CHAPTER 1 Semiconductor Diodes 2

8/2/2019 CHAPTER 1 Semiconductor Diodes 2

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KNL 1053ANALOG ELECTRONICS

Chapter 1 Semiconductor Diodes

MDM KASUMAWATI BT LIASDR THELAHA BIN MASRIDR WAN AZLAN BIN WAN ZAINAL ABIDIN


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TOPIC OUTLINES

SEMICONDUCTOR MATERIALCOVALENT BONDING AND INSTRINSIC MATERIALENERGY LEVELEXTRINSIC N-TYPE AND P-TYPE MATERIALSEMICONDUCTOR BIASINGZENER REGIONIDEAL VERSUS PRACTICAL DIODERESISTANCE LEVELDIODE EQUIVALENT CIRCUIT

TRANSITION AND DIFFUSION CAPACITANCEREVERSE RECOVERY GIMEDIODE SPECIFICATION SHEET


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SEMICONDUCTOR MATERIAL

Semi a range of levels midway between two limitsConductor any material that will support a generous flow of charge when avoltage source of limited magnitude ia appilied across its terminalsInsulator material that offers a very low level of conductivity under pressure

from an applied voltage sourceSemiconductor material that has a conductivity level somewhere between theextremes of an insulator and a conductor @ material that have a conductivitybetween a good conductor and that an insulator3 semiconductors used most frequently in the construction of electronic devices

are Ge, Si and GaAs



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COVALENT BONDING AND INTRINSIC MATERIALS

Every atom is composed of 3 basic particles : electron, proton and neutronIn the lattice structure, neutrons + protons form the nucleus and electrons appearin fixed orbits around the nucleusBohr model for 3 materials:



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Atomic structure of (a) silicon; (b) germanium; and (c) gallium and arsenic.



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Gallium trivalent Silicon & Germanium tetravalent Arsenic pentavalent Valence - indicate that the ionization potential required to remove any one of

these electrons from the atomic structure is significantly lower than thatrequired for any other electron in the structure



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Atom bonding is strengthened by the sharing of electrons called as covalentbonding

Covalent bonding of the silicon atom Covalent bonding of the GaAs crystal



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Covalent bond may break by external natural causes such as light energy in the form of photons and thermal energy (heat) from the surrounding medium toproduce freestate electronAt room temperature, there are approximately 1.5 X 10 10 free carriers in 1 cm 3 of intrinsic material equivalent with 15 billion electrons in a space smaller thana small sugar cubeIntrinsic any semiconductor material that has been carefully refined to reducethe number of impurities to a very low level, essentially as pure as can be madeavailable through modern technologyIntrinsic carriers free electrons in a material due only to external causes



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Semiconductor Intrinsic Carriers (per cubic cm)

GaAs 1.7 X 10 6

Si 1.5 X 10 10

Ge 2.5 X 10 13

Semiconductor n (cm 2 /Vs)

GaAs 8500

Si 1500

Ge 3900

Intrinsic Carriers

Relative Mobility Factor, n



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Ability to change the characteristics of a material is called doping process Different between conductor and semiconductor:

Semiconductor Conductor

Increase level of conductivity with theapplication of heat (negative temperaturecoefficient)

Resistance increase with the increase of heat (positive temperature coefficient)

As temperature rises, an increasing

number of valence electrons absorbsufficient thermal energy to break covalent bond and contribute to thenumber of free carriers

The numbers of carriers in a conductor do

not increase significantly withtemperature, but the vibration patternabout a relatively fixed location makes itincreasingly difficult for a sustained flowof carriers through the material



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ENERGY LEVELS

The farther an electron is from the nucleus, the higher is the energy state andany electron that has left its parent atom has a higher energy state than anyelectron in the atomic structure

Energy levels: (a) discrete levels in isolated atomic structures; (b) conduction and valence bands of aninsulator, a semiconductor, and a conductor



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ENERGY LEVELS

An electron in the valence band of silicon must absorb more energy than one inthe valence band of germanium to become a free carrier.Similarly, an electron in the valence band of GaAs must gain more energy thanone in silicon or germanium to enter the conduction band

Energy gap requirements reveals the sensitivity of each type of semiconductorto changes in temperatureIn LEDs, the wider the energy gap, greater the possibility of energy beingreleased in the form of visible or invisible light waveseV electron volts

W (energy) = QV



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EXTRINSIC MATERIAL: N-TYPE AND P-TYPEMATERIALS

Extrinsic material semiconductor material that has been subjected to thedoping processExtrinsic material n-type

- p-type



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EXTRINSIC MATERIAL:N-TYPE

Created by introducing impurity elements that have 5 valence electron such asantimony, arsenic, phosphorusDiffused impurities with five valence electrons are called donor atoms

Antimony impurity in n-type material Effect of donor impurities on the energyband structure



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EXTRINSIC MATERIAL:P-TYPE

Created by introducing impurity elements that have 3 valence electron such asboron, gallium, indiumDiffused impurities with five valence electrons are called acceptor atomsVacancy atom is called as hole

Boron impurity in p-type material



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EXTRINSIC MATERIAL: N-TYPE AND P-TYPEMATERIALS

The resulting from the doping:

p-type and n-type are electrically neutral



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EXTRINSIC MATERIAL

Electron versus hole flow

Electron versus hole flow



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EXTRINSIC MATERIAL

Majority and Minority Carriers

(a) n-type material; (b) p-type material



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SEMICONDUCTOR 3D ANIMATION
http://semiconductors%20%203d%20animation%20-electron%20and%20hole.flv/http://semiconductors%20%203d%20animation%20-electron%20and%20hole.flv/


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SEMICONDUCTOR DIODE BIASING

When two materials are joined the electrons and holes in the region of the junction will combine, resulting in a lack of free carriers in the region near the junction .The region of uncovered positive and negative ions is called the depletionregion due to the depletion of free carriers in the region .If lead are connected to the ends of each material, a two-terminal device resultswith 3 option bias available; no bias, reverse-bias and forward-bias.Bias refers to the application of an external voltage across the two terminals of the device to extract a response.



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SEMICONDUCTOR DIODE BIASINGNO APPLIED BIAS (V = 0V)

No external voltage applied.The absence of a voltage across a resistor results in zero current through it.

A p n junction with no external bias. (a) An internal distribution of charge; (b) a diode symbol, withthe defined polarity and the current direction; (c) demonstration that the net carrier flow is zero at the

external terminal of the device when V D = 0 V . MDM KASUMAWATI BT LIASDR THELAHA BIN MASRIDR WAN AZLAN BIN WAN ZAINAL ABIDIN


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If the voltage applied across the diode has the same polarity across the diode, itwill be considered a positive voltage.If the reverse, it is a negative voltage.Under no-bias conditions, any minority carriers (holes) in n-type material thatfind themselves within the depletion region will pass quickly into p-typematerial.The closer the minority carriers is to the junction, the greater is the attraction forthe layer of negative ions and the less is the opposition offered by the positiveions in the depletion region of n-type material.



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The majority carriers (electrons) of n-type material must overcome the attractiveforces of the layer of positive ions in n-type material.The shield of negative ions in p-type material migrate into the area beyond thedepletion region of p-type material.However, the number of majority carriers is so large in n-type material thatthere will invariably be a small number of majority carriers with sufficientkinetic energy to pass through the depletion region into p-type material.Relative magnitudes of the flow vectors are such that the net flow in eitherdirection is zero.



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SEMICONDUCTOR DIODE BIASINGREVERSE-BIAS (VD < 0V)

External potential of V volts is applied across the p-n junction such that thepositive terminal is connected to n-type material and negative terminal isconnected to p-type material.The number of uncovered positive ions in the depletion region of n-typematerial will increase due to the large number of free electrons drawn to the

positive potential of the applied voltage.For similar reasons, the number of uncovered negative ions will increase in p-type material.The net effect, therefore is a widening of depletion region.The widening of depletion region will establish too great a barrier for majoritycarriers to overcome, effectively reducing the majority carrier flow to zero.The number of minority carriers, however, entering the depletion region will notchange, resulting in minority-carrier flow vectors of the same magnitude withno applied voltage



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The current that exists under reverse-bias condition is called the reversesaturation current and represented by I s.The reverse saturation current is seldom more than a few microamperes, exceptfor high power devices.Term saturation comes from the fact that it reaches its maximum level quicklyand does not change significantly with increases in the reverse-bias potential.



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Reverse-biased p n junction. (a) Internal distribution of charge under reverse-bias conditions; (b)reverse-bias polarity and direction of reverse saturation current



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SEMICONDUCTOR DIODE BIASINGFORWARD-BIAS (VD > 0V)

Also known as on conditionEstablished by applying the positive potential to the p-type material andnegative potential to n-type material.

Internal distribution of charge underforward-bias conditions

forward-bias polarity and direction of resulting current



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The application of a forward-bias potential V D will pressure electrons in then-type material and holes in p-type material to recombine with the ions near theboundary and reduce the width of the depletion region.The resulting minority carrier flow of electrons from p-type material and n-typematerial (and holes from n-type material to p-type material) has not changed in

magnitude (since the conduction level is controlled primarily by the limitednumber of impurities in the material)However, the reduction in the width of the depletion region has resulted in aheavy majority flow across the junction.As the applied bias increases in magnitude, the depletion region continue todecrease in width until a flood of electrons can pass through the junction,resulting in an exponential rise in current.



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General characteristics of semiconductor diode can be defined throughShockleys equation, for forward and reverse-bias region:

ID =IS (eVD/nVT -1)Is = reverse saturation currentVD = applied forward-bias across the dioden = ideality factor, which is function of the operating conditions and physical

construction; it has a range between 1 and 2 depending a wide variety of factorsVT = kT/q (V)

k = Boltzmanns constant = 1.38 X 10 -23J/KT = temperature in kelvin (273 + the temperature in 0C)q = magnitude of electronic charge = 1.6 X 10 -19C



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PN JUNCTION BIASING
http://pn%20junction.flv/http://pn%20junction.flv/


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ZENER REGION

The current increases at the a very rapid rate in a direction opposite to that of the positive voltage region.The reverse-bias potential that results in this dramatic change in characteristicsis called Zener potential , VZ

Zener region

MDM KASUMAWATI BT LIASDR THELAHA BIN MASRI

DR WAN AZLAN BIN WAN ZAINAL ABIDIN


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ZENER REGION

As the voltage across the diode increases in the reverse-bias region, the velocityof the minority carriers responsible for reverse saturation current Is will alsoincrease.Eventually, their velocity and associated kinetic energy (Wk=1/2mv 2)will besufficient to release additional carriers through collisions with otherwise stableatomic structures.That is, the ionization process will result whereby valence electrons absorbsufficient energy to leave the parent atom.These additional carriers can then aid the ionization process to the point where ahigh avalanche current is established and the avalanche breakdown regiondetermined.




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ZENER REGION

The avalanche region (VZ) can be brought closer to the vertical axis byincreasing the doping levels in p-and n-type materialHowever, as VZ decreases to a very low levels, such as -5V, another mechanismcalled Zener breakdown , will contribute to the sharp change in thecharacteristics.It occurs, a strong electric field in the region of the junction that can disrupt the

bonding forces within the atom and generate carriers. Although the Zener breakdown mechanism is a significant contributor only atlower levels of VZ, this sharp change in the characteristic at any level is called

Zener region , and diodes employing this unique characteristic of p-n junctionare called Zener diodes .




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ZENER REGION

The maximum reverse-bias potential that can be applied before entering theZener region is called the peak inverse voltage (PIV) or peak reverse voltage(PRV).In order to get higher PIV, diodes with same characteristics can be connected inseries.Diodes are also connected in parallel to increase the current-carrying capacity.At a fixed temperature, the reverse saturation current of a diode increases withan increase in the applied reverse bias.




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ZENER REGION

Comparison of Ge, Si, and GaAs diodes




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ZENER REGION

Temperature can have a marked effect on the characteristics of a semiconductordiode.In forward-bias region the characteristics of a silicon diode shift to the left at arate of 2.5mV per centigrade degree increase in temperatureIn the reverse-bias region the reverse saturation current of a silicon diodedoubles for every 10 0C rise in temperatureThe reverse breakdown voltage of a semiconductor diode will increase ordecrease with temperature depending on the Zener potential




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ZENER REGION

Variation in Silicon diode characteristics with temperature change:




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IDEAL VERSUS PRACTICAL

Ideal versus actual semiconductor characteristics




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RESISTANCE LEVEL

3 difference level:DC or Static ResistanceAC or Dynamic ResistanceAverage AC Resistance




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RESISTANCE LEVELDC OR STATIC RESISTANCE

The application of a dc voltage to a circuit containing a semiconductor diodewill result in an operating point on the characteristic curve that will not changewith time.

RD = V D /IDIn general, the higher the current through a diode, the lower is the dc resistancelevel




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RESISTANCE LEVELDC OR STATIC RESISTANCE

Example:Determine the dc resistance levels for the diode;a. ID = 2mAb. ID = 20mA

c. VD = -10V




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RESISTANCE LEVELAC OR DYNAMIC RESISTANCE

Sinusoidal input is appliedThe varying input will move the instantaneous operating point up and down aregion of the characteristics and thus defines a specific change in current andvoltage.The point with no applied varying signal is called Q- point, which means stillor unvarying will define a particular change in voltage and current that can beused to determine the ac or dynamic resistance for diode characteristics.In general, the lower the Q-point of operation (smaller current or lowervoltage), the higher is ac resistance

rd = Vd/ Id




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Ac or dynamic resistance:




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Example:For the characteristic below;a. Determine the ac resistance at ID = 2mAb. Determine the ac resistance at ID = 25mA

c. Compare the results of parts (a) and (b) to the dc resistance at each currentlevel




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rd = 26mV/ID




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RESISTANCE LEVELAVERAGE AC RESISTANCE

If the input signal is sufficiently large to produce a broad swing, the resistanceassociated with the device is called average ac resistanceBy definition; the resistance determined by a straight line drawn between thetwo intersection established by the maximum and minimum values of inputvoltage.

rav = Vd/ Id pt to pt




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RESISTANCE LEVELAVERAGE AC RESISTANCE

As with the dc and ac resistance levels, the lower the level of currents used todetermine the average resistance, the higher is the resistance level.




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DIODE EQUIVALENT CIRCUITS

An equivalent circuit is a combination of elements properly chosen to bestrepresent the actual terminal characteristics of a device or system in a particularoperating region.3 types:

Piecewise-Linear Equivalent CircuitSimplified Equivalent CircuitIdeal Equivalent Circuit




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DIODE EQUIVALENT CIRCUITSPIECEWISE-LINEAR EQUIVALENT CIRCUIT

Defining the piecewise-linear equivalentcircuit using straight-line segments toapproximate the characteristic curve.

Components of the piecewise-linear equivalentcircuit




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DIODE EQUIVALENT CIRCUITSSIMPLIFIED EQUIVALENT CIRCUIT

Simplified equivalent circuit for the silicon semiconductor diode




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TRANSITION AND DIFFUSION CAPACITANCE

Every electronic or electrical device is frequency sensitiveTransition or depletion-region capacitance is applied in reverse-bias regionDiffusion or storage capacitance is applied in forward-bias region




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REVERSE RECOVERY TIME

Important for high-speed switching applications.It is the sum of two interval;

trr = ts (storage time) + tt (transition time)Most commercially available switching diodes have trr in the range of a fewnanosecond to 1s.




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DIODE SPECIFICATION SHEETS




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SEMICONDUCTOR DIODE NOTATION




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VARIOUS TYPES OF DIODES

[(a) Courtesy of Motorola Inc.; (b) and (c) Courtesy International Rectifier Corporation.]




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END OF CHAPTER 1

THANK YOU

MDM KASUMAWATI BT LIAS

CHAPTER 1 Semiconductor Diodes 2

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