1 Present by:- Bakori smit b, ( )3 RD E.C. Electronic Device & circuit

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Outline 1.1 Introduction 1.2 Basic Semiconductor Concepts 1.3 The pn Junction 1.4 Analysis of diode circuits 1.5 Applications of diode circuits 3

Introduction  The diode is the simplest and most fundamental nonlinear circuit element.  Just like resistor, it has two terminals.  Unlike resistor, it has a nonlinear current-voltage characteristics.  Its use in rectifiers is the most common application. 4

Physicle structure 5 The most important region, which is called pn junction, is the boundary between n-type and p-type semiconductor.

Symbol and Characteristic for the Ideal Diode 6 (a) diode circuit symbol; (b) i– v characteristic; (c) equivalent circuit in the reverse direction; (d) equivalent circuit in the forward direction.

Characteristics  Conducting in one direction and not in the other is the I-V characteristic of the diode.  The arrowlike circuit symbol shows the direction of conducting current.  Forward biasing voltage makes it turn on.  Reverse biasing voltage makes it turn off. 7

1.2 Basic Semiconductor Concepts  Intrinsic Semiconductor  Doped Semiconductor  Carriers movement 8

Intrinsic Semiconductor  Definition A crystal of pure and regular lattice structure is called intrinsic semiconductor.  Materials  Silicon---today’s IC technology is based entirely on silicon  Germanium---early used  Gallium arsenide---used for microwave circuits 9

10 Two-dimensional representation of the silicon crystal. The circles represent the inner core of silicon atoms, with +4 indicating its positive charge of +4q, which is neutralized by the charge of the four valence electrons. Observe how the covalent bonds are formed by sharing of the valence electrons. At 0 K, all bonds are intact and no free electrons are available for current conduction.

11 At room temperature, some of the covalent bonds are broken by thermal ionization. Each broken bond gives rise to a free electron and a hole, both of which become available for current conduction.

 Thermal ionization  Valence electron---each silicon atom has four valence electrons  Covalent bond---two valence electrons from different two silicon atoms form the covalent bond  Be intact at sufficiently low temperature  Be broken at room temperature  Free electron---produced by thermal ionization, move freely in the lattice structure.  Hole---empty position in broken covalent bond,can be filled by free electron, positive charge 12 Intrinsic Semiconductor(cont’d)

 Carriers A free electron is negative charge and a hole is positive charge. Both of them can move in the crystal structure. They can conduct electric circuit. 13

Recombination Some free electrons filling the holes results in the disappearance of free electrons and holes. Thermal equilibrium At a certain temperature, the recombination rate is equal to the ionization rate. So the concentration of the carriers is able to be calculated. 14

Important notes: has a strong function of temperature. The high the temperature is, the dramatically great the carrier concentration is. At room temperature only one of every billion atoms is ionized. Silicon’s conductivity is between that of conductors and insulators. Actually the characteristic of intrinsic silicon approaches to insulators. 15

Doped Semiconductor  Doped semiconductors are materials in which carriers of one kind predominate.  Only two types of doped semiconductors are available.  Conductivity of doped semiconductor is much greater than the one of intrinsic semiconductor.  The pn junction is formed by doped semiconductor. 16

Doped Semiconductor(cont’d) n type semiconductor  Concept Doped silicon in which the majority of charge carriers are the negatively charged electrons is called n type semiconductor.  Terminology  Donor---impurity provides free electrons, usually entirely ionized.  Positive bound charge---impurity atom donating electron gives rise to positive bound charge  carriers ○ Free electron---majority, generated mostly by ionized and slightly by thermal ionization. ○ Hole---minority, only generated by thermal ionization. 17

18 A silicon crystal doped by a pentavalent element. Each dopant atom donates a free electron and is thus called a donor. The doped semiconductor becomes n type.

p type semiconductor  Concept Doped silicon in which the majority of charge carriers are the positively charged holes is called p type semiconductor.  Terminology  acceptor---impurity provides holes, usually entirely ionized.  negatively bound charge---impurity atom accepting hole give rise to negative bound charge  carriers ○ Hole---majority, generated generated mostly by ionized and slightly by thermal ionization. ○ Free electron---minority, only generated by thermal ionization. 19

20 A silicon crystal doped with a trivalent impurity. Each dopant atom gives rise to a hole, and the semiconductor becomes p type.

Carrier concentration for n type a) Thermal equilibrium equation b) Electric neutral equation 21

Carrier concentration for p type a) Thermal equilibrium equation b) Electric neutral equation 22

Because the majority is much great than the minority, we can get the approximate equations shown below: for n type for p type 23

Doped Semiconductor(cont’d)  Conclusion  Majority carrier is only determined by the impurity, but independent of temperature.  Minority carrier is strongly affected by temperature.  If the temperature is high enough, characteristics of doped semiconductor will decline to the one of intrinsic semiconductor. 24

 Doping compensation 25 n type semiconductor is generated by donor diffusion, then injecting acceptor into the specific area(assuming ) forms p type semiconductor. The boundary between n and p type semiconductor is the pn junction. This is the basic step for VLSI fabrication technology. NDND NANA

Carriers Movement There are two mechanisms by which holes and free electrons move through a silicon crystal.  Drift --- The carrier motion is generated by the electrical field across a piece of silicon. This motion will produce drift current.  Diffusion --- The carrier motion is generated by the different concentration of carrier in a piece of silicon. The diffused motion, usually carriers diffuse from high concentration to low concentration, will give rise to diffusion current. 26

Diffusion and Diffusion Current  diffusion 27 A bar of intrinsic silicon (a) in which the hole concentration profile shown in (b) has been created along the x-axis by some unspecified mechanism.

Diffusion and Diffusion Current where are the diffusion constants or diffusivities for hole and electron respectively. * The diffusion current density is proportional to the slope of the the concentration curve, or the concentration gradient. 28

pn Junction  The pn junction under open-circuit condition  I-V characteristic of pn junction  Terminal characteristic of junction diode.  Physical operation of diode.  Junction capacitance 29

pn Junction Under Open-Circuit Condition  Usually the pn junction is asymmetric, there are p + n and pn +.  The superscript “+” denotes the region is more heavily doped than the other region. 30

pn Junction Under Open-Circuit Condition 31 Fig (a) shows the pn junction with no applied voltage (open-circuited terminals). Fig.(b) shows the potential distribution along an axis perpendicular to the junction.

I-V Characteristics 32 The diode i– v relationship with some scales expanded and others compressed in order to reveal details

Terminal Characteristic of Junction Diodes  The Forward-Bias Region, determined by  The Reverse-Bias Region, determined by  The Breakdown Region, determined by 33

The pn Junction Under Forward- Bias Conditions 34  The pn junction excited by a constant- current source supplying a current I in the forward direction.  The depletion layer narrows and the barrier voltage decreases by V volts, which appears as an external voltage in the forward direction.

The pn Junction Under Forward- Bias Conditions 35 Minority-carrier distribution in a forward-biased pn junction. It is assumed that the p region is more heavily doped than the n region; N A >> N D.

The pn Junction Under Forward- Bias Conditions Excess minority carrier concentration:  Exponential relationship  Small voltage incremental give rise to great incremental of excess minority carrier concentration. 36

The pn Junction Under Forward- Bias Conditions Distribution of excess minority concentration: Where are called excess-minority-carrier lifetime. 37

The pn Junction Under Forward- Bias Conditions The total current can be obtained by the diffusion current of majority carriers. 38

The pn Junction Under Forward- Bias Conditions The saturation current is given by : 39

The pn Junction Under Forward- Bias Conditions I-V characteristic equation: Exponential relationship, nonlinear. I s is called saturation current, strongly depends on temperature. or 2 ， in general V T is thermal voltage. 40

The pn Junction Under Forward- Bias Conditions assuming V 1 at I 1 and V 2 at I 2 then: * For a decade changes in current, the diode voltage drop changes by 60mv (for n=1) or 120mv (for n=2). 41

The pn Junction Under Forward- Bias Conditions  Turn-on voltage A conduction diode has approximately a constant voltage drop across it. It’s called turn-on voltage.  Diodes with different current rating will exhibit the turn-on voltage at different currents.  Negative TC, 42 For silicon For germanium

The pn Junction Under Forward- Bias Conditions 43  The pn junction excited by a constant-current source I in the reverse direction.  To avoid breakdown, I is kept smaller than I S.  Note that the depletion layer widens and the barrier voltage increases by V R volts, which appears between the terminals as a reverse voltage.

The pn Junction Under Forward- Bias Conditions I-V characteristic equation: Where I s is the saturation current, it is proportional to n i 2 which is a strong function of temperature. 44 Independent of voltage 。

The pn Junction In the Breakdown Region 45 The pn junction excited by a reverse-current source I, where I > I S. The junction breaks down, and a voltage V Z, with the polarity indicated, develops across the junction.

The pn Junction In the Breakdown Region  Supposing, the current source will move holes from p to n through the external circuit.  The free electrons move through opposite direction.  This result in the increase of barrier voltage and decrease almost zero of diffusion current.  To achieved the equilibrium, a new mechanism sets in to supply the charge carriers needed to support the current I. 46

Breakdown Mechanisms  Zener effect  Occurs in heavily doping semiconductor  Breakdown voltage is less than 5v.  Carriers generated by electric field---field ionization.  TC is negative.  Avalanche effect.  Occurs in slightly doping semiconductor  Breakdown voltage is more than 7v.  Carriers generated by collision.  TC is positive. 47

Breakdown Mechanisms Remember: pn junction breakdown is not a destructive process, provided that the maximum specified power dissipation is not exceeded. 48

Zener Diode 49 Circuit symbol The diode i– v characteristic with the breakdown region shown in some detail.

Zener Diode  Diffusion Capacitance  Charge stored in bulk region changes with the change of voltage across pn junction gives rise to capacitive effect.  Small-signal diffusion capacitance  Depletion capacitance  Charge stored in depletion layer changes with the change of voltage across pn junction gives rise to capacitive effect.  Small-signal depletion capacitance 50

Diffusion Capacitance According to the definition: The charge stored in bulk region is obtained from below equations: 51

Diffusion Capacitance The expression for diffusion capacitance: 52 Forward-bias, linear relationship Reverse-bias, almost inexistence

Diffusion Capacitance According to the definition: Actually this capacitance is similar to parallel plate capacitance. 53

Diffusion Capacitance  A more general formula for depletion capacitance is :  Where m is called grading coefficient.  If the concentration changes sharply,  Forward-bias condition,  Reverse-bias condition, 54

Junction Capacitance Remember: a) Diffusion and depletion capacitances are incremental capacitances, only are applied under the small-signal circuit condition. b) They are not constants, they have relationship with the voltage across the pn junction. 55

Analysis of Diode Circuit  Models  Mathematic model  Circuit model  Methods of analysis  Graphical analysis  Iterative analysis  Modeling analysis 56

The Diode Models Mathematic Model ： The circuit models are derived from approximating the curve into piecewise-line. 57 Forward biased Reverse biased

The Diode Models Circuit Model a) Simplified diode model b) The constant-voltage-drop model c) Small-signal model d) High-frequency model e) Zener Diode Model 58

Simplified Diode Model 59 Piecewise-linear model of the diode forward characteristic and its equivalent circuit representation.

The Constant-Voltage-Drop Model 60 The constant-voltage-drop model of the diode forward characteristics and its equivalent-circuit representation.

Small-Signal Model Symbol convention:  Lowercase symbol, uppercase subscript stands for total instantaneous qualities.  Uppercase symbol, uppercase subscript stands for dc component.  Lowercase symbol, lowercase subscript stands for ac component or incremental signal qualities.  Uppercase symbol, lowercase subscript stands for the rms(root-mean-square) of ac. 61

Small-Signal Model 62 Development of the diode small-signal model. Note that the numerical values shown are for a diode with n = 2.

Small-Signal Model Incremental resistance: *The signal amplitude sufficiently small such that the excursion at Q along the i- v curve is limited to a short, almost linear segment. 63

High-Frequency Model High frequency model 64

Zener Diode Model 65

Method of Analysis 66  Load line  Diode characteristic  Q is the intersect point  Visualization

Application of Diode Circuits  Rectifier circuits  Half-wave rectifier  Full-wave rectifier ○ Transformer with a center-tapped secondary winding ○ Bridge rectifier  The peak rectifier  Voltage regulator  Limiter 67

Half-Wave Rectifier 68 (a)Half-wave rectifier. (b)Equivalent circuit of the half-wave rectifier with the diode replaced with its battery-plus-resistance model.

Half-Wave Rectifier 69 (c) Transfer characteristic of the rectifier circuit. (d) Input and output waveforms, assuming that

full-Wave Rectifier 70 (a)circuit (b)transfer characteristic assuming a constant-voltage-drop model for the diodes

Full-Wave Rectifier 71 (c) input and output waveforms.

The Bridge Rectifier 72 (a) circuit

The Bridge Rectifier 73 (b) input and output waveforms

Peak Rectifier 74  Voltage and current waveforms in the peak rectifier circuit with.  The diode is assumed ideal.

Voltage Regulator We define: 75

Limiter 76

Limiter 77 Applying a sine wave to a limiter can result in clipping off its two peaks.

Soft Limiting 78

79 Created by:bakori smit