introduction semi devices transistors ics

8/3/2019 Introduction Semi Devices Transistors ICs

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Introduction toIntroduction to

Semiconductor DevicesSemiconductor Devices


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PrerequisitesPrerequisites

To understand this presentation, you shouldbe able to: Draw the structure of an atom, including electrons,

protons, and neutrons.

Define resistance and conductance. Label an electronic schematic, indicating current flow.

Define Ohms and Kirchhoffs laws.

Describe the characteristics of DC and AC (sine wave)

voltages.


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Student Learning OutcomesStudent Learning Outcomes

Upon completion of viewing this presentation, youshould be able to: Define active and passive components.

Name three main categories of active semiconductor devices.

Name the two elements of a diode and state the primary

operational characteristics of a diode.

Define bias and name the two types of bias and their effects on

diode operation.

Explain the concept of a transistor.

Explain how a transistor can switch or amplify.

Define integrated circuit.


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Passive ComponentsPassive Components

Semiconductor materials can be used to form almost

any kind of electronic component, including passivecomponents.

Passive components like resistors, capacitors orinductors are very common. A resistoris made with a piece of N or P-type

semiconductor material doped to the appropriateresistance level.

A capacitoris made with two plates of a highlyconductive semiconductor material separated by a puresemiconductor material or another insulator like glass

silicon dioxide (SiO2). An inductoris made by making a spiral of highlyconductive semiconductor material.


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Active Electronic ComponentsActive Electronic Components

An active electronic component either amplifiesor switches. The most common active components are diodes

and transistors.

Both diodes and transistors are easy to make with

semiconductors With semiconductor materials it is possible to

create complete circuits of active and/orpassive components wired together. These circuits are formed on a single chip of silicon

and are called integrated circuits (ICs).


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DiodesDiodes

A semiconductor diode is

formed with pieces of Nand P-type material are

joined. The P material is called the

anode.

The N material is called thecathode.

The resulting structure iscalled a PN junction.

A PN junction (or diode) isa switch or component

through which electronswill flow easily in onedirection but not in theopposite direction.


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Biasing a PN JunctionBiasing a PN Junction

To get current to flow in PN

junction or diode, you have toapply an external voltage calledbias.

With this connection, currentonly flows freely from cathodeacross the junction to the

anode. Youd say the switch is closed

when electrons can flow throughthe diode.

Note: The current flow may beso high that an externalresistance R is usually neededto minimize the current flow to alevel a diode can withstand.

This arrangement is calledforward bias.


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Reverse BiasReverse Bias

A diode with theexternal voltage

polarity reversed is

called reverse bias.

With this connection nocurrent will flow.

Youd say the switch is

open when electrons

cant flow through the

diode.


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Diode SymbolDiode Symbol

The PN junction forms a

diode.

To represent the diode in

schematic diagrams, we

use the symbol shown on

the right. Note the designations for

the anode and cathode.


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Diode Current FlowDiode Current Flow

The direction of current flow (electrons) is shown by the

arrow.


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APractical DiodeAPractical Diode

A practical diode has two wire leads. Note the end with the band circling the body is the

cathode end.

Wire Leads


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Forward and Reverse BiasForward and Reverse Bias

Examples of both forward and reverse biaseddiodes:


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Practical DiodesPractical Diodes

Diodes, like other electronic components, arenot perfect.

Diodes have an upper current limit that ifexceeded will destroy the diode.

You can get diodes with current limits of a fewmilliamperes up to hundreds or thousands ofamperes.

Diodes also have an upper voltage limit when

reverse biased. This is the reverse break down voltage which

may be only a few volts or hundreds orthousands of volts.


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Diode Voltage DropDiode Voltage Drop

Diodes also have a voltage drop

while they are conducting. This iscalled forward voltage drop. It is inthe 0.5 to 0.9 volt range for silicondiodes. A typical value is 0.7 volts.

The forward drop in a germaniumdiode is in the 0.2 to 0.4 volt range.

Diodes also have a thresholdvoltage approximately equal to theforward voltage drop. This is theminimum amount of forward voltagethat must be applied to get the diodeto conduct.

For example if you apply a forward

bias of less than abut 0.7 volts to asilicon diode, it will not conduct. Assoon as the bias voltage rises about0.7 volts the diode will conduct.


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TransistorsTransistors

A transistor is a 3-terminal

semiconductor device that isused to amplify or switch.

By applying an external DCvoltage, current will flow fromterminal 1 through the device toterminal 3.

A resistor is used to set thecurrent level.

A voltage or current applied toterminal 2 is used to control howmuch current flows fromterminals 1 to 3.

A very small voltage or current

variation at terminal 2 canproduce a very large currentvariation between terminals 1and 3.


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How Transistors SwitchHow Transistors Switch

The transistor is connected in

series with a resistor. With no (zero) voltage on

terminal 2, no current will flow inthe transistor. The transistoracts like an open switch. Theoutput voltage is 3 volts as seen

through the resistor R. If a large enough voltage is

applied to terminal 2, thetransistor will conduct heavilyand act like a very lowresistance between terminals 1

and 3. It then acts like a closedswitch. The output voltage isnear zero.


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How a TransistorAmplifiesHow a TransistorAmplifies

The circuit is the same as the

switch. With a small voltage on terminal

2, a large variation in currentfrom terminals 1 to 3 occurs.

For example, if a sine wave isapplied to the input, the currentthrough the transistor will be asine wave and it will produce asine wave voltage across theresistor and the transistor.

Since the current variation in thetransistor is very large, the output voltage is larger than the

smaller input voltage. The transistor amplifier is said to

have gain.


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How a TransistorAmplifies (continued)How a TransistorAmplifies (continued)

A key point to note in the

amplifier is that the transistordoes not actually make the input

voltage bigger. Instead, the

small input controls the larger

current through the device

produced by the external DCvoltage.

The transistor just generates a

larger separate version of the

input voltage in the output.


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Types ofTransistorsTypes ofTransistors

There are two commonly used

types of transistors, metal oxidesemiconductor field effecttransistors (MOSFETs) andbipolar junction transistors(BJTs) called bipolars.

MOSFETs are the most widely

used although BJTs are stillused in selected applications.

There are sub categories ofeach type and you will learnhow each works in a latercourse.


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Integrated CircuitsIntegrated Circuits

Integrated circuits (ICs)

are semiconductor devicesthat are complete circuitsmade up of transistors,diodes, capacitors,resistors and inductors.

The complete circuit ismade on a single piece ofsilicon called a chip.

See Figure.

Any circuit from a simpleamplifier to a quad coremicroprocessor used in aPC can be made.


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Most electronicequipment is made upof a collection of ICsand a small selectionof discrete passive

components likeresistors andcapacitors connectedtogether on a printed

circuit board (PCB). Figure shows a typical

board.


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In SummaryIn Summary

The simplest semiconductor device is a diode made by forming a

junction between a P and N-type semiconductor material.

The diode will pass current from cathode to anode but not in the other

direction.

If the anode is made positive and the cathode negative, the diode is

said to be forward biased and current will flow. The reverse condition

is called reverse bias and no current flows. A three terminal (element) semiconductor device is called a transistor.

The voltage on one element controls the current between the other two

elements.

A transistor is used to switch voltages or currents or to amplify small

signals into larger ones. MOSFETs and BJTs are the two major types of transistors

introduction semi devices transistors ics

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