{"id":479,"date":"2017-04-20T12:00:55","date_gmt":"2017-04-20T16:00:55","guid":{"rendered":"http:\/\/www.circuitcrush.com\/?p=479"},"modified":"2021-06-30T21:16:35","modified_gmt":"2021-07-01T01:16:35","slug":"how-diodes-work","status":"publish","type":"post","link":"https:\/\/www.circuitcrush.com\/how-diodes-work\/","title":{"rendered":"How Diodes Work &#8211; An Introduction"},"content":{"rendered":"<h1><strong>How Diodes Work \u2013 An Introduction<\/strong><\/h1>\n<p>This post explores the basics of how diodes work.<\/p>\n<p>A diode is the most basic useful semiconductor device. It has two leads and acts as a one-way gate for electric current. Diodes have a multitude of uses, some of which we\u2019ll touch on later.<\/p>\n<p>We\u2019ll start our tutorial on how diodes work with a very quick overview of basic semiconductor physics. This will serve as the basis for which all diodes work.<\/p>\n<p><!--more--><\/p>\n<h1><strong>How Diodes Work \u2013 Semiconductor Physics 101<\/strong><\/h1>\n<p>Semiconductors are neither good conductors or insulators. However, their resistance can be controlled by a process called doping to either increase or decrease the resistance. This is why semiconductors are so useful.<\/p>\n<p>A pure semiconductor must be modified by doping to give it desirable qualities. Doping is adding impurity atoms to the material.<\/p>\n<p>There are two main types of semiconductor material in electronics: n-type and p-type.<\/p>\n<p>The doping of the silicon (or germanium) will determine if the material is n-type or p-type.<\/p>\n<h2><strong>N-type and P-type Material<\/strong><\/h2>\n<p>To produce an n-type <span class=\"nanospell-typo\" data-cke-bogus=\"true\">semiconducting<\/span> material, the manufacturer will add varying amounts of arsenic, phosphorous, antimony, bismuth and other <span class=\"nanospell-typo\" data-cke-bogus=\"true\">pentavalent<\/span> elements.<\/p>\n<p><span class=\"nanospell-typo\" data-cke-bogus=\"true\">Pentavalent<\/span> means that there are five electrons in the outer most shell (valence shell) of the atom. When a <span class=\"nanospell-typo\" data-cke-bogus=\"true\">pentavalent<\/span> substance is added to pure silicon, four of the five electrons will be bound the silicon atoms, while the fifth will be a free electron. The figure below illustrates this.<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_480\" aria-describedby=\"caption-attachment-480\" style=\"width: 495px\" class=\"wp-caption aligncenter\"><img fetchpriority=\"high\" decoding=\"async\" class=\"wp-image-480 size-full\" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/Semiconductor-physics.jpg\" alt=\"Semiconductor physics n-type material\" width=\"495\" height=\"360\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Semiconductor-physics.jpg 495w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Semiconductor-physics-150x109.jpg 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Semiconductor-physics-300x218.jpg 300w\" sizes=\"(max-width: 495px) 100vw, 495px\" \/><figcaption id=\"caption-attachment-480\" class=\"wp-caption-text\">Figure 1: The arsenic atom is in the middle. Notice the extra electron.<\/figcaption><\/figure>\n<p>That fifth electron in the picture above is not part of the covalent bonds (like the other four are) and requires a small amount of energy to break free.<\/p>\n<p>To produce an p-type <span class=\"nanospell-typo\" data-cke-bogus=\"true\">semiconducting<\/span> material, the manufacturer will add varying amounts of aluminum, gallium, boron, indium and others.<\/p>\n<p>The scenario is similar to the one in the picture, only instead of an extra electron we now have an extra hole.<\/p>\n<p>Holes are like positive charges. Another way to think of a hole is the absence of an electron. Looking at the diagram above, imagine that the As atom is now an Al atom. All electrons in the valance shell of the Al atom will participate and bind to the silicon atoms. But because Al (and the other materials) are all trivalent atoms, there will be an empty space or a \u201chole\u201d where the fourth electron would normally be.<\/p>\n<p>Holes appear to move in the opposite direction of electrons. This is because every time an electron moves into a hole, it creates another hold behind it.<\/p>\n<h2><strong>How Diodes Work: The Depletion Region<\/strong><\/h2>\n<p>N-type and p-type material are of little use by themselves.<\/p>\n<p>Together, they form a P-N junction.<\/p>\n<p>It\u2019s important to note that the manufacturer does not create separate p-type and n-type material and then glue them together. Rather, a single piece of silicon would have each half doped accordingly.<\/p>\n<p>The area on either side of the diode junction is the depletion region (a.k.a. space charge region). This area is depleted of free electrons and holes.<\/p>\n<p>This occurs when the two materials meet. Electrons from the n-type side rush over to fill empty holes on the p-type side. This action \u201cuncovers\u201d positive charges in the n-region because holes are left behind when the electrons migrate to the other side.<\/p>\n<p>Similarly, holes \u201cmove\u201d from the p-side over to the n-type material and uncover negative charges. Refer to the figure below.<\/p>\n<figure id=\"attachment_481\" aria-describedby=\"caption-attachment-481\" style=\"width: 422px\" class=\"wp-caption aligncenter\"><img decoding=\"async\" class=\"wp-image-481 \" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/Diode-depletion-region.png\" alt=\"how diodes work: the depletion region\" width=\"422\" height=\"211\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Diode-depletion-region.png 318w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Diode-depletion-region-150x75.png 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Diode-depletion-region-300x150.png 300w\" sizes=\"(max-width: 422px) 100vw, 422px\" \/><figcaption id=\"caption-attachment-481\" class=\"wp-caption-text\">Figure 2: first we see the P-N junction. Next, we see holes and electrons diffuse to the other side. Finally, the depletion region sets up.<\/figcaption><\/figure>\n<p>Though the whole diode is electrically neutral, the charges in the depletion region set up a barrier potential. For silicon, this is about 0.7 V (at room temperature) and for germanium it\u2019s about 0.3 V. This barrier <span class=\"nanospell-typo\" data-cke-bogus=\"true\">potential<\/span> is also known as the \u201cdiode drop\u201d and is why there is a drop of 0.7 V across a silicon diode in an energized circuit.<\/p>\n<h2><strong>How Diodes Work: Are You Biased? <\/strong><\/h2>\n<p>Because of the barrier potential, a voltage of a certain amplitude and polarity is necessary for conduction (the 0.7 V for silicon). These voltages are the bias voltages. Bias voltages control the width of the depletion region. This controls the resistance of the P-N junction, and the amount of current that can pass through the diode.<\/p>\n<p>Diodes can operate in either forward bias or reverse bias mode. Forward biasing a diode allows current to flow and reverse biasing a diode blocks current from flowing. Let\u2019s discuss this a bit more.<\/p>\n<h1><strong>Diodes: The Practical Side <\/strong><\/h1>\n<p>All the above describes the theory on how diodes work. But, chances are you won\u2019t be fabricating your own P-N material or diodes at your bench.<\/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 decoding=\"async\" class=\"aligncenter  wp-image-4238\" src=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/FB_Cover2.png\" alt=\"\" width=\"376\" height=\"143\" 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: 376px) 100vw, 376px\" \/><\/p>\n<p>The schematic symbol for a typical diode is below.<\/p>\n<figure id=\"attachment_482\" aria-describedby=\"caption-attachment-482\" style=\"width: 454px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-482\" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/Diode-Schematic-Symbol.png\" alt=\"Schematic symbol for a diode.\" width=\"454\" height=\"176\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Diode-Schematic-Symbol.png 454w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Diode-Schematic-Symbol-150x58.png 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Diode-Schematic-Symbol-300x116.png 300w\" sizes=\"(max-width: 454px) 100vw, 454px\" \/><figcaption id=\"caption-attachment-482\" class=\"wp-caption-text\">Figure 3: Schematic symbol for a diode.<\/figcaption><\/figure>\n<p>The anode is the positive side (p-type material) and the cathode is the negative side (n-type material). An easy way to remember this is that the line on the cathode side resembles a negative sign that has been flipped on its end.<\/p>\n<p>When a voltage source connects to the diode with the negative end of the source on the cathode and the positive end on the anode, the diode is forward biased. In other words, making the cathode more negative with respect to the anode forward biases the diode.<\/p>\n<p>Earlier, you may have come to the conclusion that a diode works like a switch.<\/p>\n<p>This is true.<\/p>\n<p>When you forward bias a diode, the \u201cswitch\u201d closes and current flows through it, if the potential applied is enough to overcome the barrier potential set up in the depletion region.<\/p>\n<p>The simple diagram below illustrates this.<\/p>\n<figure id=\"attachment_483\" aria-describedby=\"caption-attachment-483\" style=\"width: 463px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-483\" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-Diodes-Work.jpg\" alt=\"How diodes work\" width=\"463\" height=\"343\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-Diodes-Work.jpg 463w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-Diodes-Work-150x111.jpg 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-Diodes-Work-300x222.jpg 300w\" sizes=\"(max-width: 463px) 100vw, 463px\" \/><figcaption id=\"caption-attachment-483\" class=\"wp-caption-text\">Figure 4: Forward biasing a diode. The resistor limits current to a safe value.<\/figcaption><\/figure>\n<p>If we want to know the current flowing through the diode, we can use the formula<\/p>\n<p>I = (V<sub>source<\/sub> \u2013 V<sub>diode<\/sub>) \/ R<\/p>\n<p>In this case, the diode is silicon, so we get<\/p>\n<p>(10 \u2013 0.7) \/ 1k = 9.3 mA<\/p>\n<h2><strong>How Diodes Work: Reverse Biasing<\/strong><\/h2>\n<p>When we reverse bias a diode, we make the cathode more positive with respect to the anode. This is like turning the 10 V source shown in figure 4 180 degrees. The positive end of the source would now be at the cathode and the negative end is at the anode.<\/p>\n<p>Doing this will increase the width of the depletion region. This happens because free electrons in the n region are drawn to the positive terminal of the source, leaving behind holes (positive charges). Also, the electrons from the negative terminal of the source are attracted to the holes in the p region. They fill the holes in this region near the junction creating negative ions.<\/p>\n<p>Note that a very small current known as a leakage current still flows after reverse biasing a diode, but since it is on the order of <span class=\"nanospell-typo\" data-cke-bogus=\"true\">nanoamps<\/span> or smaller it is negligible.<\/p>\n<p>If you\u2019re having trouble picturing this, the image below can help.<\/p>\n<figure id=\"attachment_484\" aria-describedby=\"caption-attachment-484\" style=\"width: 605px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-484 size-full\" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-Diodes-Work-Depletion-Widening.jpg\" alt=\"How diodes work: the depletion region\" width=\"605\" height=\"449\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-Diodes-Work-Depletion-Widening.jpg 605w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-Diodes-Work-Depletion-Widening-600x445.jpg 600w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-Diodes-Work-Depletion-Widening-150x111.jpg 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-Diodes-Work-Depletion-Widening-300x223.jpg 300w\" sizes=\"(max-width: 605px) 100vw, 605px\" \/><figcaption id=\"caption-attachment-484\" class=\"wp-caption-text\">Figure 5: How a diode works in reverse bias.<\/figcaption><\/figure>\n<p>The widening of the depletion region will continue until the barrier potential matches the potential of the voltage source, as shown in the picture.<\/p>\n<h1><strong>Diode Voltage-Current Characteristics<\/strong><\/h1>\n<p>Refer to the picture below for the following discussion.<\/p>\n<figure id=\"attachment_485\" aria-describedby=\"caption-attachment-485\" style=\"width: 662px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-485\" src=\"http:\/\/www.circuitcrush.com\/wp-content\/uploads\/Diode-I-V-Curve.jpg\" alt=\"Diode voltage-current characteristics\" width=\"662\" height=\"642\" srcset=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Diode-I-V-Curve.jpg 662w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Diode-I-V-Curve-600x582.jpg 600w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Diode-I-V-Curve-150x145.jpg 150w, https:\/\/www.circuitcrush.com\/wp-content\/uploads\/Diode-I-V-Curve-300x291.jpg 300w\" sizes=\"(max-width: 662px) 100vw, 662px\" \/><figcaption id=\"caption-attachment-485\" class=\"wp-caption-text\">Figure 6: Diode voltage-current characteristic curve.<\/figcaption><\/figure>\n<p>The top right quadrant of the graph shows the diode in forward bias (as seen in the simple circuit in the inset) and the lower left quadrant shows it in reverse bias.<\/p>\n<p>Notice the point on the x-axis (positive direction) where the current suddenly rises. This is about 0.7 V for silicon diodes. This also resembles the shape of a human knee and is therefore known as the knee voltage. The knee voltage is just another name for the diode\u2019s internal barrier potential. This is why we see a huge spike in current once the voltage reaches 0.7 V. Of course, a given diode can only take so much current, so it will burn out if the current increases too much, as we can see in the graph.<\/p>\n<p>Looking at the reverse characteristics of the diode in the bottom left quadrant of the graph, we can see that diodes does its job and limits current flow to almost zero until we reach 50 V.<\/p>\n<p>This point also resembles a knee.<\/p>\n<p>The spot on the reverse voltage scale at which the diode breaks down and conducts is known as the breakdown voltage. This voltage, which varies depending on the type of diode, is also known as the peak inverse voltage (<span class=\"nanospell-typo\" data-cke-bogus=\"true\">PIV<\/span>). Just as in the forward case, if current continues to increase the diode will eventually burn out.<\/p>\n<p>Notice that curve as a whole is non-linear. We assume a barrier potential of 0.7 V for silicon diodes, but the exact value involves considering the non-linearity of the diode (and ambient temperature) and uses <span class=\"nanospell-typo\" data-cke-bogus=\"true\">exponentials<\/span> and more complicated math.<\/p>\n<p>I won\u2019t bore you with the exact equations, but when using them you\u2019d find the value to be sufficiently close to 0.7 V. The constant voltage drop model (assuming 0.7 V for silicon) is fine for most applications. Also, using the constant drop model enables rapid analysis of circuits employing diodes. If you were to use the exponential model, you\u2019d want to use a SPICE program.<\/p>\n<p>There are a few other models like the piecewise linear model which attempts to <span class=\"nanospell-typo\" data-cke-bogus=\"true\">linearize<\/span> parts of the curve, the ideal model which assumes no barrier potential, and the small signal model which is useful for finding the signal component of the diode voltage.<\/p>\n<p>Perhaps we\u2019ll talk more about these models in a future post. For now, know that the constant drop model is the one you\u2019ll usually use.<\/p>\n<h1><strong>Practical Applications of Diodes<\/strong><\/h1>\n<p>One of the most common applications for diodes is rectifying AC into DC. The power supply on your bench likely employs a bridge rectifier consisting of four diodes.<\/p>\n<p>Diodes also find a home in clipping circuits (a.k.a. limiters) which clip parts of a waveform and <span class=\"nanospell-typo\" data-cke-bogus=\"true\">clampers<\/span> which add a DC component to a waveform. <span class=\"nanospell-typo\" data-cke-bogus=\"true\">Clampers<\/span> are also known as DC restorers.<\/p>\n<p>Voltage multiplication is another common use of diodes (think of a stun gun).<\/p>\n<p>A special type of diode known as a <span class=\"nanospell-typo\" data-cke-bogus=\"true\">Zener<\/span> diode operates in the reverse breakdown region where it can serve as a simple voltage regulator. A voltage regulator ensures a certain voltage output regardless of what the load is like.<\/p>\n<p>Diodes also make good transient suppressors. When working with relays, it is usually a good idea to connect a diode across the coil to absorb the transient that occurs when the <span class=\"nanospell-typo\" data-cke-bogus=\"true\">coil\u2019s<\/span> magnetic field collapses.<\/p>\n<p>And there are other uses for diodes, some of which we\u2019ll revisit in more detail in the future.<\/p>\n<h2><strong>How Diodes Work: That\u2019s a Wrap<\/strong><\/h2>\n<p>Diodes are the most basic semiconductor device with a ton of uses.<\/p>\n<p>There are also many different types of diodes out there that you\u2019re likely to run into or at least hear about.<\/p>\n<p>Some of these include the <span class=\"nanospell-typo\" data-cke-bogus=\"true\">Zener<\/span>, the <span class=\"nanospell-typo\" data-cke-bogus=\"true\">Schottky<\/span>, PIN diodes, tunnel diodes, <span class=\"nanospell-typo\" data-cke-bogus=\"true\">varactors<\/span>, <span class=\"nanospell-typo\" data-cke-bogus=\"true\">LEDs<\/span>, laser diodes, and even more.<\/p>\n<p>What clever uses have you found for diodes? Comment and tell us!<\/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 size-full wp-image-4238\" src=\"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/FB_Cover2.png\" alt=\"\" width=\"828\" height=\"315\" 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: 828px) 100vw, 828px\" \/><\/p>\n<p><em>References:<\/em><\/p>\n<ol>\n<li>Cook, Nigel P. Introductory DC\/<span class=\"nanospell-typo\" data-cke-bogus=\"true\">ACs<\/span> Electronics, 4th Ed. Prentice Hall, 1999. Print.<\/li>\n<li><span class=\"nanospell-typo\" data-cke-bogus=\"true\">Sedra<\/span>, Adel S. &amp; Smith, Kenneth C. Microelectronic Circuits, 5th Ed. Oxford University Press, 2004. Print.<\/li>\n<\/ol>\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>How Diodes Work \u2013 An Introduction This post explores the basics of how diodes work. A diode is the most basic useful semiconductor device. It has two leads and acts as a one-way gate for electric current. Diodes have a multitude of uses, some of which we\u2019ll touch on later. We\u2019ll start our tutorial on [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":490,"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":[79],"tags":[80],"class_list":{"0":"post-479","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-diodes","8":"tag-diodes","9":"entry"},"jetpack_sharing_enabled":true,"jetpack_featured_media_url":"https:\/\/www.circuitcrush.com\/wp-content\/uploads\/How-Diodes-Work_2.jpg","_links":{"self":[{"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/posts\/479","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=479"}],"version-history":[{"count":2,"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/posts\/479\/revisions"}],"predecessor-version":[{"id":4342,"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/posts\/479\/revisions\/4342"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/media\/490"}],"wp:attachment":[{"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/media?parent=479"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/categories?post=479"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.circuitcrush.com\/wp-json\/wp\/v2\/tags?post=479"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}