Chapter 1 semiconductor diodes

Val de Loire Program p.4 - 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..

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CHAPTER 1:

SEMICONDUCTOR DIODES

Table of Contents

1.1 INTRODUCTION 4

1.2 THE IDEAL DIODE 5

1.2.1 The Ideal Analysis Procedure 6

1.3 DIODE TERMINAL CHARACTERISTICS 9

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

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1.11 TUNNEL DIODES 30

Table of Figures Fig 1-1 Construction and symbol of diode 4

Fig 1-2 Common diode 5

Fig 1-3 Ideal diode 6

Fig 1-5 Ex 1.1 7

Fig 1-6 Ex 1.2 8

Fig 1-7 Diode terminal characteristics 9

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

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Fig 1-20 Zener diode symbol and characteristics 27Fig 1-21 Varicap characteristic - C pF versus   V 28 R

Fig 1-22 Schottky diode characteristic 29Fig 1-23 LED 29Fig 1-24 Tunnel diode 30

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- 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

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+ 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

1-2(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 reverse-

biased (Fig 1-3)

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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

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quantities using any method of circuit analysis Voltage v Dmust 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 = 5 volts (forward bias)

b) VA = -5 volts (reverse bias)

Fig 1-5 Ex 1.1 Solution

a) With VA > 0 the diode is in forward bias and is acting like a perfect conductor so:

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The analysis is simplified if a Thévenin equivalent is found for the circuit to the left of terminals a, b; the result is





1 1

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,





Th D

v i

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Step 4: If v S  0, then i D  0 and the result of step 3 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 i D 0,v Li R D L0 Since

  0

v v , the reverse bias of the diode is verified

1.3 DIODE TERMINAL CHARACTERISTICS

Fig 1-7 Diode terminal characteristics

Equation for diode junction current:

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The nonideality factor is typically close to 1, but approaches 2 for

devices with high current densities It is assumed to be 1 in the rest of this course

In forward bias, current is closely approximated by

Notice there is a strong dependence on temperature

We can approximate the diode equation for I D  I : 0 /

0

D T

v nV D

I I e

In reverse bias (when v  0by at least V ), then : T I D  I 0

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-

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biased p-n junction; for a forward-biased p-n junction, a larger diffusion capacitance (typically several hundred picofarads) that is directly

proportional to the forward current should be included in the model

Recalling that absolute zero is 273 C0 , we write

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Thévenin equivalent exists for it Then the two simultaneous equations to

be solved graphically for i and D v D are: the diode characteristic

Fig 1-8 Graphical analysis

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(b)

(c)

Fig 1-9 Ex.1.5 Solution

The circuit may be reduced to that of Fig 1-9(c), with

1 1

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Then, with these values the load line must be superimposed on the diode characteristic, as in Fig 1-9(b) The desired solution, i D  3mA

and v D  0.75V , is given by the point of intersection of the two plots

Example 1.6 If all sources in the original linear portion of a

network vary with time, then v Th is also a time-varying source In reduced form [Fig 1-10(a)], one such network has a Thévenin voltage that is a triangular wave with a 2-V peak Find i and D v D for this network

Solution

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Fig 1-10 Ex 1.6

Since the load line is continually changing, it is referred to as a

dynamic load line The solution, a plot of i , differs drastically in form D

from the plot of v Th because of the nonlinearity of the diode

Example 1.7 If both dc and time-varying sources are present in the

original linear portion of a network, then v Th is a series combination of a

dc and a time-varying source Suppose that the Thévenin source for a particular network combines a 0.7-V battery and a 0.1-Vpeak sinusoidal source, as in Fig 1-11(a) Find i and D v D for the network

Solution

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Fig 1-11 Ex 1.7

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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

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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 v D V F, and as an ideal battery if v tends to be D

greater than V F V is usually selected as 0.6 to 0.7 V for a Si diode and F

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 R is introduced, as in Fig 1-13(b) F

- If the diode reverse current i D  0 cannot be neglected, the additional refinement (R plus an ideal diode) of Fig 1-13(c) is R

introduced

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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 v Th 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 v d to the diode ac current i will be constant and equal d

Where r d 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

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If the frequency of the ac signal is large, a capacitor can be placed in parallel with r d 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

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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:

D d

D

v r

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The total diode current is obtained by superposition and checks well with that found in Example 1.7:



  36 8 sin

108rad s and the diffusion capacitance is known to be 5000 pF /

Solution

From Example 1.8, r d 2.5 The diffusion capacitance C acts in d

parallel with r d to give the following equivalent impedance for the diode,

as seen by the ac signal:

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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,



0

maximum variation in output voltage

average value of output voltage

L r

L

v F

V

A small value, say F r  0.05, is usually attainable and practical

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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 v ithat is greater than V b

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Fig 1-18 Clipping

And passes as the output signal v o any portion of v ithat is less than

b

V As you can see, v D is negative when v i V , causing the ideal diode b

to act as an open circuit With no path for current to flow through R, the value of v i appears at the output terminals as v o However, when v i V , b

the diode conducts, acting as a short circuit and forcing v o V Figure 1- b 18(b), the transfer graph or transfer characteristic for the circuit, show

the relationship between the input voltage, here taken as v i  2V bsinwt ,

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

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initially uncharged, the ideal diode D is forward-biased for 0 t T/ 4, and its acts as a short circuit while the capacitor charges to v C V p At

 / 4

t T , D open-circuits, breaking the only possible discharge path for the capacitor Thus, the value v C V p is preserved; since v i can never exceed V , D remains reverse-biased for all p t T/ 4, giving

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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

1 The reverse voltage breakdown is rather sharp The breakdown voltage can be controlled through the manufacturing process so it has a reasonably predictable value

2 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 i Z  0.1I to ensure Z

the constancy of v Z

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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

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)

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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