SIGNIFICANT EQUATIONS

When you consider the number of examples and problems in the text along with the length of the text material, this statistic clearly suggests that the text is as error-free as possible. The section on solar cells now includes a detailed examination of the materials used, additional response curves and a number of new practical applications.

BRIEF CONTENTS

Bipolar Junction Transistors 129

Determinations and Conversion Equations (Exact

Appendix D: Solutions to Selected

Transition and Diffusion Capacitance 30

Diode Specification Sheets 32

Semiconductor Diode Notation 35

Networks with a dc and ac Source 88

Class B Amplifier Operation 695
Class B Amplifier Circuits 699
Power Transistor Heat Sinking 709
Class C and Class D Amplifiers 712
Comparator Unit Operation 722
Digital–Analog Converters 729
Timer IC Unit Operation 732
Voltage-Controlled Oscillator 736
Feedback Connection Types 752
Practical Feedback Circuits 758
Tuned Oscillator Circuit 771
Unijunction Oscillator 777
General Filter Considerations 784
Discrete Transistor Voltage Regulation 791
Schottky Barrier (Hot-Carrier) Diodes 811
Varactor (Varicap) Diodes 815
Liquid-Crystal Displays 829
Silicon-Controlled Rectifier 841
Basic Silicon-Controlled Rectifier Operation 842
SCR Characteristics and Ratings 843
Silicon-Controlled Switch 849
Unijunction Transistor 857
Programmable Unijunction Transistor 869

Appendix A: Hybrid Parameters—Graphical Determinations and Conversion Equations (Exact and Approximate)

CHAPTER OBJECTIVES ●

COVALENT BONDING AND INTRINSIC MATERIALS ●

Between the two is an energy gap that the electron in the valence band must overcome to become a free carrier. Similarly, an electron in the valence band of gallium arsenide must gain more energy than one in silicon or germanium to enter the conduction band.

Insulator

Semiconductor

Conductor

Each is a member of a subset group of elements in the Periodic Table of Elements, also called Group V, because each has five valence electrons. Each is a member of a subgroup of elements in the Periodic Table of Elements, also called Group III, because each has three valence electrons.

Electron versus Hole Flow

Majority and Minority Carriers

SEMICONDUCTOR DIODE

The closer the minority carrier is to the junction, the greater the attraction for the layer of negative ions, and the smaller the resistance of the positive ions in the depletion region of the n-type material. Again, the same type of discussion can be applied to most carriers (holes) of the p-type material.

Note in the above use of the terms reverse saturation current and reverse current. As the voltage across the diode increases in the reverse bias region, the rate of the minority carriers responsible for the reverse saturation current I s will also increase.

Ge, Si, and GaAs

However, when V BV decreases to very low levels, such as ⫺5 V, another mechanism, called Zener analysis, will contribute to the sharp change in the characteristic. The shape of the curve in the reverse bias region is also quite similar for each material, but note the measurable difference in the magnitude of the typical reverse saturation currents.

Temperature Effects

In the reverse-bias region, the reverse current of a silicon diode doubles for every 10°C rise in temperature. Consider for a moment how large the reverse saturation current would be if we started with a Ge diode with a saturation current of 1 mA and applied the same doubling factor.

Summary

Ideally, if the semiconductor diode is to act like a closed switch in the forward-bias region, the resistance of the diode should be 0. In the reverse-bias region, its resistance must be fi⍀ to represent the open-circuit equivalent.

DC or Static Resistance

First impressions may indicate that the commercial unit gives a poor impression of the ideal transition. Due to the shape and location of the curve for the commercial unit in the forward bias region, there will be a resistance associated with the diode that is greater than 0.

AC or Dynamic Resistance

The derivative of the function at a point is equal to the slope of the tangent line drawn at that point. If we find the derivative of the general equation (1.2) for the semiconductor diode with respect to the applied forward bias and then invert the result, we will have an equation for the dynamic or ac resistance in that region.

Average AC Resistance

Typically, the AC resistance of a diode in the active region will vary from approx. 1 ⍀ to 100. Occasionally the AC resistor will make the transition from the high value at 2 mA to the lower value at 17 mA.

Summary Table

One technique to obtain an equivalent circuit for a diode is to approximate the characteristics of the device with straight line segments, as shown in figure. For the sloped portion of the equivalence, the average AC resistance as introduced in section 1.8 is: the resistance level appearing in the equivalent circuit of Fig.

Simplified Equivalent Circuit

DIODE EQUIVALENT 29 CIRCUITS V D ⫽ 0.8 V, we know that for silicon a shift of 0.7 V is required before the characteristics. If the characteristics or specification sheet for a diode is not available, the resistance rav can be approximated by the ac resistance rd.

Ideal Equivalent Circuit

TRANSITION AND DIFFUSION CAPACITANCE ●

VKbID (1.10) where t T is the minority carrier lifetime - the time it takes for a minority carrier, such as a hole, to recombine with an electron in the n-type material. The electrons in the p-type material and holes passing through the n-type material establish a large number of minority carriers in each material.

DIFFUSED SILICON PLANAR

SEMICONDUCTOR DIODE NOTATION ●

Anode refers to the higher or positive potential and cathode refers to the lower or negative terminal.

DIODE TESTING ●

Diode Checking Function

Ohmmeter Testing

For the reverse-bias situation, the reading must be quite high, which requires a high resistance scale on the meter, as indicated in Fig. A high resistance reading in either direction indicates an open (defective device) condition, while a very low resistance current reading in either direction is likely to indicate a shorted device.

Curve Tracer

The term nominal used in the specification of the Zener voltage simply indicates that it is a typical average value. This is the current that will define the dynamic resistance Z ZT and appears in the general equation for the power rating of the device.

Important Conclusions and Concepts

In the reverse-bias region, the diode current is the very small reverse saturation current. The AC resistance of a diode is sensitive to the shape of the curve in the region of interest and decreases for higher levels of diode current or voltage.

Equations

The direction of conduction for a Zener diode is opposite to that of the arrow in the symbol, and the Zener voltage has an opposite polarity to that of a forward-biased diode. The result is the use of the term PSpice in the descriptions to follow when starting an analysis using OrCAD software.

OrCAD

50 Those of you who have used both programs before will find that the changes are minor and mainly appear in the front end and in the generation of specific data and plots. You will find that the OrCAD software has a wider field of study, but the Multisim software generates screens that better match the actual lab experience.

Multisim

The reason for including two programs stems from the fact that both are used throughout the educational community. The demo version of OrCAD is free from Cadence Design Systems, Inc. and can be downloaded directly from the EMA Design Automation, Inc. website, [email protected].

PROBLEMS

PARALLEL AND SERIES–PARALLEL

Solution: For an applied voltage, the "pressure" of the source acts to establish a current through each diode in the same direction as shown in the figure. The moment the rising supply voltage reaches 0.7V, the silicon diode will "turn on" and maintain the 0.7V level, because the characteristic at this voltage is vertical - the current of the silicon diode simply rises to the defined level.

SINUSOIDAL INPUTS; HALF-WAVE

Sketch the output v o and determine the dc output level for the network in fig. A negative sign means that the polarity of the output is opposite to the polarity defined in fig.

PIV (PRV)

Bridge Network

The important result is that the polarity across the load resistor R is the same as in Figure 2.54, creating a second positive pulse as shown in Figure. If silicon is used instead of ideal diodes, as shown in Figure 2.58, the application of Kirchhoff's voltage law around the conduction path can result in

Center-Tapped Transformer

For the negative part of the input, the roles of the diodes are reversed and vo appears as shown in the figure. Depending on the orientation of the diode, the positive or negative region of the applied signal is "clipped".

Series

Take careful note of where the output voltage is defined
Try to develop an overall sense of the response by simply noting the “pressure”
Determine the applied voltage (transition voltage) that will result in a change of state for the diode from the “off” to the “on” state
It is often helpful to draw the output waveform directly below the applied voltage using the same scales for the horizontal axis and the vertical axis

Determine the applied voltage (transition voltage) that will result in a change of state for the diode from the "off" to the "on" state. For the "on" region, as shown in Fig. 2.72, the diode is replaced by a short-circuit equivalent, and the output voltage is defined by.

Parallel

In this example the output is defined across the series combination of the 4-V sup- ply and the diode, not across the resistor R
The polarity of the dc supply and the direction of the diode strongly suggest that the diode will be in the “on” state for a good portion of the negative region of the input
Start the analysis by examining the response of the portion of the input signal that will forward bias the diode
During the period that the diode is in the “on” state, assume that the capac- itor will charge up instantaneously to a voltage level determined by the surrounding
Assume that during the period when the diode is in the “off” state the capac- itor holds on to its established voltage level
Check that the total swing of the output matches that of the input

NETWORKS WITH A DC AND AC SOURCE ●

For input voltages less than 3.3 V, the diode will be in the "on" state and the network of Fig. The analysis will start with the period t1St2 of the input signal since the diode is in the short circuit state.

DC Source

DC 89 AND AC SOURCE NETWORKS For such a system it is particularly important that the Superposition Theorem can be applied. The answer of any ac and dc source network can be found by finding the answer for each source independently and then combining the results.

AC Source

How does the power absorbed by the LED compare to the 6-V Zener diode. Determine the state of the Zener diode by removing it from the network and calculate the voltage across the resulting open circuit.

96 To obtain the minimum load resistance of Fig. 2.112 to determine what will turn on the Zener diode, simply calculate the value of R L that will result in a load voltage V L ⫽ V Z. Any load resistance value greater than the R L obtained from Eq. 2.20) will ensure that the Zener diode is in the "on" state and that the diode can be replaced by its VZ source equivalent. The condition defined by Eq. 2.20) establishes the minimum R L, but in turn specifies the maximum I L as.

EXAMPLE 2.28 Determine the range of values of V i that the Zener diode of Fig will maintain. Voltage multiplier circuits are used to maintain a relatively low transformer peak voltage while increasing the peak output voltage to two, three, four or more times the peak rectified voltage.

Voltage Doubler

The maximum inverse voltage across each diode is 2 V m , as is the case for the filter capacitor circuit. In summary, half-wave or half-wave voltage doubler circuits provide twice the peak voltage of the secondary transformer, while requiring no central transformer and only a 2 V m PIV rating for the diodes.

Voltage Tripler and Quadrupler

In operation, capacitor C 1 charges through diode D 1 to a peak voltage Vm during the positive half cycle of the transformer secondary voltage. Capacitor C 2 charges up to twice the peak voltage, 2 V m , developed by the sum of the voltages across capacitor C 1 and the transformer during the negative half cycle of the transformer secondary voltage.

Rectification

Also note that the polarity of the voltage across the coil during the "build-up" phase is opposite to that during the "release". The rate at which the current collapses will be controlled by the resistance of the coil and the diode.

Polarity Insurance

2.137c will conduct the diode and ensure that no more than 0.7 V appears across the terminals of the system, protecting it from excessive voltages with the wrong polarity. This simple design protects the sensitive movement against wrong polarity voltages greater than 0.7 V.

Controlled Battery-Powered Backup

For either polarity, the difference between the applied voltage and the load or diode voltage will appear across the series source or network resistance.

Polarity Detector

In the forward bias mode, the additional diodes D 1 and D 2 reduce the voltage across the LEDs to more common operating levels.

Displays

Setting Voltage Reference Levels

Establishing a Voltage Level Insensitive to the Load Current

AC Regulator and Square-Wave Generator

To determine the state of a diode, initially simply think of it as a resistor and find the polarity of the voltage across it and the direction of normal current through it. For typical half-wave and half-wave bridge rectifiers, this is the maximum value of the applied signal.

Cadence OrCAD

Demo . The resulting screen has only a few active keys on the top toolbar

TRANSISTOR CONSTRUCTION ●

A New Simulation dialog box appears, first asking for the name of the simulation. However, the collector current consists of two components: the majority and the minority carriers, as shown in Fig.

Alpha (A)

The characteristics clearly show that IC ⬵ IE = 3 mA. The effect of changing V CB is negligible and IC continues to be 3 mA. AC Mode For AC situations where the operating point shifts on the characteristic curve, the AC alpha is defined by. In most cases, the sizes of a ac and a dc are quite close to each other, allowing one size to be used for the other.

Biasing

AC mode For AC situations where the operating point moves on the characteristic curve, an AC alpha is defined by. The AC alpha is formally called the common-base, short-circuit, amplification factor, for reasons that will become more obvious when we examine transistor equivalent circuits in Chapter 5.

Breakdown Region

For purposes of linear amplification (least distortion), the cutoff for the common-emitter configuration will be determined by I C ⫽ I CEO. For the common emitter configuration the same approach can be taken, resulting in the approximate equivalent of Fig.

Beta (B)

COMMON-COLLECTOR CONFIGURATION ●

For all practical purposes, the output characteristics of the common-collector configuration are the same as those of the common-emitter configuration. For the input circuit of the common-collector configuration, the common-emitter-base characteristics are sufficient to obtain the necessary information.

TRANSISTOR SPECIFICATION SHEET

In the small-signal characteristics, the level of h fe (b ac ) is provided along with a plot of how it varies with collector current in Fig. The vertical sensitivity is 2 mA/div, which leads to the scale on the left side of the monitor's display.

Transistor Testers

TRANSISTOR HOUSING 151 AND TERMINAL IDENTIFICATION of approximately 0.7 V with the red (positive) wire connected to the base and the black (negative). Similarly, with the emitter open, the forward and reverse bias conditions of the base-collector junction can be monitored.

Ohmmeter

TRANSISTOR CASING AND

A reversal of the leads should result in an OL indication to represent the reverse-biased junction. As with the diode IC package, the indentation in the top surface reveals the number 1 and 14 pins.

Transistors are three-terminal three-layer semiconductor devices that have a base or center layer that is much thinner than the other two layers. The two outer layers are both n- or p-type materials, with the sandwich layer of the opposite type.

If we choose a point in the middle of the characteristics defined by V CE ⫽ 4 V and I B ⫽ 60 mA as shown in the figure. Changing R C to any level will not affect the level of I B or IC as long as we stay in the active region of the device.

Transistor Saturation

Once ICsat is known, we have some idea of the maximum possible collector current for the chosen design and the level it will stay below if we expect linear amplification. The design of Example 4.1 resulted in ICQ = 2.35 mA, which is far from the saturation level and about half the maximum value for the design.

Load-Line Analysis

Load line analysis: (a) the grid; b) the characteristics of the equipment. steeper the slope of the network load line. If the level of IB is changed by varying the value of RB, the Q point moves up or down the load line as shown in Fig.

Base–Emitter Loop

The analysis will be performed by first examining the base-emitter loop and then using the results to examine the collector-emitter loop.

Collector–Emitter Loop

EMITTER PREPARATION 171 CONFIGURATION The single-subscript voltage V E is the voltage from emitter to ground and is deter-.

Improved Bias Stability

EMITTER-BIAS 173 CONFIGURATION Now the BJT collector current increases by about 81% due to the 100% increase in b. Note that I B decreased, helping to maintain the value of I C - or at least reducing the overall change in I C due to the change in b.

Saturation Level

VOLTAGE-DIVIDER BIAS CONFIGURATION ●

As noted earlier, there are two methods that can be applied to analyze the voltage divider configuration. The first thing demonstrated is the exact method, which can be applied to any voltage divider configuration.

Exact Analysis

EXAMPLE 4.8 Determine the DC bias VCE and current IC for the voltage divider configuration of Fig.

Approximate Analysis

Because Ri = (b + 1)RE ⬵ bRE is the condition that will define whether the approximate approach can be applied. In other words, if b times the value of R E is at least 10 times the value of R 2 , the approximate approach can be applied with a high degree of accuracy.