> For the complete documentation index, see [llms.txt](https://jamesbrayy.gitbook.io/atar/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jamesbrayy.gitbook.io/atar/engineering/electrical/semiconductors.md). # semiconductors *** ## rectifier diodes #### *overview* * ![](/files/2UyM6HeuuzWTdh7FNvAD) * rectifier diodes are the electrical equivalent of a one-way valve * they are classified as a semiconductor because they conduct electricity in one direction but not in the other * most rectifier diodes are made from silicon * rectifier diodes require a specific voltage to operate which is typically around 0.6 V #### *theory* * ``` ![](../../.gitbook/assets/image_2.b5a67f39.png) ``` * *a diode consists of three main regions:* * P-type semiconductor material, which has an excess of holes (positively charged) * depletion layer, which has a lack of electrons and holes (neutral charge) * N-type semiconductor material, which has an excess of electrons (negatively charged) * *when a diode is connected to a power supply with the correct polarity, then we say that it is forward-biased:* * applying a voltage in this direction will push the free electrons towards the empty holes, which in turn reduces the width of the depletion layer. * this means the free electrons will be able to cross the depletion later and therefore current can flow. * *when a diode is connected to a power supply with an incorrect polarity, then we say that it is reverse-biased.* * applying a voltage in this direction will be pull the free electrons away from the empty holes, which in turn increases the width of the depletion layer. * this means that the free electrons will not be able to cross the depletion layer and therefore current can’t flow. #### *issues with rectifier diodes* * **voltage drop:** diodes have a forward voltage drop and this will reduce the output voltage of the rectified signal * **ripple voltage:** regardless of whether it is half-wave or full-wave rectification, there is still a ripple in the output voltage * **reverse leakage:** in an ideal diode, there should be no current flowing in the reverse direction when a reverse voltage is applied. however, real-world diodes exhibit a phenomenon called reverse leakage, where a tiny amount of current does flow in the reverse direction * **temperature dependence:** the characteristics of diodes are temperature-dependent. operating diodes outside their specified temperature range can affect their performance and reliability * **power dissipation:** diodes dissipate power as heat. this can lead to temperature rise and reduced overall efficiency * **overcurrent and overvoltage protection:** diodes can be susceptible to damage from overcurrent conditions or overvoltage spikes. in critical circuits, additional protection components may be necessary to prevent diode failure * **aging and degradation**: like all semiconductor devices, rectifier diodes can experience aging and degradation over time. factors such as operating conditions, temperature cycling, and current/voltage stress can contribute to gradual performance degradation and eventual failure ## applications of rectifier diodes #### *protection against the reverse connection of a DC power supply* * if the DC power supply is connected with the correct polarity, then the diode would be forward-biased, and current will be able to flow through the circuit. * if it was connected with an incorrect polarity, then the diode would be reverse-biased, and this would block current from flowing though the circuit. * ![](/files/G9t9h0wQt87w77AwPk6r) #### *protection against back E.M.F. when switching inductive devices* * for any circuits that contain an inductor (i.e. a solenoid, relay, or motor), a problem can occur when the circuit is switched off or experiences a power interruption. * when this happens, The inductor will release its stored energy as a very short lived but rather strong current that can damage various components in the circuit. * however, if we place a rectifier diode in parallel with the inductor, then there is an alternative pathway for the current to circulate through so that it can dissipate as heat over time instead. * when a diode is used in this manner, it is often referred to as a flywheel diode or a flyback diode. * *inductors:* * an inductor (also known as a coil, choke, or reactor) is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. * they are components that typically consists of an insulated wire wound into a coil around a core. #### *back EMF* * when current flows through an inductor, it builds up a magnetic field/ stores magnetic energy. if we suddenly stop the current (for example, by turning off a switch), the magnetic field collapses. * this creates a large voltage spike known as back emf. * a rectifier diode can be connected in parallel with the inductor to provide a safe path for the energy to dissipate. * this prevents the voltage spike from harming other components in the circuit (in this case the transistor). * ![](/files/Vyd3h1KUve3iK2qAqzis) #### *AC rectification* * *half-wave rectification:* * each time the voltage reverses to become negative, the resulting negative current is blocked by the diode (called a rectifier). * only the positive half of the voltage can pass across the load, which results in a pulsating DC output. * ![](/files/WwmHgolbTMUTSgBmWFkb) * *full-wave rectification:* * each time the voltage reverses to become negative, the resulting negative current has its direction reversed by the arrangement of diodes (called a bridge rectifier). * both the positive and negative half of the voltage can pass across the load, which results in a constant DC output. * ![](/files/Jkrneiinn60jzhz3Uk0u) ## light-emitting diodes (LEDs) #### *overview* * light-emitting diodes will emit light when forward biased but not when they are reversed biased * light-emitting diodes emit different coloured light depending on the semiconductor material used in construction * the voltage they require typically varies from 1.8-3.3 V * ![](/files/8hldYHdAt27tGhE1HO9b) #### *applications* * show whether power is on/off (light = on, no light = off) * indicate a particular instruction (red = stop, yellow = wait, green = go) * displays used in marketing/ advertisement (billboards, signs) * back lighting for personal devices (televisions, phones, computers) ## transistors #### *overview* * a transistor is a semiconductor device used to amplify and switch electronic signals and electrical power * there are many types of transistors available for use in circuits, but for this course we only need to know about negative-positive-negative (NPN) bipolar junction transistors (BJTs) * this name relates to the internal structure of the transistor; three layers of doped silicon that are negative-positive-negative #### *function* * **amplifier:** when it works as an amplifier, it takes in a tiny electric current at one end and produces a much bigger electric current at the other. * in other words, it’s like a current booster * this is useful for things like hearing aids. A hearing aid has a tiny microphone in it that picks up sounds from the world around you and turns them into fluctuating electric currents * these are fed into a transistor that boosts them and powers a tiny loudspeaker, so you hear a much louder version of the sounds around you * **switch:** when it works as a switch, a small electric current flowing through one part of a transistor makes a big current flow through another part of it * in other words, the small current switches on the larger one * this is essentially how all computer chips work * for example, a memory chip contains hundreds of millions or even billions of transistors, each of which can be switched on or off individually * since each transistor can be in two distinct states, it can store two different numbers, zero and one. With billions of transistors, a chip can store billions of zeros and ones ## transistor properties #### *gain* * the ratio of collector current to base current is known as transistor gain (), and is a dimensionless value * It is sometimes also referred to as the beta ratio * for any transistor is determined during its initial design and cannot be changed after manufacture #### *modes of operation* * *cut-off:* * when a transistor is in the fully-off state (like an open switch) * , and thus there is no collector or emitter current either * is equal to the supply voltage * ![](/files/Ti7hrpvG0yjygtQOMeQy) * *forward-active:* * when a transistor is somewhere between the cut-off and saturated state * the region between cut-off and saturation is known as the forward-active region * the transistor will function normally in this region * in the active state, collector current is times the base current * ![](/files/6B03ezWTIdvbXFvh1GWS) * *saturated:* * when a transistor is fully conductive between emitter and collector (passing as much current as the power supply will allow) * the base current asks for more collector current than can be produced by the power supply * the collector current is at a maximum * the collector and emitter are effectively a short circuit * the transistor behaves as though a switch has been closed between the collector and emitter * ![](/files/FL4WpcsnRGLTMjaDWc6v) ## example transistor question * ![](/files/btST5ddbp5wUyT3G1WHg) * calculate the voltage across the 120 kΩ resistor when the relay is activated (the transistor is on) * calculate the power dissipated in the 120 kΩ resistor when the relay is activated * determine the current required to operate the relay ***