Diode Fundamentals

7/29/2019 Diode Fundamentals

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Diode Fundamentals


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Important issues

Formation of thepn Junction

Energy Band Diagrams

Concepts of Junction Potential

Modes of thepn Junction

Derivation of the IV Characteristics of apn Junction Diode

Linear Piecewise Models

Breakdown Diode

Special Types ofpn Junction SemiconductorDiodes

Applications of Diode


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INTRODUCTION

The origin of a wide range of electronic devices being used can be tracedback to a simple device, thepn junction diode.

The pn junction diode is formed when a p-type semiconductor impurity is

doped on one side and an n-type impurity is doped on the other side of a single

crystal.

All the macro effects of electronic devices, i.e., wave shaping, amplifying orregenerative effects, are based on the events occurring at the junction of thep

n device.

Most modern devices are a modification or amalgamation of pn devices in

various forms.

Prior to the era of semiconductor diodes, vacuum tubes were beingextensively used. These were bulky, costly and took more time to start

conducting because of the thermo-ionic emission.

The semiconductor diodes and the allied junction devices solved all these

problems.


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What Are Diodes Made Out Of?

Silicon (Si) and Germanium (Ge) are the two most

common single elements that are used to make

Diodes. A compound that is commonly used isGallium Arsenide (GaAs), especially in the case of

LEDs because of its large bandgap.

Silicon and Germanium are both group 4

elements, meaning they have 4 valence electrons.

Their structure allows them to grow in a shape

called the diamond lattice.

Gallium is a group 3 element while Arsenide is a

group 5 element. When put together as a

compound, GaAs creates a zincblend lattice

structure.

In both the diamond lattice and zincblend lattice,each atom shares its valence electrons with its

four closest neighbors. This sharing of electrons is

what ultimately allows diodes to be build. When

dopants from groups 3 or 5 (in most cases) are

added to Si, Ge or GaAs it changes the properties

of the material so we are able to make the P- andN-type materials that become the diode.

Si

+4

Si

+4

Si

+4

Si

+4

Si

+4

Si

+4

Si

+4

Si

+4

Si

+4

The diagram above shows the 2D

structure of the Si crystal. The

light green lines represent the

electronic bonds made when thevalence electrons are shared.

Each Si atom shares one electron

with each of its four closest

neighbors so that its valence band

will have a full 8 electrons.


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N-Type Material

N-Type Material:When extra valence electrons are

introduced into a material such as silicon

an n-type material is produced. Theextra valence electrons are introduced

by putting impurities or dopants into the

silicon. The dopants used to create an

n-type material are Group V elements.

The most commonly used dopants from

Group V are arsenic, antimony and

phosphorus.

The 2D diagram to the left shows theextra electron that will be present when

a Group V dopant is introduced to a

material such as silicon. This extra

electron is very mobile.

+4+4

+5

+4

+4+4+4

+4+4


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P-Type Material

P-Type Material: P-type material is produced when the

dopant that is introduced is from GroupIII. 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 usedGroup III dopants are 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 suchas silicon. This hole is quite mobile in the

same way the

extra electron is mobile ina n-type material.

+4+4

+3

+4

+4+4+4

+4+4


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FORMATION OF THE pn

JUNCTIONWhen donor impurities are introduced into one side and acceptors into the otherside of a single crystal semiconductor through various sophisticated microelectronic

device-fabricating techniques, apn junction is formed.

The presence of a concentration gradient between two materials in such intimate

contact results in a diffusion of carriers that tends to neutralize this gradient. Thisprocess is known as the diffusion process.

The nature of the pn junction so formed may, in general, be of two types:

A step-graded junction:- In a step-graded semiconductor junction, the impurity

density in the semiconductor is constant.

A linearly-graded junction:- In a linearly-graded junction, the impurity density

varies linearly with distance away from the junction.

A semiconductorpn junction


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ENERGY BAND DIAGRAMS It is assumed that a junction is made up of uniformly dopedp-type and n-type

crystals forming a step-graded junction.

Thepn Junction at Thermal Equilibrium

p-type and n-type semiconductors just beforecontact

From the discussion of the law of mass action, the carrier concentrations on either

side away from the junction are given by:

(where pn is the hole concentration in n-type semiconductors, np is the electron

concentration in p-type semiconductors; nn and pp are the electron and hole

concentrations in n- and p-type semiconductors respectively.)


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The energy band diagram of ap

n junction under thecondition of thermal equilibrium

Band structure ofp

n junction

ENERGY BAND DIAGRAMS


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CONCEPTS OF JUNCTION

POTENTIAL Space-charge Region

The non-uniform concentration of holes and electrons at the junction gives

rise to a diffusive flow of carriers.

Since the electron density is higher in the n-type crystal than in the p-type

crystal, electrons flow from the n-type to the p-type and simultaneously, due to

reversibility, the holes flow from the p-type to the n-type.

The result of this migration of carriers is that the region near the junction of

the n-type is left with a net positive charge (only ionized donor atoms) while

that of the p-type is left with a net negative charge (only ionized acceptor

atoms).

This diffusive mechanism of migration of the carriers across the junction

creates a region devoid of free carriers, and this region is called the space-

charge region, the depletion region or the transition region.


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The junction, as noted above, has three major properties:1. There is a space charge and an electric field across the junction,

which in turn indicates that the junction is pre-biased (i.e., there

exists a built-in potential, a very important concept,

2. The impure atoms maintaining the space charge are immobile in

the temperature range of interest (at very high temperatures, the

impurities become mobile). The pre-biased condition can bemaintained indefinitely;

3. The presence of any free electron or hole is strictly forbidden.

Built-in and Contact Potentials

This diffusive flow process results in a space-charge region and an

electric field.

The resulting diffusion current cannot build up indefinitely because anopposing electric field is created at the junction.

The homogeneous mixing of the two types of carriers cannot occur in

the case of charged particles in a pn junction because of the

development of space charge and the associated electric field E0.


POTENTIAL


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The electrons diffusing from the n-type to the p-type leave behinduncompensated donor ions in the n-type semiconductor, and the holes

leave behinduncompensated acceptors in thep-type semiconductors.

This causes the development of a region of positive space charge near the

n-side of the junction and negative space charge near the p-side. The

resulting electric field is directed from positive charge towards negative

charge.

Thus, E0 is in the direction opposite to that ofthe diffusion current for each

type of carrier.

Therefore, the field creates a drift component of current from n to p,

opposing the diffusion component of the current.

Since no net current can flow across the junction at equilibrium, the

current density due to the drift of carriers in the E0field must exactly

cancel the current densitydue to diffusion of carriers.

Moreover, since there can be no net build-up of electrons or holes on

either side as a function of time, the drift and diffusion current densities

must cancel for each type of carrier.


POTENTIAL


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POTENTIAL Therefore, the electric field E0 builds up to the point where the net

current density is zero at equilibrium.

The electric field appears in the transition region of length L about the

junction, and there is an equilibrium potential difference V0 across L

(known as contact potential).

In the electrostatic potential diagram, there is a gradient in potential

in the direction opposite to E0. In accordance with the followingfundamental relation:

The contact potential appearing across L under condition of zero

external bias is a built-in potential barrier, in that it is necessary for

the maintenance of equilibrium at the junction.

It does not imply any external potential. V0 is an equilibrium quantity,

and no net current can resultfrom it. In general, the contact potential

is the algebraic sum of the built-in potential and the applied voltage.

The variations in the contact potential under the condition of applied

bias are given in the subsequent sections.


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Assuming that the field is confined within the space-charge region L,

the potential barrier Vdand the field E0 are related by:

It should be noted that a voltmeter cannot measure this electrostatic

potential since the internal field is set up to oppose the diffusion

current and also since the built-in potential is cancelled exactly by the

potential drop across the contact. The barrier energy corresponding to barrier potential Vdis expressed

as EB = eVd. The value ofEB can be changed by doping change. The value

of EB is different for different semiconductors.


POTENTIAL


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Effect of Doping on Barrier Field

The width of the depletion region is inversely proportional to the

doping strength, as a larger carrier concentration enables the same

charge to be achieved over a smaller dimension.

It should be noted that the depletion charge for different doping is

not constant.

The barrier field is normally independent of the doping concentration

except under conditions of heavy doping, which may alter the band-gap itself, thereby modifying the barrier field.

The value ofVd in terms of the hole and electron concentrations can be

derived in the following manner.


POTENTIAL


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POTENTIAL At thermal equilibrium, the non-degenerate electron concentrations for

the n-type and p-type can be written as:

where Ecn, Ecp, Efn, and Efp are the conduction and Fermi level energies of

the n-type and p-type semiconductors, respectively, and Nc is the

effective density-of-states.

The Fermi levels are given by:

At equilibrium condition, the Fermi level must be constant throughout

the entire crystal.

Otherwise, because of the availability of lower energy levels, a flow of

carriers would result. The Fermi levels, therefore, must line up at theequilibrium.


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MODES OF THE pn JUNCTION

There are two modes of switching of apn junction diode.

Forward-biasedpn junction

When the positive terminal

of a battery is connected to

the p-type side and the

negative terminals to the n-

type side of a p

n junction,

the junction allows a large

current to flowthrough it due

to the low resistance level

offered by the junction. In

this case the junction is said

to be forward biased.

Energy band diagram of Forward-

biasedpn junction


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Reverse-biasedpn junction

When the terminals of the

battery are reversed i.e.,

when the positive terminal is

connected to the n-type side

and the negative terminal isconnected to the p-type side,

the junction allows a very

little current to flow through

it due to the high resistance

level offered by the junction.

Under this condition, the p

n junction is said to be

reverse-biased.

Energy band diagram of Reverse-

biasedpn junction



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Thepn Junction with External Applied Voltage

If an external voltage Va is applied across the p

n junction, the height of

the potential barrier is eitherincreased or diminished as compared to Va,

depending upon the polarity of the applied voltage.

The energyband distribution, with applied external voltage, is shown in

below figure. For these non-equilibrium conditions, the Fermi level can

no longer be identified. In order to describe the behaviour of the pn

junction, quasi- Fermi levels are introduced.


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Rectifying Voltage

Current Characteristics of ap

n Junction If the polarity of the applied voltage is such that the p-type region is

made negative with respect to the n-type, the height of the potential-barrier

is increased.

Under this reverse-biased condition, it is relativelyharder for the majority

of the carriers to surmount the potential-barrier.

The increase in the potential barrier height is essentially equal to theapplied voltage.

Under an external applied voltage, the carrier concentrations near the

junction are:

(where, the plus and minus signs are for the reverse-biased and the

forward-biased conditions.)


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The injected or extracted minority-carrier concentrations near the junction

can be written as:

The plus sign is for the forward-biased case where minority carriers are

injected. The minus sign is for the reverse-biased case where minority

carriers are extracted.

Electron and hole carriers at the boundaries of ap

njunction under an externally appliedvoltage

The concentration of

the carriers on the

boundaries, for the

usual cases, Na >> ni and

under an external

applied voltage V is

shown in right side

figure.

i f i d


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Properties of Diodes

VD = Bias Voltage ID = Current through

Diode. ID is Negative

for Reverse Bias and

Positive for Forward

Bias

IS = Saturation Current

VBR = Breakdown

Voltage

V = Barrier PotentialVoltage

VD

ID (mA)

(nA)

VBR

~V

IS


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Properties of Diodes

The transconductance curve on the previous slide is characterized by

the following equation:ID = IS(e

VD/VT 1) As described in the last slide, ID is the current through the diode, IS is

the saturation current and VD is the applied biasing voltage.

VT is the thermal equivalent voltage and is approximately 26 mV atroom temperature. The equation to find VT at various temperatures

is:

VT = kT/ q

k = 1.38 x 10-23

J/K T = temperature in Kelvin q = 1.6 x 10-19

C 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 is around 2 for low currents and goes down to about 1

at higher currents


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The Junction CapacitanceTwo types of idealized

junctions, which are

approximated closely in

practice. These are:

1. The abrupt or

step junction,which results from

the alloying

technique.

2. The graded

junction, which

results from the

crystal-growing

technique.

The profiles of charge density, potential, and electric

field in an abrupt junction

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Diode Fundamentals