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Sensors Technology – MED4 ST10 – Diode Diode Lecturer: Smilen Dimitrov 1 ST10 – Diode Introduction • The model that we introduced for ST 2 ST10 – Diode Introduction • We have discussed – The units of voltage, current and resistance, from both a microscopic and macroscopic (electric circuits) perspective – The definition of an elementary electric circuit, Ohm’s law and Kirschoff Laws – Solving and measurement of voltage divider circuit and more complicated circuits - and applications in sensors – Resistive based sensors – AC current, capacitors, and capacitive based sensors • This time we discuss the diode as an electronic element, and as a basis for different types of sensors 3 ST10 – Diode Semiconductors - intrinsic silicon • • • Diodes and transistors represent electronic components, which in essence are semiconductor structures So far, distinction between: conductor and insulator materials; and – distinction within conductors, between ideal conductors and (ohmic) resistive materials. (semiconductors were left out) In terms of electrical conductance, substances are described as: – Conductors allow electricity to flow through them (mercury, iron, copper, gold) – Insulators prevent electricity from flowing through them (sulphur, etc. Paper, rubber and most plastics) – Semi-conductors are poor conductors, or poor insulators. They allow the partial flow of electricity (Carbon, Silicon, Germanium) 4 ST10 – Diode Semiconductors - intrinsic silicon • • Many materials can have free electrons on normal temperatures - however, in not as great number as metals (resistors); in metals, almost all valence electrons are free Some materials have no free electrons (insulators) • Semiconductors are a class of materials that have conductive properties that fall somewhere between conductors and insulators • Dominant characteristic of semiconductors elements: (Si, Ge) - those are elements with valency 4 – that is, they have four electrons in the valence shell. • This is situation in pure silicon – a semiconductor that is chemically pure is known as an intrinsic semiconductor 5 ST10 – Diode Intrinsic silicon - properties • Silicon has a crystal structure – periodic arrangement of atoms • Silicon has a 3D tetrahedral arrangement - the 2D representation is just an approximation 6 ST10 – Diode Intrinsic silicon - properties • the fact that a silicon atom might have an unfilled valence bond (that is, it is a positive ion), here is treated like a positive movable charge, known as a hole. • Important notion in semiconductors is that both holes and free electrons contribute to electric current. – Holes contribute because they can be seen as movable - as neighbouring electrons may progressively fill the hole, and create the effect that the hole moves in direction of an electric field (opposite to the direction of the electrons) 7 ST10 – Diode Doped semiconductors - P and N • used as so-called doped semiconductors - certain atoms with different valency are added to the 4-valency intrinsic semiconductor, which then changes the conductivity of the material. The added material is known as a dopant. • If 3-valent dopant is used (such as boron), then a single covalent bond from the neighbouring 4-valent silicon remains unfilled, which is a hole. This results with a semiconductor material which has excess of holes, and is known as a P-type • If a 5-valent dopant is used (such as antimony), then a single electron from the dopant can not participate in a bond, so it becomes a free electron. This results with a semiconductor material which has excess of free electrons, known as a N-type semiconductor. Impurity atoms with 5 valence electrons (pentavalent impurities donors) Impurity atoms with 3 valence electrons (trivalent impurities acceptors) 8 ST10 – Diode Doped semiconductors - P and N • When the electron in pure silicon crosses the gap, it leaves behind an electron vacancy or 'hole' in the regular silicon lattice. Under the influence of an external voltage, both the electron and the hole can move across the material. • • • In an n-type semiconductor, the dopant contributes extra electrons, dramatically increasing the conductivity. In a p-type semiconductor, the dopant produces extra vacancies or holes, which likewise increase the conductivity. It is however the behavior of the p-n junction which is the key to the enormous variety of solid-state electronic devices. 9 ST10 – Diode Diode (PN junction) • Basic structure - formed by a contact between a P and N type semiconductor - which is known as a PN junction, or commonly a diode. • When p-type and n-type materials are placed in contact with each other, the junction behaves very differently than either type of material alone. Current will flow readily in one direction (forward biased) but not in the other (reverse biased), creating the basic diode. This non-reversing behavior arises from the nature • of the charge transport process in the two types of materials. • Near the junction, electrons diffuse across to combine with holes, creating a 'depletion region' (zone). 10 ST10 – Diode Diode (PN junction) • a PN junction is visualised though an energy level sketch (The upward direction in the diagram represents increasing electron energy.). Conductivity of the junction can be visualised too • When reverse voltage [reverse bias or inverse polarization] is applied (that is, the N-type is on higher positive potential than P-type), then the depletion region grows, and there is even less possibility of free charges crossing, and current flowing. When direct voltage [forward bias or direct polarization] is applied (that is, P-type is on higher potential than N-type) the depletion region gets smaller, and free charges gain enough energy to cross the barries so current can flow. (In this case the resistance is very small – almost 0.) • 11 ST10 – Diode Diode (PN junction) - visualisation • The behavior of the PN junction is visualised on several resources on the Internet - however, most of them use the energy band diagrams, whose proper interpretation is beyond the scope of the course 12 ST10 – Diode Diode (PN junction) - Hydraulic analogy • Very simplified analogy to a diode in relation to water flow - would be a one way valve • In electronics, a diode is a component that restricts the direction of movement of charge carriers. It allows an electric current to flow in one direction, but essentially blocks it in the opposite direction. Thus the diode can be thought of as an electronic version of a check valve. If a water valve has a rubber flap it can be blown out permanently by too much reverse bias, which is similar to the real thing. • 13 ST10 – Diode Diode as an electronic element • Diode - implies a small signal device with current typically in the milliamp range • Rectifier - implies a power device, conducting from1 to 1000 amps or even higher. • Many diodes or rectifiers are identified as 1NXXXX • A semiconductor diode consists of a PN junction and has two(2) terminals, an anode (+) and a cathode (-). Current (technical direction!) flows from anode to cathode within the diode. (diode is a polar element, since it has a polarity [+] or [-] assigned to its terminals) • In order to analyse circuits with a diode, we need a model of the diode – - a model that will specify the diode branch equation: the dependence between voltage across, and current through the diode – (or in other words, the I-U characteristic of the diode). 14 ST10 – Diode Diode as an electronic element • Commonly used model (branch equation) - Shockley ideal diode equation I Is e VD VT 1 where – I is the diode current, – IS is a scale factor called the saturation current, – VD is the voltage across the diode – VT is the thermal voltage: 26mV @ T=25°C (room temp) • no linear dependency between voltage and current as in ohmic elements so a diode is not an ohmic element • the Shockley equation doesn't account for the processes involved in reverse breakdown • in the reverse bias region for a normal diode, the current through the device is very low (in the µA range) for all reverse voltages upto a point called the peak-inverse-voltage (PIV). Beyond this point a process called reverse breakdown occurs which causes the device to be damaged along with a large increase in current. (though, deliberate for Zener and avalanche diodes) 15 ST10 – Diode Diode as an electronic element • Shockley equation – too complex for quick analysis – so it is simplified: A diode's I-V characteristic can be approximated by two regions of operation. – Below a certain difference in potential between the two leads, the depletion layer has significant width, and the diode can be thought of as an open (non-conductive) circuit. – As the potential difference is increased, at some stage the diode will become conductive and allow charges to flow, at which point it can be thought of as a connection with zero (or at least very low) resistance. This simplification lacks the diode turn-on voltage - one needs to invest certain energy (that is, forward voltage bias) in order to compensate and lower the depletion zone, so that current can start flowing. Once current starts flowing, this turnon voltage can be approximated to stay constant 16 ST10 – Diode Diode as an electronic element • • In a normal silicon diode at rated currents, the voltage drop across a conducting diode is approximately 0.6 to 0.7 volts. – The value is different for other diode types - Schottky diodes can be as low as 0.2 V and – light-emitting diodes (LEDs) can be 1.4 V or more depending on the current.(Blue LEDs can be up to 4.0 V, depending on the type and current.) Including the diode turn-on voltage in the switch model, we arrive at the socalled large-signal diode model 17 ST10 – Diode Measuring (testing) diodes • reason for measuring diodes is to find the proper pins of a diode - and to also make sure it is working properly Testing a diode with a DIGITAL multimeter – Digital multimeters have a special setting for testing a diode, usually labelled with the diode symbol. – Connect the red (+) lead to the anode and the black (-) to the cathode. The diode should conduct and the meter will display a value (usually the voltage across the diode in mV, 1000mV = 1V). – Reverse the connections. The diode should NOT conduct this way so the meter will display 'off the scale' (usually blank except for a 1 on the left). 18 ST10 – Diode Diode - construction • Diodes are produced using a variety of chemical processes. 19 ST10 – Diode Basic circuits - Half wave rectifier • • Half wave rectifier is the simplest diode circuit - it is a series connection of a diode and a resistor. A rectifier is an electrical device that converts alternating current to direct current, a process known as rectification. 20 ST10 – Diode Basic circuits - Half wave rectifier • Half wave rectifier is the simplest diode circuit - it is a series connection of a diode and a resistor. Vi Vd Vo 0 -> I Vi Vd R I Vi Vd R a) When Vi < VdON (or Vi < 0.7V): b) When Vi > VdON (or Vi > 0.7V): – – - the diode is directly polarised (forward biased) so it conducts (it is modelled with an 0.7V voltage generator) – - the current I flowing in the circuit is then calculated by . – - the output voltage Vo = Vi - VdON = Vi 0.7V - the diode is inversely polarised (reverse biased) so it does not conduct (it is modelled with an open switch) – - the current I is zero – - the output voltage Vo is zero 21 ST10 – Diode Basic circuits - Half wave rectifier • One way to visualise this circuits operation, related to a water flow analogy • Also, Falstad applets 22 ST10 – Diode Clipper • Clipper – is a simplest implementation of a “distortion” audio effect a) When Vi < ViON –> ID = 0 so just a voltage divider VD R2 V V E VdON R1 R2 i - but how big is ViON? R2 V VE R1 R2 i Vi VdON VE R1 R2 R2 23 ST10 – Diode Clipper • Clipper – is a simplest implementation of a “distortion” audio effect Vi VdON VE R1 R2 R2 a) When true b) When false – - the diode is directly polarised (forward biased) so it conducts (it is modelled with an 0.7V voltage generator) – - the diode is inversely polarised (reverse biased) so it does not conduct (it is modelled with an open switch) – - the voltage output is fixed – - the voltage output is given by voltage on Vo = VdON + VE divider 24 ST10 – Diode Peak (envelope) detector • Peak detector - very useful circuit - it can help extract the amplitude of an AC signal. – It is useful in getting the envelope of high frequency sugnals as a DC signal - then it is possible to use high frequency sensor output with low speed DAQ hardware like an Arduino. 25 ST10 – Diode Peak (envelope) detector • Peak detector Need Vx – the input voltage when diode starts conducting (will not be solved here) a) When Vi > Vx – - the diode is directly polarised (forward biased) so it conducts (it is modelled with an 0.7V voltage generator) – - the voltage output is fixed on Vo = Vi - VD = Vi - 0.7 As soon as the input voltage has dropped a little below the maximum kept by the capacitor, the diode becomes reverse biased, so it turns off and ID becomes zero. b) When Vi < Vimax – – - the diode is inversely polarised (reverse biased) so it does not conduct (it is modelled with an open switch) Vis - the voltage output o Vi max e t RC 26 ST10 – Diode Peak (envelope) detector • Peak detector in Falstad applet simulator 27 ST10 – Diode Diode logic – AND and OR circuit • Diodes and resistors can be used for implementing the most basic digital logic circuits OR cir cui t AND circ uit 28 ST10 – Diode Light emitting diodes • Special kind of diodes are the light emitting diodes - or LED's. – this device converts electric energy into a different physical quantity (light), and as such it has a definition opposite of a sensor - a LED diode is an actuator! • when LED's are forward biased (and producing light), their resistance is extremely small, so we can treat it as a closed switch (wire). – Thus, connecting a LED directly to a source in forward bias, will short circuit the source and most likely blow up the diode !!!!!! 29 ST10 – Diode Light emitting diodes - interfacing • • A resistor must always be present in series with a LED diode, in order to limit the current. – Then, another important thing is to determine the correct polarity of the LED terminals. Problem is finding the proper resistor value for the resistor - Typically, LEDs will need from 20 - 30 mA to shine A sample calculation of the dropping resistor is included … – Most leds operate at 1.7V although this is not always the case and it is wise to check. – The dropping resistor is simply the net of supply voltage minus the 1.7V led voltage, then divided by the led brightness current expressed as 'amps' (ohms law) 30 ST10 – Diode Sensing application - Photodiode and interfacing • A photodiode consists of an active p-n junction which is operated in reverse bias. When light falls on the junction, a reverse current flows which is proportional to the illuminance. – Photodiodes can be used under either zero bias (photovoltaic mode) or reverse bias (photoconductive mode). • Requirement for reverse current (very small) – makes interfacing more difficult; must use some sort of amplification/buffering (opamp, transistor) 31