BJT.<\/p>\n<p>Before we give the names of the MOSFET\u2019s three terminals it may be useful to know what MOSFET stands for.<\/p>\n<p>MOSFET stands for <em>metal oxide semiconductor field effect transistor<\/em>. A MOSFET is a type of <em>FET<\/em> or <em>field effect transistor. <\/em>There are other less common types of field effect transistors out there such as the <em>JFET<\/em> (<em>junction field effect transistor<\/em>), but for now we\u2019re just going to talk about MOSFETs.<\/p>\n<p>The terminals on a MOSFET are the <em>gate<\/em>, <em>drain<\/em>, and <em>source<\/em>.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-3792 size-medium\" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/MOSFET-300x300.jpg\" alt=\"Typical MOSFET\" width=\"300\" height=\"300\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/MOSFET-300x300.jpg 300w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/MOSFET-150x150.jpg 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/MOSFET.jpg 600w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/MOSFET-100x100.jpg 100w\" sizes=\"(max-width: 300px) 100vw, 300px\" \/><\/p>\n<p style=\"text-align: left;\"><strong><em>Figure 6: a typical MOSFET.<\/em><\/strong><\/p>\n<p>The gate <em>usually<\/em> appears on the left pin if the transistor is facing you with the drain in the middle and the source on the right. The gate of the MOSFET turns it on and off just the like base of a BJT turns it on and off.<\/p>\n<p>And, like the BJT, MOSFETs come in two flavors: n-channel (the schematic symbol in figure 7 depicts an n-channel device) and p-channel. However, there is a big difference between the base of a BJT and gate of a MOSFET. While current entering the base controls the BJT, voltage on the gate controls the MOSFET. This gives the MOSFET an advantage: it consumes very little current (like nano or pico amps which is just leakage current), and thus power while BJTs consume more.<\/p>\n<p>There are other advantages, too. MOSFETs can be made smaller and they are relatively easy to manufacture. Plus, they have a high input impedance.<\/p>\n<p>A disadvantage is that MOSFETs can be sensitive to static electricity, so you need to take care when handling them. An antistatic wrist strap helps with this.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-3793 \" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/What-is-a-Transistor-MOSFET.png\" alt=\"What is a MOSFET?\" width=\"388\" height=\"352\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/What-is-a-Transistor-MOSFET.png 500w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/What-is-a-Transistor-MOSFET-150x136.png 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/What-is-a-Transistor-MOSFET-300x272.png 300w\" sizes=\"(max-width: 388px) 100vw, 388px\" \/><\/p>\n<p style=\"text-align: left;\"><strong><em>Figure 7: a typical n-channel MOSFET and its schematic symbol. If this were a p-channel MOSFET, the arrow would be pointing away from the gate instead of towards it. The schematic symbols for MOSFETs may appear slightly different depending on what the source is, but it\u2019s easy to tell them apart either way.<\/em><\/strong><\/p>\n<p>&nbsp;<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-3794 size-large\" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/MOSFET-Schematic-Symbol-1024x576.jpg\" alt=\"MOSFET-Schematic-Symbol\" width=\"1024\" height=\"576\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/MOSFET-Schematic-Symbol-1024x576.jpg 1024w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/MOSFET-Schematic-Symbol-150x84.jpg 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/MOSFET-Schematic-Symbol-300x169.jpg 300w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/MOSFET-Schematic-Symbol-768x432.jpg 768w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/MOSFET-Schematic-Symbol-600x338.jpg 600w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/MOSFET-Schematic-Symbol.jpg 1280w\" sizes=\"(max-width: 1024px) 100vw, 1024px\" \/><\/p>\n<p style=\"text-align: left;\"><strong><em>Figure 8: for further clarification, we can see 2 different schematic symbols for the same n-channel enhancement type device. The substrate is just a fancy term for the main body of the transistor.<\/em><\/strong><\/p>\n<p>There is yet another important consideration when choosing MOSFETs. In addition to being n-channel or p-channel type, they can also be either depletion-mode or enhancement-mode MOSFETs. This gives us a total of four possible MOSFET types: n-channel depletion-mode, p-channel depletion-mode, n-channel enhancement-mode, and p-channel enhancement mode.<\/p>\n<p>The fact that there are four choices sounds bewildering, but the enhancement-mode MOSFETs are more common than the depletion-mode version. And, as is a similar case with BJTs, the n-channel MOSFETs are more common than the p-channel variety. But remember, things exist for a reason, and since all MOSFET varieties have their special little place in the world of electronics it\u2019s important to be aware of all four MOSFET types.<\/p>\n<p>Since this is an introductory treatment of transistors, we won\u2019t go into any more detail on enhancement vs depletion mode MOSFETs here.<\/p>\n<h3><strong>How MOSFETs are Made: Their Structure<br \/>\n<\/strong><\/h3>\n<p>While N-type and P-type semiconductor material compose MOSFETs, their structure is a bit different than that of a BJT. Refer to figures 9 and 10 for the following discussion.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-3795 size-full\" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/Types-of-MOSFETs.jpg\" alt=\"Structure of a MOSFET\" width=\"480\" height=\"376\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Types-of-MOSFETs.jpg 480w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Types-of-MOSFETs-150x118.jpg 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Types-of-MOSFETs-300x235.jpg 300w\" sizes=\"(max-width: 480px) 100vw, 480px\" \/><\/p>\n<p style=\"text-align: left;\"><strong><em>Figure 9: basic structure of n-channel and p-channel enhancement MOSFETs with their schematic symbols. These symbols include the substrate and a 4<sup>th<\/sup> terminal, but don\u2019t let that confuse you.<\/em><\/strong><\/p>\n<p>Like the BJT, we can see that the different types of semiconductor material form a sandwich \u2013 sort of. A closer look reveals that the substrate (or body), whether N-type of P-type, spans the whole length of the device unlike the material in a BJT.<\/p>\n<p>Also, unlike the BJT, there is no base. Instead, the gate is a very thin layer of silicon dioxide. This is what makes MOSFETs vulnerable to static \u2013 high voltage can punch a hole right through the thin gate destroying\u00a0 the transistor. But this design has some pros; it offers a high input impedance (it\u2019s essentially a capacitor) and enables voltage on the gate to control the device.<\/p>\n<p>On top of the gate, source, and drain lie metal contacts for the electrical connections.<\/p>\n<p>Let\u2019s take a look at a cross section of one of these babies.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-3796 \" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-MOSFETs-Are-Made.jpg\" alt=\"How MOSFETs work\" width=\"601\" height=\"766\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-MOSFETs-Are-Made.jpg 702w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-MOSFETs-Are-Made-118x150.jpg 118w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-MOSFETs-Are-Made-235x300.jpg 235w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-MOSFETs-Are-Made-600x765.jpg 600w\" sizes=\"(max-width: 601px) 100vw, 601px\" \/><\/p>\n<p style=\"text-align: left;\"><strong><em>Figure 10: cross section of the structure of a MOSFET.<\/em><\/strong><\/p>\n<p>Figure 10 shows us a nifty 3D view of the cross section of an n-channel enhancement type MOSFET on top and a flat view on the bottom. The n<sup>+<\/sup> indicates heavily doped N-type material. The device is fabricated on a P-type substrate which is a single crystal silicon wafer. The n<sup>+ <\/sup>regions are created in the substrate. Then a thin layer of silicon dioxide (a great insulator) is deposited on the substrate\u2019s surface. Metal is then placed on top of that to form the electrical connection. Metal also goes on the drain and source regions for electrical connections.<\/p>\n<p>For the purposes of this introductory article, that\u2019s all we need to know about the structure of the MOSFET for now.<\/p>\n<h2><strong>Transistor Operation: Two Modes<br \/>\n<\/strong><\/h2>\n<p>We\u2019ve already seen how the BJT finds use in two different modes. They can be used as an amplifier or as a switch. This depends on whether it\u2019s cutoff, in active mode, or saturation.<\/p>\n<p>We already know that to use a BJT as a switch we need to run it in cutoff (turns the \u201cswitch\u201d off) or saturation (turns the \u201cswitch\u201d on).<\/p>\n<p>If we want to use a BJT as an amplifier, we need to run it in active mode. Amplifier design is a whole other huge subject on its own, so that\u2019s all were going to say about it here.<\/p>\n<p>Similarly, the MOSFET has the same uses. They can be used as either a switch or an amplifier.<\/p>\n<p>MOSFETs operate in a cutoff and a saturation region, but, unlike the BJT, the third region is the triode (or ohmic, as some people call it) region. To use it as a switch, we need to operate the MOSFET in the cutoff and triode regions. If we want to use it as an amplifier, we need to operate it in saturation mode. This can be confusing because while the three modes or regions of operation between a BJT and a MOSFET are similar, they don\u2019t map to each other perfectly.<\/p>\n<p>In digital circuits, MOSFETs are used as switches. In fact, modern digital ICs including microcontrollers and microprocessors employ MOSFETs (along with capacitors, resistors, and other parts) as their most basic building block.<\/p>\n<p>BJTs aren\u2019t used in most modern digital circuits because of their power requirements and other reasons. However, <em>TTL<\/em> or <em>transistor to transistor logic<\/em> chips, which use BJTs, are still around and make great tools for learning, experimentation and prototyping. And, while MOSFETs dominate the digital world, BJTs dominate the analog world finding a home in op amps, amplifiers, audio, motor control, radio, and much more.<\/p>\n<h2><strong>How Transistors Work \u2013 Wrapping Up<br \/>\n<\/strong><\/h2>\n<p>We\u2019ve now answered some basic questions.<\/p>\n<p>We know that vacuum tubes were in use before transistors and we have a little transistor history.<\/p>\n<p>We also answered the questions <em>what is a transistor?<\/em> and <em>how does a transistor work?<\/em><\/p>\n<p>The basic structure and operation of the two most common transistors you\u2019re going to see \u2013 BJTs and MOSFETs \u2013 were discussed.<\/p>\n<p>But there is much more to say about transistors, as was hinted in the beginning of the post.<\/p>\n<p>Future articles will dive a bit deeper into a particular transistor type, use, or mode of operation.<\/p>\n<p>Until then, drop me a comment and let me know what types of transistors you find yourself using most often. Or just let me know if you have any questions about transistors. Maybe I\u2019ll answer it in the next transistor related article.<\/p>\n<h2 style=\"text-align: center;\">Become the Maker you were born to be. Try <a href=\"https:\/\/learnarduinonow.com\">Arduino Academy<\/a> for FREE!<\/h2>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-4238\" src=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/FB_Cover2.png\" alt=\"\" width=\"473\" height=\"180\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/FB_Cover2.png 828w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/FB_Cover2-300x114.png 300w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/FB_Cover2-150x57.png 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/FB_Cover2-768x292.png 768w\" sizes=\"(max-width: 473px) 100vw, 473px\" \/><\/p>\n<p style=\"text-align: center;\"><em><strong>\u00a0<\/strong><\/em><\/p>\n<a target=\"_blank\" href=\"https:\/\/www.drpeterscode.com\/index.php\"><img src=\"https:\/\/www.circuitcrush.com\/wp-content\/plugins\/dpabottomofpostpage\/apixel1x1.jpg\" ><\/a><table><\/table>","protected":false},"excerpt":{"rendered":"<p>When someone poses the question what is a transistor or how does a transistor work they can literally find tons of material that answers those questions and gives more detail than perhaps one would care for. Indeed, many books exist on the subject of transistors, semiconductor physics, transistor characteristics, and their operation. They range from [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":3798,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_genesis_hide_title":false,"_genesis_hide_breadcrumbs":false,"_genesis_hide_singular_image":false,"_genesis_hide_footer_widgets":false,"_genesis_custom_body_class":"","_genesis_custom_post_class":"","_genesis_layout":"","_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[168],"tags":[170,171,169],"class_list":{"0":"post-3785","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-transistors","8":"tag-bjt","9":"tag-mosfet","10":"tag-transistors","11":"entry"},"jetpack_sharing_enabled":true,"jetpack_featured_media_url":"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/What-is-a-Transistor.jpg","_links":{"self":[{"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/posts\/3785","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/comments?post=3785"}],"version-history":[{"count":20,"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/posts\/3785\/revisions"}],"predecessor-version":[{"id":4260,"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/posts\/3785\/revisions\/4260"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/media\/3798"}],"wp:attachment":[{"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/media?parent=3785"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/categories?post=3785"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/tags?post=3785"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}