Università degli Studi di Roma Tor Vergata Dipartimento di Ingegneria Elettronica Analogue Electronics Paolo Colantonio A.A. 2015-16
Introduction: materials Conductors e.g. copper or aluminum have a cloud of free electrons (at all temperatures above absolute zero). If an electric field is applied electrons will flow causing an electric current Insulators e.g. polythene electrons are tightly bound to atoms, so, only a few can break free to conduct electricity Semiconductors e.g. silicon or germanium at very low temperatures these have the properties of insulators as the material warms up some electrons break free and can move about, and it takes on the properties of a conductor albeit a poor one however, semiconductors have several properties that make them distinct from conductors and insulators 2 32
Pure semiconductors The Silicon is one of the most adopted semiconductor to realize electronic devices, both discrete or integrated It is constituted by a tetrahedral cell, with an atom in each vertexes, linked to the other by covalent bounds between the 4 electrons of each atom At 0 Kelvin, each electron is linked to its atom and the conductivity is null Increasing material temperature, thermal vibration results in some bonds being broken, generating free electrons which move about These leave behind holes (positive charge carries) which accept electrons (negative charge carriers) from adjacent atoms and therefore, also move about At room temperatures there are few charge carriers, thus pure semiconductors are poor conductors (this is intrinsic conduction) The atomic structure of silicon Temperature 3 32
Doping of semiconductors The addition of small amounts of impurities drastically affects the properties of a semiconductor. This process is known as doping. Adding Phosphorus P (e.g. pentavalent material) into the crystal lattice of Silicon Si (e.g. tetravalent material), four of P s valence electrons are tightly bound by the covalent bonding. The fifth electron is only weakly bound and is therefore almost free to move within the lattice and contribute to an electric current. 4 32
Doping of semiconductors When the added materials form an excess of electrons, they are called donor impurities and produce a n type semiconductor When the added materials form an excess of holes, they are called acceptor impurities and produce a p type semiconductor Both n type and p type materials have much greater conductivity than pure semiconductors: extrinsic conduction The dominant charge carriers in a doped semiconductor (e.g. electrons in n type material) are called majority charge carriers. The other type are minority charge carriers The overall doped material is electrically neutral 5 32
Device technology Excepting bulk devices, which exploit basic properties of semiconductors, the large amount of electronic devices are based on the junction of different semiconductor materials with different doping profile, i.e. with different concentration of impurity (e.g., p n junction), or by metal and semiconductors (e.g. Schottky junction). The modern electronic technology employ a variety of semiconductor Pure e.g. constituted by a single atomic specie, Composed e.g. constituted by different atomic elements like Gallium Arsenide (GaAs), Indium Phosphorus (InP), Gallium Nitride (GaN), Silicon Carbide (SiC), Silicon Germanium (SiGe). 6 32
pn junctions When p type and n type materials are joined, this forms a pn junction The majority charge carriers on each side diffuse across the junction where they combine with (and remove) the charge carriers of the opposite polarity Hence, around the junction there are few free charge carriers and we have a depletion layer (also called a spacecharge layer) The diffusion of positive charge in one direction and negative charge in the other produces a charge imbalance, resulting in a potential barrier across the junction 7 32
Potential barrier The barrier opposes the flow of majority charge carriers and only a small number have enough energy to surmount it This generates a small diffusion current The barrier encourages the flow of minority carriers and any that come close to it will be swept over This generates a small drift current For an isolated junction these two currents must balance each other and the net current is zero 8 32
Forward and Reverse bias Forward bias If the p type side is made positive with respect to the n type side the height of the barrier is reduced, thus more majority charge carriers have sufficient energy to surmount it the diffusion current therefore increases while the drift current remains the same there is thus a net current flow across the junction which increases with the applied voltage Reverse bias if the p type side is made negative with respect to the n type side the height of the barrier is increased, thus the number of majority charge carriers that have sufficient energy to surmount it rapidly decreases the diffusion current therefore vanishes while the drift current remains the same thus the only current is a small leakage current caused by the (approximately constant) drift current the leakage current is usually negligible (a few na) 9 32
Forward and reverse currents The current flowing through a pn junction can be approximately related to the applied voltage by the expression I is the current through the junction I S is a constant called the reverse saturation current q is the electronic charge V is the applied voltage k is Boltzmann s constant, T istheabsolutetemperature is a constant in the range 1 to 2 determined by the junction material (e.g. approximately 1 for Ge and 1.3 for Si). For most purposes we can assume =1 10 32
Diode IV characteristic At room temperature (T=300K) V T 26mV Forward bias Reverse bias 11 32
Diodes An ideal diode passes electricity in one direction but not in the other A pn junction is not an ideal diode, but it does have a characteristic that approximates to such a device (semiconductor diode) 12 32
Generally have a turn on voltage of about 0.5 V Generally have a conduction voltage of about 0.7 V Have a breakdown voltage that depends on their construction perhaps 75 V for a small signal diode perhaps 400 V for a power device have a maximum current that depends on their construction perhaps 100 ma for a small signal diode perhaps many amps for a power device Silicon Diodes K A 13 32
Application of diodes One application of diodes is in rectification Example: half-wave rectifier In practice, no real diode has ideal characteristics but semiconductor pn junctions make good diodes 14 32
Sometimes we represent a diode by an equivalent circuit. Models have different levels of sophistication Diode equivalent circuits 15 32
Sometimes we represent a diode by an equivalent circuit. Models have different levels of sophistication Diode equivalent circuits 16 32
Diode equivalent circuits 17 32
Diode equivalent circuits 18 32
Diode equivalent circuits 19 32
Diode equivalent circuits 20 32
Diode characteristics Conduction Voltage In a real diode I V characteristic it can be noted the existence of a voltage V ON, below of which the current is negligible (less 1% of diode maximum current). V ON 0.2V for Ge V ON 0.7V for Si Saturation current For negative voltage (reverse bias) the current is I S (generally negligible) ma for Si na for Ge Logarithmic scale behavior Increasing the forward voltage V, the voltage drop across the semiconductor becomes significant and the device behaves like a resistor 21 32
Diode characteristics Effects of temperature There is an intrinsic relationship with the operating temperature T For a given current I, the voltage V is inversely proportional to T For a silicon diode (similar for germanium), V decreases by about 2 mv per C The diode current is also affected by the reverse saturation current, which increases with temperature I S increases by about 7% per C I S twined for T=10 C 22 32
Diode capacitive behavior Areversed biased diode has two conducting regions separated by an insulating depletion region This structure resembles a capacitor Variations in the reverse bias voltage change the width of the depletion layer and hence the capacitance, namely Transient Capacitance C T Aforward biased diode shows a capacitive behavior due to the injection of minority charges across the junction The resulting Diffusion Capacitance C D is greater than C T 23 32
Diode transient behaviour When a varying signal is applied to a diode, the electrical response of diode shows a transient behavior. Considering the following situation 24 32
Reverse breakdown If a large reverse voltage is applied across a pn junction, a breakdown phenomenon can be observed, that can be caused by two mechanisms Zener breakdown In devices with heavily doped p and n type regions the transition from one to the other is very abrupt and the depletion region is few nanometers thick This produces a very high field strength across the junction that can pull electrons from their covalent bonds, resulting in a large reverse current The current produced by Zener breakdown must be limited by external circuitry to prevent damage to the diode The breakdown voltage is largely constant and the Zener breakdown normally occurs below 5V The breakdown voltage decreases very slightly with increasing temperature (the energy required to break the covalent bonds is reduced being increased the energy acquired by each electrons) 25 32
Reverse breakdown Avalanche breakdown Occurs in diodes with more lightly doped materials, where the field strength across junction is insufficient to pull electrons from their atoms, but is sufficient to accelerate the electrons within the depletion layer The electrons loose energy by colliding with atoms If they have sufficient energy they can liberate other electrons, leading to an avalanche effect Usually occurs at voltages above 5V The voltage at which avalanche breakdown occurs increase with junction temperature (the increased temperature increase the thermal vibration, thus increasing the collision before the electrons reach the sufficient velocity to break the covalent bonds) 26 32
Special purpose diodes Varactor Areversed biased diode has two conducting regions separated by an insulating depletion region This structure resembles a capacitor Variations in the reverse bias voltage change the width of the depletion layer and hence the capacitance This produces a voltage dependent capacitor The Varactor are diodes constructed to emphasize such capacitive behavior They are used in applications such as automatic tuning circuits Symbol Equivalent circuit R r R s R r = reverse bias resistance R s = semiconductor bulk resistance C T = reverse bias capacitance C T 27 32
Special purpose diodes Zener Uses the relatively constant reverse breakdown voltage to produce a voltage reference Breakdown voltage is called the Zener voltage, V Z Symbol 28 32
Special purpose diodes Zener It is used as a voltage stabilizer, since the voltage across the Zener is practically fixed to V Z value 29 32
Impossibile trovare nel file la parte immagine con ID relazione rid4. Special purpose diodes LED The Light Emitting Diode (LED) is a semiconductor diode constructed in such a way that in the pn junction, when it is forward biased, the charges injected in the region where they are minoritary (holes in region n type and electrons in region p type) will recombine emitting light. Such phenomenon happens in the depletion layer region A range of semiconductor material can be used to produce infrared or visible light of various colors Typical devices use materials such as gallium arsenide (GaAs), gallium phosphide (GaP) or gallium arsenide phosphide (GaAsP) Symbol Material Light length Color (nm) GaAs 910 Infrared GaP 560 Green GaAsP 650 Red 30 32
Photodiode They are devices able to transform light in electrical current Without light the photodiode behaves like a normal diode, with an I V characteristic crossing the origin axes By lighting the pn junction reverse biased, with a proper incident radiating length (i.e. the energy of the incident photon shall be larger than the energy gap), there is an increase of reverse current due to the increase of free charges The efficiency of the illumination is a function of the light spot distance from the pn junction Special purpose diodes Symbol 31 32
Solar cells If the pn junction of a photodiode is used in the fourth quadrant, being IV negative then electrical energy is produced I ph short circuit current proportional to the illumination intensity V ph photovoltaic potential With Siliconsilicon 32 32