Department of Physics

• HINDU COLLEGE GUNTUR

Presented By P.V.RAMU

2

N-Type Material

antimony and phosphorus.

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

3

N-Type Material:

When extra valence electrons are introduced into a material such as silicon an n-type

material is produced. The electrons are introduced

extra valence by putting

impurities or dopant into the silicon. The

dopant used to create an n-type material are

Group V elements. The most commonly used dopant from Group V are arsenic,

+4

+4

+5

+4

+4

+4

+4

+4

+4

3/24/2023

P-Type Material

aluminum, boron, and gallium.

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

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P-Type Material:

P-type material is produced when the dopant

that is introduced is from Group III. Group

III elements have only 3 valence electrons and therefore there is an electron missing. This creates a hole (h+), or a positive charge

that can move around in the material.

Commonly used Group III dopant are

+4

+4

+3

+4

+4

+4

+4

+4

+4

The PN Junction

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Steady State

P

n

- - - - -

- - - - -

- - - - -

- - - - -

+ + + + +

+ + + + +

+ + + + +

+ + + + +

Metallurgical

Na Junction Nd

Space Charge Region

ionized

acceptors

ionized

donors

E-Field

+ +

_ _

h+ diffusion

e- diffusion

h+ drift ==

== e- drift

When no external source is connected to the pn

junction, diffusion and

drift balance each other

out for both the holes

and electrons

Space Charge Region: Also called the depletion region. This region includes the net positively and negatively charged regions. The space charge region does not have any free carriers. The width of the space charge region is denoted by W in pn junction formula’s.

Metallurgical Junction: The interface where the p- and n-type materials meet.

​

Na & Nd: Represent the amount of negative and positive doping in number of carriers per centimeter cubed. Usually in the range of 10¹⁵ to 10²⁰.

The Biased PN Junction

P

n

+

_

App Electric

lied

Field

Metal

Contact

“Ohmic

Contact”

(Rs~0)

+

_

I

^Vapplied

The pn junction is considered biased when an external voltage is applied.

There are two types of biasing: Forward bias and Reverse bias.

These are described on then next slide.

The Biased PN Junction

Forward Bias:

In forward bias the depletion region shrinks slightly in

width. With this shrinking the energy required for

charge carriers to cross the depletion region decreases

exponentially. Therefore, as the applied voltage

increases, current starts to flow across the junction. The barrier potential of the diode is the voltage at which appreciable current starts to flow through the diode. The barrier potential varies for different materials.

Reverse Bias:

Under reverse bias the depletion region widens. This

causes the electric field produced by the ions to cancel

out the applied reverse bias voltage. A small leakage

current, Is (saturation current) flows under reverse bias

conditions. This saturation current is made up of electron-hole pairs being produced in the depletion region. Saturation current is sometimes referred to as scale current because of it’s relationship to junction

temperature.

^Vapplied ^{> 0}

^Vapplied ^{< 0}

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Properties of Diodes

Figure 1.10 – The Diode Transconductance Curve²

• V_D = Bias Voltage
• I_D = Current through Diode. I_D is Negative for Reverse Bias and Positive for Forward Bias
• I_S = Saturation Current
• V_BR = Breakdown Voltage
• V_φ = Barrier Potential Voltage

V_D

I_D

(mA)

(nA)

^VBR

~V_φ

I_S

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Properties of Diodes

The Shockley Equation

• The transconductance curve on the previous slide is characterized by the following equation:

I_D = I_S(e^VD/ηVT – 1)

• As described in the last slide, I_D is the current through the diode, I_S is

the saturation current and V_D is the applied biasing voltage.

• V_T is the thermal equivalent voltage and is approximately 26 mV at room

temperature. The equation to find V_T at various temperatures is:

V_T = kT

q

k = 1.38 x 10^-23 J/K T = temperature in Kelvin q = 1.6 x 10^-19 C

• η is the emission coefficient for the diode. It is determined by the way the diode is constructed. It somewhat varies with diode current. For a silicon diode η is around 2 for low currents and goes down to about 1 at higher currents

Types of Diodes and Their Uses

PN Junction

Diodes:

Are used to allow current to flow in one direction

while blocking current flow in the opposite

direction. The pn junction diode is the typical diode that has been used in the previous circuits.

A

K

Schematic Symbol for a PN Junction Diode

P

n

Representative Structure for a PN Junction Diode

Zener Diodes:

Are specifically designed to operate under reverse

breakdown conditions. These diodes have a very accurate and specific reverse breakdown voltage.

A

K

Schematic Symbol for a Zener Diode

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Types of Diodes and Their Uses

Light-Emitting

Diodes:

Light-emitting diodes are designed with a very large band gap so movement of carriers across their

depletion region emits photons of light energy.

Lower band gap LEDs (Light-Emitting Diodes) emit infrared radiation, while LEDs with higher band gap energy emit visible light. Many stop lights are now starting to use LEDs because they are extremely bright and last longer than regular bulbs for a relatively low cost.

A

K

Schematic Symbol for a Light-Emitting Diode

The arrows in the LED representation indicate emitted light.

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Types of Diodes and Their Uses

Photodiodes:

While LEDs emit light, Photodiodes are sensitive to

received light. They are constructed so their pn

junction can be exposed to the outside through a clear window or lens.

In Photoconductive mode the saturation current

increases in proportion to the intensity of the

received light. This type of diode is used in CD players.

In Photovoltaic mode, when the pn junction is

exposed to a certain wavelength of light, the diode generates voltage and can be used as an energy

source. This type of diode is used in the

production of solar power.

A

K

A

K

Schematic Symbols for Photodiodes

λ

12

13