Chapter 1 semiconductor diodes

CHAPTER 1: SemiConductor Diodes CHAPTER 1: SEMICONDUCTOR DIODES Table of Contents 1.1 INTRODUCTION 1.2 THE IDEAL DIODE 1.2.1 The Ideal Analysis Procedure 1.3 DIODE TERMINAL CHARACTERISTICS 1.4 GRAPHICAL ANALYSIS 11 1.5 EQUIVALENT CIRCUIT ANALYSIS 17 1.5.1 Piecewise-Linear Techniques 17 1.5.2 Small-Signal Techniques 19 1.6 APPLICATIONS 22 1.6.1 Rectifier applications 22 1.6.2 Waveform filtering 23 1.6.3 Clipping and Clamping operations 24 1.7 ZENER DIODE 27 1.8 VARACTOR DIODE 28 1.9 SCHOTTKY BARRIER DIODE 28 1.10 LIGHT-EMITTING DIODE (LED) 29 Val de Loire Program p.1 CHAPTER 1: SemiConductor Diodes 1.11 TUNNEL DIODES 30 Table of Figures Fig 1-1 Construction and symbol of diode Fig 1-2 Common diode Fig 1-3 Ideal diode Fig 1-5 Ex 1.1 Fig 1-6 Ex 1.2 Fig 1-7 Diode terminal characteristics Fig 1-8 Graphical analysis 12 Fig 1-9 Ex.1.5 13 Fig 1-10 Ex 1.6 15 Fig 1-11 Ex 1.7 16 Fig 1-12 Diode models 17 Fig 1-13 Piecewise-linear techniques 19 Fig 1-14 Dynamic resistance 20 Fig 1-15 Ex 1.8 21 Fig 1-16 Half-wave rectifier 23 Fig 1-17 Waveform filtering 24 Fig 1-18 Clipping 25 Fig 1-19 Clamping 26 Val de Loire Program p.2 CHAPTER 1: SemiConductor Diodes Fig 1-20 Zener diode symbol and characteristics 27 Fig 1-21 Varicap characteristic - C  pF  versus VR 28 Fig 1-22 Schottky diode characteristic 29 Fig 1-23 LED 29 Fig 1-24 Tunnel diode 30 Val de Loire Program p.3 CHAPTER 1: SemiConductor Diodes CHAPTER 1: SEMICONDUCTOR DIODES 1.1 INTRODUCTION - A diode may be defined as near-unidirectional conductor whose state of conductivity is determined by the polarity of its terminal voltage: Two-terminal device that conducts current freely in one direction but blocks current flow in the opposite direction - The two electrodes are the anode which must be connected to a positive voltage with respect to the other terminal, the cathode in order for current to flow (a) (b) Fig 1-1 Construction and symbol of diode - When a p-type material is connected to an n-type material, a junction is formed + Holes from p-type diffuse to n-type region + Electrons from n-type diffuse to p-type region Val de Loire Program p.4 CHAPTER 1: SemiConductor Diodes + Through these diffusion processes, recombination takes place + Some holes disappear from p-type + Some electrons disappear from n-type - A depletion region consisting of bound charges is thus formed Charges on both sides cause electric field → potiential V0 - Potiential acts as barrier that must be overcome for holes to diffuse into the n-region and electrons to diffuse into the p-region 1.2 THE IDEAL DIODE - The symbol for the common or rectifier diode is shown in Fig 12(a) Fig 1-2 Common diode - The device has two terminals, labeled anode (p-type) and cathode (n-type), which makes understandable the choice of diode as its name - The ideal diode is a perfect two-state device that exhibits zero impedance when forward-biased and infinite impedance when reversebiased (Fig 1-3) Val de Loire Program p.5 CHAPTER 1: SemiConductor Diodes Fig 1-3 Ideal diode - Note that since either current or voltage is zero at any instant, no power is dissipated by an ideal diode - In many circuit applications, diode forward voltage drops and reverse currents are small compared to other circuit variables; then, sufficiently accurate results are obtained if the actual diode is modeled as ideal 1.2.1 The Ideal Analysis Procedure The ideal diode analysis procedure is as follows: Step 1: Assume forward bias, and replace the ideal diode with a short circuit Step 2: Evaluate the diode current iD , using any linear circuitanalysis technique Step 3: If iD  , the diode is actually forward-biased, the analysis is valid, and step is to be omitted Step 4: If iD  , the analysis so far is invalid Replace the diode with an open circuit, forcing iD  , and solve for the desired circuit Val de Loire Program p.6 CHAPTER 1: SemiConductor Diodes quantities using any method of circuit analysis Voltage vD must be found to have a negative value Example 1.1: Assume the diode in the circuit below is ideal Determine the value of ID if a) VA = volts (forward bias) b) VA = -5 volts (reverse bias) Fig 1-5 Ex 1.1 Solution a) With VA > the diode is in forward bias and is acting like a perfect conductor so: ID = VA/RS = V / 50  = 100 mA b) With VA < the diode is in reverse bias and is acting like a perfect insulator, therefore no current can flow and ID = Example 1.2 Find voltage vL in the circuit of Fig 1-6(a), where D is an ideal diode Solution Val de Loire Program p.7 CHAPTER 1: SemiConductor Diodes The analysis is simplified if a Thévenin equivalent is found for the circuit to the left of terminals a, b; the result is vTh  R1 vs R1  RS and ZTh  RTh  R1 ||RS  R1RS R1  RS Fig 1-6 Ex 1.2 Step 1: After replacing the network to the left of terminals a,b with the Thévenin equivalent, assume forward bias and replace diode D with a short circuit, as in Fig 1-6 (b ) Step 2: By Ohm’s law, iD  vTh RTh  RL Step 3: If vS  , then iD  and vL  iD RL  RL vTh RL  RTh Val de Loire Program p.8 CHAPTER 1: SemiConductor Diodes Step 4: If vS  , then iD  and the result of step is invalid Diode D must be replaced by an open circuit as illustrated in Fig 1-6 (c) , and the analysis performed again Since now iD  0, vL  iDRL  Since vD  vS  , the reverse bias of the diode is verified 1.3 DIODE TERMINAL CHARACTERISTICS Fig 1-7 Diode terminal characteristics Equation for diode junction current: iD  I e vD / nVT  1 A Where VT  kT / q  thermal voltage V  I  reverse saturation current A Val de Loire Program p.9 CHAPTER 1: SemiConductor Diodes vD  voltage applied to diode V  q  electronic charge 1.60  1019C    k  Boltzman ' s constant 1.38  10 23 J / K    T  absolute temperature K  n  nonideality factor dimensionless  I is typical between 10 18 and 10 19 A , and is strongly temperaturedependent The nonideality factor is typically close to 1, but approaches for devices with high current densities It is assumed to be in the rest of this course In forward bias, current is closely approximated by  ID  I e vD /nVT   whereVT  kT / q , assumed n 1 Notice there is a strong dependence on temperature We can approximate the diode equation for I D  I : I D  I 0e vD / nVT In reverse bias (when v  by at least VT ), then : I D  I In breakdown, reverse current increases rapidly …a vertical line If high-frequency analysis (above 100kHz) or switching analysis is to be performed, it may be necessary to account for the small depletion capacitance (typically several picofarads) associated with a reverse- Val de Loire Program p.10 CHAPTER 1: SemiConductor Diodes Fig 1-11 Ex 1.7 Val de Loire Program p.16 CHAPTER 1: SemiConductor Diodes 1.5 EQUIVALENT CIRCUIT ANALYSIS Fig 1-12 Diode models 1.5.1 Piecewise-Linear Techniques In piecewise-linear analysis, the diode characteristic curve is approximated with straight-line segments Here we shall use only the Val de Loire Program p.17 CHAPTER 1: SemiConductor Diodes three approximations shown in Fig 1-13, in which combinations of ideal diodes, resistors, and batteries replace the actual diode - The simplest model, in Fig.1-13(a), treats the actual diode as an infinitive resistance for vD VF , and as an ideal battery if vD tends to be greater than VF VF is usually selected as 0.6 to 0.7 V for a Si diode and 0.2 to 0.3 V for a Ge diode - If greater accuracy in the range of forward conduction is dictated by the application, a resistor RF is introduced, as in Fig 1-13(b) - If the diode reverse current iD   cannot be neglected, the additional refinement ( RR plus an ideal diode) of Fig 1-13(c) is introduced Val de Loire Program p.18 CHAPTER 1: SemiConductor Diodes Fig 1-13 Piecewise-linear techniques 1.5.2 Small-Signal Techniques Small-signal analysis can be applied to the diode circuit of Fig 1-15 if the amplitude of the ac signal vTh is small enough so that the curvature of the diode characteristic over the range of the operation (from b to a) may be neglected Then the diode voltage and current may each be written as the sum of a dc signal and an undistorted ac signal Furthermore, the ratio of the diode ac voltage vd to the diode ac current id will be constant and equal to vd 2Vdm vD a  vD b vD    id 2I dm iD a  iD b iD  rd Q Where rd is known as the dynamic resistance of the diode It follows (from a linear circuit argument) that the ac signal components may be determined by analysis of the “small-signal” circuit of Fig 1-14 Val de Loire Program p.19 CHAPTER 1: SemiConductor Diodes If the frequency of the ac signal is large, a capacitor can be placed in parallel with rd to model the depletion or diffusion capacitance as discussed in Section 1.3 The dc or quiescent signal components must generally be determined by graphical methods since, overall, the diode characteristic is nonlinear Fig 1-14 Dynamic resistance Example 1.8 For the circuit of Fig 1-15(a), determine iD Val de Loire Program p.20 CHAPTER 1: SemiConductor Diodes Fig 1-15 Ex 1.8 Solution The Q-point current I DQ has been determined as 36 mA (see Example 1.7) The dynamic resistance of the diode at the Q point can be evaluated graphically: rd  0.37  0.33  vD   2.5  iD 0.044  0.028 Now the small-signal circuit of Fig 1-14 can be analyzed to find id : id  Val de Loire Program vTh 0.1sin t   0.008 sin t A RTh  rd 10  2.5 p.21 CHAPTER 1: SemiConductor Diodes The total diode current is obtained by superposition and checks well with that found in Example 1.7: iD  I DQ  id  36  sin t mA Example 1.9 For the circuit of Fig 1-14, determine iD if   108 rad / s and the diffusion capacitance is known to be 5000 pF Solution From Example 1.8, rd  2.5  The diffusion capacitance C d acts in parallel with rd to give the following equivalent impedance for the diode, as seen by the ac signal:  Zd  rd || jx d  rd ||   j  C d  rd 2.5   12   jC drd  j 10  5000  10  2.5   1.56  51.34  0.974  j 1.218 In the time domain, with I DQ as found in Example 1.7, we have  iD  I DQ  id  36  9.1cos 108 t  83.67  mA 1.6 APPLICATIONS 1.6.1 Rectifier applications Rectifier circuits are two-port networks that capitalize on the nearly one-way conduction of the diode: An ac voltage is impressed upon the input port, and a dc voltage appears at the output port Val de Loire Program p.22 CHAPTER 1: SemiConductor Diodes The simplest rectifier circuit (Fig 1-16) contains a single diode It is commonly called a half-wave rectifier because the diode conducts over the positive or the negative half of the input-voltage waveform Fig 1-16 Half-wave rectifier 1.6.2 Waveform filtering The output of a rectifier alone does not usually suffice as a power supply, due to its variation in time The situation is improved by placing a filter between the rectifier and the load The filter acts to suppress the harmonics from the rectified waveform and to preserve the dc component A measure of goodness for rectified waveforms, both filtered and unfiltered, is the ripple factor, Fr  maximum variation in output voltage vL  average value of output voltage VL A small value, say Fr  0.05 , is usually attainable and practical Val de Loire Program p.23 CHAPTER 1: SemiConductor Diodes Fig 1-17 Waveform filtering 1.6.3 Clipping and Clamping operations Diode clipping circuits separate an input signal at a particular dc level and pass to the output, without distortion, the desired upper or lower portion of the original waveform They are used to eliminated amplitude noise or to fabricate new waveforms from an existing signal Example 1.10 Figure 1-18(a) shows a positive clipping circuit, which removes any portion of the input signal vi that is greater than Vb Val de Loire Program p.24 CHAPTER 1: SemiConductor Diodes Fig 1-18 Clipping And passes as the output signal vo any portion of vi that is less than Vb As you can see, vD is negative when vi  Vb , causing the ideal diode to act as an open circuit With no path for current to flow through R , the value of vi appears at the output terminals as vo However, when vi  Vb , the diode conducts, acting as a short circuit and forcing vo  Vb Figure 118(b), the transfer graph or transfer characteristic for the circuit, show the relationship between the input voltage, here taken as vi  2Vb sin wt , and the output voltage Clamping is a process of setting the positive or negative peaks of an input ac waveform to a specific dc level, regardless of any variation in those peaks Example 1.11 An ideal clamping circuit is shown in Fig 1-19(b), and a triangular ac input waveform in Fig 1-19(a) If the capacitor C is Val de Loire Program p.25 CHAPTER 1: SemiConductor Diodes initially uncharged, the ideal diode D is forward-biased for  t  T / , and its acts as a short circuit while the capacitor charges to vC Vp At t  T / , D open-circuits, breaking the only possible discharge path for the capacitor Thus, the value vC Vp is preserved; since vi can never exceed Vp , D remains reverse-biased for all t  T / , giving vo  vD  vi Vp The function vo is sketched in Fig 1-19(c); all positive peaks are clamped at zero, and the average value is shifted from to Vp Fig 1-19 Clamping Val de Loire Program p.26 CHAPTER 1: SemiConductor Diodes 1.7 ZENER DIODE Fig 1-20 Zener diode symbol and characteristics The Zener diode or reference diode, whose symbol is shown in Fig 1-20(a), finds primary usage as a voltage regulator or reference The forward conduction characteristic of a Zener diode is much the same as that of a rectifier diode; however, it usually operates with a reverse bias, for which its characteristic is radically different The reverse voltage breakdown is rather sharp The breakdown voltage can be controlled through the manufacturing process so it has a reasonably predictable value When a Zener diode is in reverse breakdown, its voltage remains extremely close to the break-down value while the current varies from rated current  I Z  to 10 percent or less of rated current A Zener regulator should be designed so that iZ  0.1I Z to ensure the constancy of vZ Val de Loire Program p.27 CHAPTER 1: SemiConductor Diodes 1.8 VARACTOR DIODE Varactor [also called varicap, VVC (voltage-variable capacitance), or tuning] diodes are semiconductor, voltage-dependent, variable capacitors Their mode of operation depends on the capacitance that exists at the p-n junction when the element is reverse-biased Fig 1-21 Varicap characteristic - C  pF  versus VR 1.9 SCHOTTKY BARRIER DIODE Schottky diodes are also called surface-barrier, or hot-carrier diode Its areas of application were first limited to the very high frequency range due to its quick response time (especially important at high frequencies) and a lower noise figure (a quantity of real important in high-frequency applications) Val de Loire Program p.28 CHAPTER 1: SemiConductor Diodes Fig 1-22 Schottky diode characteristic 1.10 LIGHT-EMITTING DIODE (LED) Light-emitting diode (LED) is a diode that will give off visible light when it is energized In any forward-biased p-n junction there is, within the structure and primarily close to the junction, a recombination of holes and electrons This recombination requires that the energy possessed by the unbound free electron be transferred to another state In silicon and germanium the greater percentage is given up in the form of heat and the emitted light is insignificant In other materials, such as gallium arsenide phosphide (GaAsP) or gallium phosphide (GaP), the number of photons of light energy emitted is sufficient to create a very visible light source Fig 1-23 LED Val de Loire Program p.29 CHAPTER 1: SemiConductor Diodes 1.11 TUNNEL DIODES The tunnel diode was first introduced by Leo Esaki in 1958 Its characteristics, show in Fig 1-24, are different from any diode discussed thus far in that it has a negative-resistance region In this region, an increase in terminal voltage results in a reduction in diode current Fig 1-24 Tunnel diode Val de Loire Program p.30 ... 15 Fig 1- 11 Ex 1. 7 16 Fig 1- 12 Diode models 17 Fig 1- 13 Piecewise-linear techniques 19 Fig 1- 14 Dynamic resistance 20 Fig 1- 15 Ex 1. 8 21 Fig 1- 16 Half-wave... Fig 1- 5 Ex 1. 1 Fig 1- 6 Ex 1. 2 Fig 1- 7 Diode terminal characteristics Fig 1- 8 Graphical analysis 12 Fig 1- 9 Ex .1. 5 13 Fig 1- 10 Ex 1. 6 15 ... a 0 .1- Vpeak sinusoidal source, as in Fig 1- 11( a) Find iD and vD for the network Solution Val de Loire Program p .15 CHAPTER 1: SemiConductor Diodes Fig 1- 11 Ex 1. 7 Val de Loire Program p .16 CHAPTER