Kristin Ackerson, Virginia Tech EE Spring 2002. A DIODE IS A SEMICONDUCTER DEVISE, IT A ACTIVE COMPONENT WHOSE PROVIDE BEST FLOW OF CURRENT. IT IS A PN

Kristin Ackerson, Virginia Tech EEKristin Ackerson, Virginia Tech EESpring 2002Spring 2002

A DIODE IS A SEMICONDUCTER DEVISE , IT A ACTIVE A DIODE IS A SEMICONDUCTER DEVISE , IT A ACTIVE COMPONENT WHOSE PROVIDE BEST FLOW OF CURRENT.COMPONENT WHOSE PROVIDE BEST FLOW OF CURRENT.

IT IS A PN JUNCTION FORMED BY TOGETHER P AND N TYPE IT IS A PN JUNCTION FORMED BY TOGETHER P AND N TYPE MATERIAL. ONE SIDE OF THE JUNCTION CONTAIN DONER MATERIAL. ONE SIDE OF THE JUNCTION CONTAIN DONER

IMPURITY AND OTHER SIDE HAS ACCEPTER. AND A SINGLE IMPURITY AND OTHER SIDE HAS ACCEPTER. AND A SINGLE JUNCTION MADE BY P TYPE AND N TYPE MATERIAL KNOWN AS JUNCTION MADE BY P TYPE AND N TYPE MATERIAL KNOWN AS

DIODE.DIODE.

N-Type MaterialN-Type Material


N-Type Material:N-Type Material: When extra valence electrons are introduced When extra valence electrons are introduced into a material such as silicon an n-type into a material such as silicon an n-type material is produced. The extra valence material is produced. The extra valence electrons are introduced by putting electrons are introduced by putting impurities or dopants into the silicon. The impurities or dopants into the silicon. The dopants used to create an n-type material dopants used to create an n-type material are Group V elements. The most commonly are Group V elements. The most commonly used dopants from Group V are arsenic, used dopants from Group V are arsenic, antimony and phosphorus. antimony and phosphorus.

The 2D diagram to the left shows the extra The 2D diagram to the left shows the extra electron that will be present when a Group V electron that will be present when a Group V dopant is introduced to a material such as dopant is introduced to a material such as silicon. This extra electron is very mobile. silicon. This extra electron is very mobile.

P-Type MaterialP-Type Material


P-Type Material:P-Type Material: P-type material is produced when the dopant P-type material is produced when the dopant that is introduced is from Group III. Group that is introduced is from Group III. Group III elements have only 3 valence electrons III elements have only 3 valence electrons and therefore there is an electron missing. and therefore there is an electron missing. This creates a hole (h+), or a positive charge This creates a hole (h+), or a positive charge that can move around in the material. that can move around in the material. Commonly used Group III dopants are Commonly used Group III dopants are aluminum, boron, and gallium.aluminum, boron, and gallium.

The 2D diagram to the left shows the hole The 2D diagram to the left shows the hole that will be present when a Group III dopant that will be present when a Group III dopant is introduced to a material such as silicon. is introduced to a material such as silicon. This hole is quite mobile in the same way the This hole is quite mobile in the same way the extra electron is mobile in a n-type material. extra electron is mobile in a n-type material.

The PN JunctionThe PN Junction


Steady StateSteady State11

PP nn

- - - - - -- - - - - -

- - - - - -- - - - - -

- - - - - -- - - - - -

- - - - - -- - - - - -

- - - - - -- - - - - -

+ + + + + + + + + + ++

+ + + + + + + + + + ++

+ + + + + + + + + + ++

+ + + + + + + + + + ++

+ + + + + + + + + + ++

NaNa NdNdMetallurgical Metallurgical

JunctionJunction

Space Charge Space Charge RegionRegionionized ionized

acceptorsacceptorsionized ionized donorsdonors

E-FieldE-Field

++++__ __

h+ drifth+ drift h+ diffusionh+ diffusion e- diffusione- diffusion e- drifte- drift== ==

The Biased PN JunctionThe Biased PN Junction


PP nn

++__

Applied Applied Electric FieldElectric Field

Metal Metal ContactContact

““Ohmic Ohmic Contact”Contact”

(Rs~0)(Rs~0)

++__

VVappliedapplied

II

The pn junction is considered biased when an external voltage is applied. The pn junction is considered biased when an external voltage is applied. There are two types of biasing: Forward bias and Reverse bias. There are two types of biasing: Forward bias and Reverse bias.

These are described on then next slide.These are described on then next slide.

The Biased PN JunctionThe Biased PN Junction


Forward Bias:Forward Bias: In forward bias the depletion region shrinks slightly in In forward bias the depletion region shrinks slightly in width. With this shrinking the energy required for width. With this shrinking the energy required for charge carriers to cross the depletion region decreases charge carriers to cross the depletion region decreases exponentially. Therefore, as the applied voltage exponentially. Therefore, as the applied voltage increases, current starts to flow across the junction. increases, current starts to flow across the junction. The barrier potential of the diode is the voltage at The barrier potential of the diode is the voltage at which appreciable current starts to flow through the which appreciable current starts to flow through the diode. The barrier potential varies for different diode. The barrier potential varies for different materials.materials.

Reverse Bias:Reverse Bias: Under reverse bias the depletion region widens. This Under reverse bias the depletion region widens. This causes the electric field produced by the ions to cancel causes the electric field produced by the ions to cancel out the applied reverse bias voltage. A small leakage out the applied reverse bias voltage. A small leakage current, Is (saturation current) flows under reverse bias current, Is (saturation current) flows under reverse bias conditions. This saturation current is made up of conditions. This saturation current is made up of electron-hole pairs being produced in the depletion electron-hole pairs being produced in the depletion region. Saturation current is sometimes referred to as region. Saturation current is sometimes referred to as scale current because of it’s relationship to junction scale current because of it’s relationship to junction temperature.temperature.

VVappliedapplied > 0 > 0

VVappliedapplied < 0 < 0

Properties of DiodesProperties of Diodes


Figure 1.10 – The Diode Transconductance CurveFigure 1.10 – The Diode Transconductance Curve22

• VVDD = Bias Voltage = Bias Voltage

• IIDD = Current through = Current through Diode. IDiode. IDD is Negative is Negative for Reverse Bias and for Reverse Bias and Positive for Forward Positive for Forward BiasBias

• IISS = Saturation = Saturation CurrentCurrent

• VVBRBR = Breakdown = Breakdown VoltageVoltage

• VV = Barrier Potential = Barrier Potential

VoltageVoltage

VVDD

IIDD (mA)(mA)

(nA)(nA)

VVBRBR

~V~V

IISS


Types of Diodes and Their UsesTypes of Diodes and Their Uses

PN Junction PN Junction Diodes:Diodes:

Are used to allow current to flow in one direction Are used to allow current to flow in one direction while blocking current flow in the opposite while blocking current flow in the opposite direction. The pn junction diode is the typical diode direction. The pn junction diode is the typical diode that has been used in the previous circuits.that has been used in the previous circuits.

AA KK

Schematic Symbol for a PN Schematic Symbol for a PN Junction DiodeJunction Diode

PP nn

Representative Structure for Representative Structure for a PN Junction Diodea PN Junction Diode

Zener Diodes:Zener Diodes: Are specifically designed to operate under reverse Are specifically designed to operate under reverse breakdown conditions. These diodes have a very breakdown conditions. These diodes have a very accurate and specific reverse breakdown voltage.accurate and specific reverse breakdown voltage.

AA KK

Schematic Symbol for a Schematic Symbol for a Zener DiodeZener Diode



Schottky Schottky Diodes:Diodes:

These diodes are designed to have a very fast These diodes are designed to have a very fast switching time which makes them a great diode for switching time which makes them a great diode for digital circuit applications. They are very common digital circuit applications. They are very common in computers because of their ability to be switched in computers because of their ability to be switched on and off so quickly. on and off so quickly. AA KK

Schematic Symbol for a Schematic Symbol for a Schottky DiodeSchottky Diode

Shockley Shockley Diodes:Diodes:

The Shockley diode is a four-layer diode while other The Shockley diode is a four-layer diode while other diodes are normally made with only two layers. diodes are normally made with only two layers. These types of diodes are generally used to control These types of diodes are generally used to control the average power delivered to a load. the average power delivered to a load.

AA KK

Schematic Symbol for a Schematic Symbol for a four-layer Shockley Diodefour-layer Shockley Diode



Light-Emitting Light-Emitting Diodes:Diodes:

Light-emitting diodes are designed with a very large Light-emitting diodes are designed with a very large bandgap so movement of carriers across their bandgap so movement of carriers across their depletion region emits photons of light energy. depletion region emits photons of light energy. Lower bandgap LEDs (Light-Emitting Diodes) emit Lower bandgap LEDs (Light-Emitting Diodes) emit infrared radiation, while LEDs with higher bandgap infrared radiation, while LEDs with higher bandgap energy emit visible light. Many stop lights are now energy emit visible light. Many stop lights are now starting to use LEDs because they are extremely starting to use LEDs because they are extremely bright and last longer than regular bulbs for a bright and last longer than regular bulbs for a relatively low cost. relatively low cost.

AA KK

Schematic Symbol for a Schematic Symbol for a Light-Emitting DiodeLight-Emitting Diode

The arrows in the LED The arrows in the LED representation indicate representation indicate

emitted light.emitted light.



Photodiodes:Photodiodes: While LEDs emit light, Photodiodes are sensitive to While LEDs emit light, Photodiodes are sensitive to received light. They are constructed so their pn received light. They are constructed so their pn junction can be exposed to the outside through a junction can be exposed to the outside through a clear window or lens.clear window or lens.

In Photoconductive mode the saturation current In Photoconductive mode the saturation current increases in proportion to the intensity of the increases in proportion to the intensity of the received light. This type of diode is used in CD received light. This type of diode is used in CD players.players.

In Photovoltaic mode, when the pn junction is In Photovoltaic mode, when the pn junction is exposed to a certain wavelength of light, the diode exposed to a certain wavelength of light, the diode generates voltage and can be used as an energy generates voltage and can be used as an energy source. This type of diode is used in the source. This type of diode is used in the production of solar power.production of solar power.

AA KK

AA KK

Schematic Symbols for Schematic Symbols for PhotodiodesPhotodiodes

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Kristin Ackerson, Virginia Tech EE Spring 2002. A DIODE IS A SEMICONDUCTER DEVISE, IT A ACTIVE COMPONENT WHOSE PROVIDE BEST FLOW OF CURRENT. IT IS A PN