semiconductor devices1

8/8/2019 Semiconductor Devices1

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A p-n junction is formed by combining N-type and P-typesemiconductors together in very close contact. The termjunction refers to the region where the two types of

semiconductor meet. It can be thought of as the borderregion between the P-type and N-type blocks as shown in thefollowing diagram:


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An external electric potential may be established acrossa pn junction with two different polarities. When a battery isused, the positive terminal may be connected to the p-side

and the negative terminal to the n-side; this is referred to as aforward bias. The opposite polarity (minus to p and plus to n)is termed reverse bias.


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The transistor is a solid state semiconductor device usedfor amplification and switching. It acts as a variable valvewhich, based on its input current (BJT) or input voltage (FET),allows a precise amount of current to flow through it from thecircuit's voltage supply.

What does a transistor actually do?

It can work either as an amplifieror a switch.


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When it works as an amplifier, it takes in a tiny electriccurrent at one end and (an input current) produces a muchbigger electric current (an output current) at the other. In

other words, it's a kind of current booster. That comes in reallyuseful in things like hearing aids, one of the first things peopleused transistors for. A hearing aid has a tiny microphone in itthat picks up sounds from the world around you and turnsthem into fluctuating electric currents. These are fed into atransistor that boosts them and powers a tiny loudspeaker, soyou hear a much louder version of the sounds around you.


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Transistors can also work as switches. A tinyelectric current flowing through one part of atransistor can make a much bigger current flow

through another part of it. In other words, the smallcurrent switches on the larger one. This is essentiallyhow all computer chips work. For example, in amemory chip, the small current turns the transistoron or off. Since the transistor can be in two distinct

states, a computer can use it to store two differentnumbers, zero and one. With lots of transistors, acomputer can lots of billions of zeros and ones anduse them to represent ordinary numbers andletters.


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Two major types:

Bipolar Junction Transistor

-semiconductor device capable ofamplifying voltage or power

Field Effect Transistor-commonly used for weak-signal

amplification (for example, for amplifyingwireless signals).


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A junction transistor consists of a thin piece of one type of

semiconductor material between two thicker layers of the oppositetype. For example, if the middle layer is p-type, the outside layers

must be n-type. Such a transistor is an NPN transistor. One of theoutside layers is called the emitter, and the other is known as the

collector. The middle layer is the base. The places where the emitterjoins the base and the base joins the collector are called junctions.


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MOSFETS are very versatile. They work very well,

both for small signal amplifiers and for high current

applications. Their advantages include:

High input resistance (DC circuits).

High input impedance (Audio and RF circuits).

Low noise. This is a particular advantage for RFamplifiers and also for the input sub-system of

sensitive audio amplifiers.

PowerMOSFETS can carry high currents (severalamps).


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An integrated circuit is a complete

electronic circuit in a package no larger

than that of a conventional low-powertransistor. The circuit consists of diodes,

transistors, resistors and capacitors.

Integrated circuits are produced with

the same technology and materials usedin making transistors and semiconductor

devices.


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The total conductivity of an ionic material is equal

to the sum of both electronic and ioniccontributions, as follows:

total = electronic + ionic


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Conductive Polymers

- plastics that conduct electricity

Conductive polymers and plastics are increasinglydesired for a growing number of sophisticated end-uses. Mostplastics are naturally non-conductive, hence their wide useas electrical insulators. Because of their ease of fabrication,however, polymers are highly desirable materials of

construction.W

here some transfer of electrical charge isdesired modifications to the polymer must be made toincrease conductivity.


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Ferroelectricity is an phenomenon that is

created when materials with qualities that make

them ideal for ferroelectric current to develop

have been placed in close proximity exhibit. The

resulting creation of electrical flow is referred to

as the creation of a dipole moment. The term

ferroelectricity is related to the concept of

ferromagnetism, which has to do with thecreation of a magnetic moment.


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Squeeze certain crystals (such as quartz) and you canmake electricity flow through them. The reverse is usuallytrue as well: if you pass electricity through the samecrystals, they "squeeze themselves" by vibrating back andforth. That's pretty much piezoelectricity in a nutshell but,for the sake of science, let's have a formal definition:

Piezoelectricity (also called the piezoelectric effect) isthe appearance of an electrical potential (a voltage, inother words) across the sides of a crystal when you subjectit to mechanical stress (by squeezing it). In practice, thecrystal becomes a kind of tiny battery with a positivecharge on one face and a negative charge on theopposite face; current flows if we connect the two facestogether to make a circuit. In the reverse piezoelectricelectric, a crystal becomes mechanically stressed(deformed in shape) when a voltage is applied across itsopposite faces.

semiconductor devices1

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