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Diode
MOSFETMOSFET
BJT
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Diode
The basic operation of a diodep The basic structure of atoms The properties of insulators conductorsThe properties of insulators, conductors, and semiconductors Covalent bonding The properties of p and n type materialsp p p yp The forward and reverse biasing of a p-n junctionjunction The application of diode
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Introduction
The basic function of a diode is to restrict current flowThe basic function of a diode is to restrict current flow to one direction.
Forward bias Reverse Bias
Bohr model of an atom
As seen in this model, electrons circle the nucleus. Atomic structure of a material determines it’s ability to conduct orit s ability to conduct or insulate.
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Conductors, Insulators, and Semiconductors
The ability of a material to conduct current isThe ability of a material to conduct current is based on its atomic structure.
The orbit paths of the electrons surroundingThe orbit paths of the electrons surrounding the nucleus are called shells.
Each shell has a defined number of electrons itEach shell has a defined number of electrons it will hold. This is a fact of nature and can be determined by the formula, 2n2.
The less complete a shell is filled to capacity the
The outer shell is called the valence shell.
The less complete a shell is filled to capacity the more conductive the material is.
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Conductors, Insulators, and Semiconductors
The valence shell determines the ability of material to conduct current.
A Copper atom has only 1 electron in it’ l i Thi k it
A Silicon atom has 4 electrons in it’s it’s valence ring. This makes it a good conductor. It takes 2n2
electrons or in this case 32 electrons to fill the valence shell
valence ring. This makes it a semiconductor. It takes 2n2 electrons or in this case or 18 electrons to fill
to fill the valence shell. the valence shell.
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Covalent BondingCovalent Bonding
Covalent bonding is a bonding of two or more atoms by the interaction of their valence electrons.
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Covalent Bonding
Certain atoms will combine in this way to form a crystal structure Silicon and Germanium atoms combine in this
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structure. Silicon and Germanium atoms combine in this way in their intrinsic or pure state.
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The process of creating N and P type
Other atoms with 5 electrons such Other atoms with 3 electrons such as
The process of creating N and P type materials is called doping.
as Antimony are added to Silicon to increase the free electrons.
Boron are added to Silicon to create a deficiency of electrons or hole charges.
N-type P-type
The Depletion Region
p region n region p region n regionp g g p g g
With the formation of the p and n materials combination
This creates the depletion region and has a barrier
of electrons and holes at the junction takes place.
region and has a barrier potential. This potential cannot be measured with a
lt t b t it ill
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Forward and Reverse Bias
Voltage source or bias connections Voltage source or bias connections are
to the p material and + to the n
Forward Bias Reverse Bias
g are + to the p material and – to the n material
Bias must be greater than 3 V for
– to the p material and + to the n material
Bias must be less than the break down Bias must be greater than .3 V for Germanium or .7 V for Silicon diodes.
voltage.
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The depletion region widens.
Ideal Diode Characteristic Curve
In this characteristicIn this characteristic curve we do not consider the voltage drop or the resistive properties. Current flow proportionallyflow proportionally increases with voltage.
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Practical Diode Characteristic Curve
In most cases we consider only the f d bi ltforward bias voltage drop of a diode. Once this voltage is g overcome the current increases proportionally withproportionally with voltage.This drop is particularly important p y p to consider in low voltage applications.
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Complex Characteristic Curve of a Di dDiode
The voltage drop is not the only loss of a diode. In some cases we must take into account other factorsaccount other factors such as the resistive effects as well as reverse breakdown.
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Diode Packagesg
Diodes come in a variety of sizes and shapes. The design and structure is determined by what type of circuit they will be used in.
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Typical diode packages and terminal identification. A is anode and K is cathode.a ode a d s cat ode
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clipperpp
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Figure 3.37 The clamped capacitor with a load resistance R.
The clamped capacitor with a load resistance Rresistance R
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Zener Diode
The basic function of zener diode is to maintain a specific p voltage across it’s terminals within given limits of line or load change. Typically it is used for providing a stable reference voltage for use in power supplies and otherreference voltage for use in power supplies and other equipment.
This particular zener circuit will work to maintain 10 V across the load
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This particular zener circuit will work to maintain 10 V across the load.
Zener Diodes A zener diode is much like a normal diode. The exception being is that it is placed in the circuit in reverse bias andbeing is that it is placed in the circuit in reverse bias and operates in reverse breakdown. This typical characteristic curve illustrates the operating range for a zener. Note that it’s forward characteristics are just like a normal diode.
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Zener Diodes
Zeners are available• Zeners are available with voltage breakdowns of 1.8 Vbreakdowns of 1.8 V to 200 V.
• This curve illustrates• This curve illustrates the minimum and maximum ranges of g current operation that the zener can ff ti l i t ieffectively maintain
it’s voltage.
Zener Diodes
As with most devices, zener diodes have given characteristics such as temperature coefficients and
i h h b id d Th dpower ratings that have to be considered. The data sheet provides this information.
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RegulationRegulation
In this simple illustration of zener regulation circuit, the zener diode will “adjust” it’s impedance based on varying inputdiode will adjust it s impedance based on varying input voltages and loads (RL) to be able to maintain it’s designated zener voltage. Zener current will increase or decrease directly with voltage input changes. The zener current will increase or decrease inversely with varying loads. Again, the zener has a finite range of operationfinite range of operation.
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Zener Limitingg
Zener diodes can used for limiting just as normal diodesZener diodes can used for limiting just as normal diodes. Recall in previous chapter studies about limiters. The difference to consider for a zener limiter is a it’s zener b kd h t i tibreakdown characteristics.
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Varactor Diodes
A varactor diode is best explained as a variable capacitorA varactor diode is best explained as a variable capacitor. Think of the depletion region a variable dielectric. The diode is placed in reverse bias. The dielectric is “adjusted” by bias changes.
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Varactor Diodes
The varactor diode can be useful in filter circuits as the adjustable component.
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Light Emitting Diodeg g
The light emitting diode (LED) emits photons asThe light-emitting diode (LED) emits photons as visible light. It’s purpose is for indication and other intelligible displays. Various impurities are added g p y p during the doping process to vary the color output.
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Optical Diodesp The seven segment display is an example of LEDs use for di l f d i l di itdisplay of decimal digits.
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Other Diode Types - LDyp The laser diode (light amplification by stimulated emission f di ti ) d h ti ( i l l ) li htof radiation) produces a monochromatic (single color) light.
Laser diodes in conjunction with photodiodes are used to retrieve data from compact discs. p
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Optical Diodes - Photo Diodep
The photodiode is used to vary current by the amount ofThe photodiode is used to vary current by the amount of light that strikes it. It is placed in the circuit in reverse bias. As with most diodes when in reverse bias, no current flows when in reverse bias, but when light strikes the exposed junction through a tiny window, reverse current increases proportional to light intensityproportional to light intensity.
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Optical Diodes - Solar Cellp
The solar cell operates on the same principleThe solar cell operates on the same principle as the photo diode. The area of the junction is larger and enough pairs are produced thatlarger, and enough pairs are produced that significant energy can be taken from the d idevice.
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Other Diode Typesyp
The Schottky diode’s significant characteristic is it’sThe Schottky diode’s significant characteristic is it s fast switching speed. This is useful for high frequencies and digital applications. It is not a typical diode in the g pp yp fact that it does not have a p-n junction, instead it consists of a heavily doped n-material and metal bound togethertogether.
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Other Diode Typesyp
h i di d l d lThe pin diode is also used in mostly microwave frequency applications. It’s variable forward series resistance characteristic is used for attenuation,resistance characteristic is used for attenuation, modulation, and switching. In reverse bias exhibits a nearly constant capacitance.
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Other Diode Typesyp
Th t l di d h ti i t It ill t llThe tunnel diode has negative resistance. It will actually conduct well with low forward bias. With further increases in bias it reaches the negative resistance range where g g current will actually go down. This is achieved by heavily doped p and n materials that creates a very thin depletion regionregion.
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Summary
Diodes, transistors, and integrated circuits are
P-materials are doped with trivalent impurities
, , g all made of semiconductor material.
N-materials are doped with pentavalent impurities
P and N type materials are joined together to form aP and N type materials are joined together to form a PN junction.
A diode is nothing more than a PN junctionA diode is nothing more than a PN junction.
At the junction a depletion region is formed. This creates barrier which requires approximately .3 V for acreates barrier which requires approximately .3 V for a Germanium and .7 V for Silicon for conduction to take place.
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Summaryy
A di d d t h f d bi d d d t
Wh d bi d di d l ith t d
A diode conducts when forward biased and does not conduct when reverse biased
When reversed biased a diode can only withstand so much applied voltage. The voltage at which avalanche current occurs is called reverse breakdown voltage.
There are three ways of analyzing a diode. These There are three ways of analyzing a diode. These are ideal, practical, and complex. Typically we use a practical diode model.
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