Chapter 1:Semiconductor Basis and Diodes1.1 1.1 Principles of SemiconductorsPrinciples of SemiconductorsCommon materials used in the development of semiconductor devices: Silicon (Si)Silicon (Si) Germanium (Ge)Germanium (Ge) GaAsGaAs2 21.1 1.1 Principles of SemiconductorsPrinciples of Semiconductors3 3Covalent bonding of the silicon atom. I Intrinsic semiconductorntrinsic semiconductor: : The single-crystal formed by pure The single-crystal formed by pure semiconductor materialssemiconductor materials HolesHoles: Vacancies in the covalent bond: Vacancies in the covalent bond Electron-hole pairsElectron-hole pairs: a free electron and a hole is generated from the : a free electron and a hole is generated from the covalent bond by thermal energycovalent bond by thermal energyThe free electrons and holes in a material due only to external causes are referred to as intrinsic carriersintrinsic carriers1.1.1 Intrinsic Semiconductors1.1.1 Intrinsic Semiconductors Two types of Two types of charged particles (Intrinsic carriers) in a charged particles (Intrinsic carriers) in a semiconductor semiconductor free electrons free electrons holesholessemiconductorIntrinsic Carriers (/cm3) room T.GaAs1.7106Si1.51010Ge1.71013Semiconductor materials have a negative temperature coefficient1.1.1 Intrinsic Semiconductors1.1.1 Intrinsic SemiconductorsIntrinsic SemiconductorsIntrinsic Semiconductors Movement of HolesMovement of Holes: by movement of covalent electrons from : by movement of covalent electrons from adjacent covalent bondsadjacent covalent bonds Electrical conductivity of intrinsic semiconductors is Electrical conductivity of intrinsic semiconductors is determined by the concentration of free electrons and holesdetermined by the concentration of free electrons and holes1.1.2 Extrinsic Semiconductors: N type and P type1.1.2 Extrinsic Semiconductors: N type and P typeThe electrical characteristics of intrinsic semiconductors are improved by adding impurity materials in a process called dopingdoping. The materials containing impurity atoms are called extrinsic extrinsic semiconductorssemiconductors, or doped semiconductorsdoped semiconductors.There are just two types of doped semiconductor materials: N N typetype: : impurities are from group V elements, e.x. impurities are from group V elements, e.x. PhosphorusPhosphorus P P typetype: : impurities are from group III elements, e.x. impurities are from group III elements, e.x. BoronBoron7 7(1) N-type Semiconductors and Carriers(1) N-type Semiconductors and CarriersA semiconductor that contains donor impurity atoms is called a N-type semiconductor.Impurities in N type materials act as Donor.Donor.The minority carriersminority carriers in N type materials are holes.The majority carriersmajority carriers in N type materials are free electrons.(2) P type Semiconductors and Carriers(2) P type Semiconductors and CarriersA semiconductor that contains acceptor impurity atoms is called a P type semiconductor.P type semiconductor.Impurities in p-type materials act as Acceptor.Acceptor.The minority carriersminority carriers in P type materials are free electrons.The majority carriersmajority carriers in P type materials are holes. N type semiconductor P type semiconductormajority carriers: free electrons holesminority carriers: holes free electronsmass-action law: ordopingN typeP typeintrinsic semiconductorextrinsic semiconductor1.21.2 PN JunctionPN JunctionOne end of a silicon or germanium crystal can be doped as a P type material and the other end as an N type material.The result is a PNPN JunctionJunction.1111Key Point: the formation of a depletion regiondepletion region around the junction.(1) PN Junction(1) PN Junction Operating Conditions Operating Conditions: : Forward BiasForward BiasForward BiasForward BiasExternal voltage is applied across the PN junction with the positive polarity on the P side and the negative polarity on the N side.The forward voltage causes the depletion layer to narrow.The electrons and holes are pushed toward the PN junction.The electrons and holes have sufficient energy to cross the PN junction.The forward bias voltage required: silicon: UD 0.7V germanium: UD 0.3VExternal voltage is applied across the PN junction with the positive polarity on the N side and the negative polarity on the P side.(2) PN Junction(2) PN Junction Operating Conditions Operating Conditions: : Reverse BiasReverse BiasReverse BiasReverse BiasThe reverse voltage causes the depletion layer to widen.The electrons in the N type material are attracted toward the positive terminal.The holes in the P type material are attracted toward the negative terminal.1313In reverse bias, the depletion layer is very large. The diodes strong positive and negative polarities create transition- or depletion-region capacitance, CT. The amount of capacitance depends on the reverse voltage applied.In forward bias diffusion capacitance (CD) exists besides barrier capacitance as the diode voltage increases. (3) (3) CapacitanceCapacitance of the PN junction of the PN junction1.3 Semiconductor Diodes1.3 Semiconductor Diodes1515PN junctionDiodeForward-biasReverse-biasForward-biasReverse-biasBreakdown is harmful for diodes, but sometimes it can be utilized.- Zener breakdown- Avalanche breakdownIV Characteristics and IV Characteristics and Shockly Shockly E Equationquation of Diodes of Diodes1616Important Parameters of Diodes Forward rating current: Peak Inverse Voltage(PIV):Reverse saturation current:There are three types to model a diode:(1)Ideal Model (2) Constant Voltage Drop Model (3) Segment Linear Model1.3.2 Models of Diodes1.3.2 Models of Diodes1717“an on-off switch” “an on-off switch with a forward voltage drop” An ideal diode conduct in one directionSemiconductor diodes (/PN junction) act differently to DC and AC currents. Different equivalent circuits to DC and AC signals are defined to simplify circuit analysis: DC, or static, resistance AC, or dynamic, resistance1.3.3 1.3.3 Equivalent Circuits of DiodesEquivalent Circuits of Diodes1818ExamplesExamplesExample1.3.1The circuit with an ideal diode is illustrated in Figure 1.3.7(b), where the input voltage is shown in Figure 1.3.7(a). Sketch the output voltage , and calculate the DC voltage from . Determine the PIV of the diode.Solutions the diode is “on” and equivalent to “short” the diode is “off” and equivalent to “open” PIV ExamplesExamplesExample1.3.2 The circuit with ideal diodes is shown in Figure 1.3.9(b) with an input voltage shown in Figure 1.3.9(a). Sketch the output voltage and calculate the DC component from . Solutions VD2 and VD4 are “on” and equivalent to “short”VD1 and VD3 is “off” and equivalent to “open”VD1 and VD3 are “on” and equivalent to “short”VD2 and VD4 is “off” and equivalent to “open” ExamplesExamplesExample 1.3.3 A Diode circuit is illustrated in Figure 1.3.11. Diodes and are both ideal silicon diodes whose knee voltage could be omitted. Determine and . Solutions Analysis method:Step 1. Make assumptions (short/on or open/off)Step 2. Analysis/Check assumptionsStep 3. Make final decisionThe Zener diode works in the diodes reverse-bias region/zener region.At some point the zener voltage is so large the diode breaks down and the reverse current increases dramatically.1.3.4 1.3.4 Zener Zener DiodesDiodesRegulation voltage/zener voltage : The reverse breakdown voltage with a regulated current range. Regulation current : The reference current for a zener diode working in regulation region. A resistor R is commonly used in a zener diode circuit to limit the current swinging between IZmax and IZmin The maximum regulation current Dynamic resistance: Main parameters:Example 1.3.5 A voltage regulation circuit with a silicon zener diode is shown in Figure 1.3.14. The regulation voltage of the zener diode is denoted by . If the DC input voltage is , discuss the output voltage . ExamplesExamplesSolutions . Analysis methods:Step1. Determine the state of the Zener diode by removing it from the network and calculating the voltage across the resulting open circuit.Step2. Substitute the appropriate equivalent and solve for the desired unknowns.1.3.5 Some 1.3.5 Some Other Types of DiodesOther Types of DiodesVaractor diodePhotodiodeLight-emitting diode (LED)Summary of Chapter 1Summary of Chapter 1Key ItemsSemiconductor basisCarriersPN junctionConstruction of a PN junctionBias a PN junction (no bias/forward/reverse) DiodesCharacteristics of a semiconductor diode (/PN junction) - Electrical conduction in only one directionDC resistance and AC resistanceEquivalent circuits for a semiconductor diode