Week 8a OUTLINE The pn Junction Diode Reference Reading
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1 Week 8a OUTLINE The pn Junction Diode Reference Reading -- Uses: Rectification, parts of transistors, light-emitting diodesand lasers, solar cells, electrically variable capacitor (varactor diode),voltage reference (zener diode)Depletion region & junction capacitanceI-V characteristicCircuit applications and analysisReference ReadingRabaey et al.Chapter to 3.2.2HambleyChapter 10.1 to 10.4
2 The pn Junction Diode Schematic diagram Circuit symbol ID + VD – p-type n-typenet acceptorconcentration NAnet donorconcentration ND+ VD –cross-sectional area ADPhysical structure:(an example)ID+VD–metalSiO2SiO2p-type SiFor simplicity, assume thatthe doping profile changesabruptly at the junction.n-type Simetal
3 Depletion RegionWhen the junction is first formed, mobile carriers diffuse across the junction (due to the concentration gradients)Holes diffuse from the p side to the n side, leaving behind negatively charged immobile acceptor ionsElectrons diffuse from the n side to the p side, leaving behind positively charged immobile donor ionsA region depleted of mobile carriers is formed at the junction.The space charge due to immobile ions in the depletion region establishes an electric field that opposes carrier diffusion.acceptor ionsdonor ions–+–+p n–+–+–+
4 Charge Density Distribution Charge is stored in the depletion region.acceptor ionsdonor ions–+–+p n–+–+–+quasi-neutral p regiondepletion regionquasi-neutral n regioncharge density (C/cm3)distance
5 Electric Field and Built-In Potential f0 –+–+p n–+–+–+electric field (V/cm)distanceNo net current flowsacross the junctionwhen the externallyapplied voltage is 0 V.potential (V)distancebuilt-in potential f0
7 Effect of Applied Voltage –+–+VDp n–+–+–+The quasi-neutral p and n regions have low resistivity, whereas the depletion region has high resistivity. Thus, when an external voltage VD is applied across the diode, almost all of this voltage is dropped across the depletion region. (Think of a voltage divider circuit.)If VD > 0 (forward bias), the potential barrier to carrier diffusion is reduced by the applied voltage.If VD < 0 (reverse bias), the potential barrier to carrier diffusion is increased by the applied voltage.
8 Forward BiasAs VD increases, the potential barrier to carrier diffusion across the junction decreases*, and current increases exponentially.The carriers that diffuse across thejunction become minority carriers inthe quasi-neutral regions; they thenrecombine with majority carriers,“dying out” with distance.VD > 0–+–+p n–+–+–+ID (Amperes)VD (Volts)* Hence, the width of the depletion region decreases.
9 Reverse BiasAs |VD| increases, the potential barrier to carrier diffusion across the junction increases*; thus, no carriers diffuse across the junction.A very small amount of reversecurrent (ID < 0) does flow, due tominority carriers diffusing from thequasi-neutral regions into the depletionregion and drifting across the junction.VD < 0–+–+p n–+–+–+ID (Amperes)VD (Volts)* Hence, the width of the depletion region increases.
10 I-V Characteristic Exponential diode equation: ID (A)VD (V)IS is the diode saturation currentfunction of ni2, AD, NA, ND, length of quasi-neutral regionstypical range of values: to A/mm2Note that e0.6/0.026 = and e0.72/0.026 = 1012 ID is in the mA range for VD in the range 0.6 to 0.7 V, typically.
14 Depletion Region Width Wj The width of the depletion region is a function of the bias voltage, and is dependent on NA and ND:If one side is much more heavily doped than the other (which is commonly the case), then this can be simplified:where N is the doping concentration on the more lightly doped side
15 Junction Capacitance VD –+–+VDp n–+–+–+charge density (C/cm3)distanceThe charge stored in the depletion region changes with applied voltage. This is modeled as junction capacitance
16 Summary: pn-Junction Diode Electrostatics A depletion region (in which n and p are each much smaller than the net dopant concentration) is formed at the junction between p- and n-type regionsA built-in potential barrier (voltage drop) exists across the depletion region, opposing carrier diffusion (due to a concentration gradient) across the junction:At equilibrium (VD=0), no net current flows across the junctionWidth of depletion regiondecreases with increasing forward bias (p-type region biased at higher potential than n-type region)increases with increasing reverse bias (n-type region biased at higher potential than p-type region)Charge stored in depletion region capacitance
17 Summary: pn-Junction Diode I-V Under forward bias, the potential barrier is reduced, so that carriers flow (by diffusion) across the junctionCurrent increases exponentially with increasing forward biasThe carriers become minority carriers once they cross the junction; as they diffuse in the quasi-neutral regions, they recombine with majority carriers (supplied by the metal contacts)“injection” of minority carriersUnder reverse bias, the potential barrier is increased, so that negligible carriers flow across the junctionIf a minority carrier enters the depletion region (by thermal generation or diffusion from the quasi-neutral regions), it will be swept across the junction by the built-in electric field“collection” of minority carriers reverse currentID (A)VD (V)
18 pn-Junction Reverse Breakdown As the reverse bias voltage increases, the peak electric field in the depletion region increases. When the electric field exceeds a critical value (Ecrit 2x105 V/cm), the reverse current shows a dramatic increase:ID (A)reverse (leakage) currentforward currentVBDbreakdown voltageVD (V)
19 Zener DiodeA Zener diode is designed to operate in the breakdown mode.ID (A)reverse (leakage) currentforwardcurrentVBDbreakdown voltageVD (V)Example:tRintegratedcircuit+vs(t)–+vo(t)–VBD = 15V
20 Other Important Diodes Light-Emitting Diodes (LEDs): Typically made of III-Vsemiconductors, such as gallium (III)-arsenide (V)GaAs. LEDs emit light of characteristic color (wavelength)when forward-biased. Turn-on voltage depends on wavelength.Laser Diodes: Similar to the LED, a laser diode hasbuilt-in mirror surfaces that cause emitted light amplitudeto build up, when diode is strongly forward-biased. Emittedradiation is coherent and monochromatic. (LASER = lightamplification by the stimulated emission of radiation)Solar Cell: Broad-area pn-diode that produces electric currentin a load that is connected to it. Photon energy, hn, of absorbedoptical radiation provides the input energy (h = Planck’s constant,6.63x10-34 Js, n = frequency of radiation).
21 Light Emitting Diode (LED) LEDs are made of compound semiconductor materialsCarriers diffuse across a forward-biased junctionand recombine in the quasi-neutral regions optical emission
22 Optoelectronic Diodes (cont’d) Light incident on a pn junction generates electron-hole pairsThe minority carriers that are generated in the depletion region, and the minority carriers that are generated in the quasi-neutral regions and then diffuse into the depletion region, are swept across the junction by the electric fieldThis results in an additional component of current flowing in the diode:where Ioptical is proportional to the intensity of the light
23 Photovoltaic (Solar) Cell ID (A)in the darkVD (V)with incident lightoperating point
24 PhotodiodeAn intrinsic region is placed between the p-type and n-type regionsWj Wi-region, so that most of the electron-hole pairs are generated in the depletion region faster response time(~10 GHz operation)ID (A)in the darkoperating pointVD (V)with incident light
25 Circuit Analysis with a Nonlinear Element Since the pn junction is a nonlinear circuit element,its presence complicates circuit analysis.(Node and loop equations become transcendental.)IRTh+V–VTh+
26 Load Line Analysis Method Graph the I-V relationships for the non-linear element and for the rest of the circuitThe operating point of the circuit is found from the intersection of these two curves.IRThI+V–VTh/RThoperating pointVTh+VVThThe I-V characteristic of all of the circuit except the non-linear element is called the load line
27 Ideal Diode Model of pn Diode Circuit symbolI-V characteristicSwitch modelIDID (A)ID+VD–+VD–forward biasreverse biasVD (V)An ideal diode passes current only in one direction.An ideal diode has the following properties:when ID > 0, VD = 0when VD < 0, ID = 0Diode behaves like a switch:closed in forward bias modeopen in reverse bias mode
28 Large-Signal Diode Model Circuit symbolI-V characteristicSwitch modelIDID (A)ID+VD–+VD–+VDonforward biasreverse biasVD (V)VDonFor a Si pn diode, VDon 0.7 VRULE 1: When ID > 0, VD = VDonRULE 2: When VD < VDon, ID = 0Diode behaves like a voltage source in series with a switch:closed in forward bias modeopen in reverse bias mode
29 How to Analyze Circuits with Diodes A diode has only two states:forward biased: ID > 0, VD = 0 V (or 0.7 V)reverse biased: ID = 0, VD < 0 V (or 0.7 V)Procedure:Guess the state(s) of the diode(s)Check to see if KCL and KVL are obeyed.If KCL and KVL are not obeyed, refine your guessRepeat steps 1-3 until KCL and KVL are obeyed.Example:If vs(t) > 0 V, diode is forward biased (else KVL is disobeyed – try it)+vR(t)–vs(t)+If vs(t) < 0 V, diode is reverse biased (else KVL is disobeyed – try it)
31 Application Example #1 (using the ideal diode model) vs(t)+vR(t)–vs(t)+tvs(t)“rectified” version of input waveform:t
32 Application Example #2 (using the ideal diode model) vs(t)+vR(t)–vs(t)+CRtvR(t)t
33 Diode Logic Diodes can be used to perform logic functions: AND gate output voltage is high only ifboth A and B are highOR gateoutput voltage is high ifeither (or both) A and B are highVccARBCACRBInputs A and B vary between 0 Volts (“low”) and Vcc (“high”)Between what voltage levels does C vary?