ELECTRONICS I

C Kawerawera 0888608859 /0999220755

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 ELECTRONICS I

PN JUNCTION DEVICE

C Kawerawera
0999220 755 0888608859

•
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 SEMICONDUCTOR THEORY

and

THE PN JUNCTION DEVICE

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 SEMICONDUCTORS
Semiconductors are materials whose electrical
conductivity lies between that of good conductor
like copper and good insulators like polythene.

Examples of semiconductor materials are silicon

(Si), germanium (Ge),elements cadmium sulphide
(Cds) and gallium arsenide(GAs), compounds.

Their conductivity is dependent on temperature.

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 In solids this bonding forms a rgularly
repeating three−dimensional pattern of
atoms called a crystal lattice.

Every atom has a half−share in eight

valence electrons giving a very good stable
arrangement.

Pure silicon and germanium are therefore

very good insulators, being perfect at near
absolute zero (−273 C).
•

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 ENERGY LEVELS

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 GENERATION OF ELECTRON-HOLE PAIRS

The freed electron and the hole left behind are called an electron-
hole pair.

As temperature increases, more heat is available and more valence

electrons acquire enough energy to jump the forbidden gap into the
conduction band.

The thermally generated electrons are available for conduction of

current. Hence the resistivity of semiconductor decreases with
increasing temperature.

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 Recombination
It is the process in which electrons and holes recombine and
causes the electron hole pair to disappear.

The thermally generated hole can be filled by an electron from a

neighbouring atom when it breaks a covalent bond. As this happens
it appears that a hole has moved from one atom to the other.
This movement of holes, which are positively charged constitute a
current flow. Current flow in a semiconductor is then composed of
electron and hole movement.

If a voltage is applied to a semiconductor the free electrons will

move from the negative terminal toward the positive terminal and
holes from positive to negative. The total current being the sum
of the two parts.

Since the electron, to be free and liberated, gains some specific

amount of energy, then at recombination the same energy must be
released either as thermal or some other form of energy

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 Extrinsic semiconductors

Doping
Tiny amounts of impurities are added to the semiconductor
materials in a controlled manner in a process called doping. The
doped material is an extrinsic semiconductor material and has its
conductivity, improved because the impurity supplies extra charge
carriers. The added material is called the dopant.

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 N-Type Semiconductor
This type is made substitutionally by doping pure semiconductor with penta-
valent (5 valent impurity) impurities: phosphorus, antimony or arsenic.

Substitutionally for the impurity forms covalent bonds with four neighboring
silicon atom but the fifth electron becomes a free conduction electron.

The impurity atom of either phosphorus, antimony, arsenic is called a donor

atom because it donates an electron, and the remaining ion is a positive ion.

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 P-Type Semiconductor
This type is made substitutionally by doping pure semiconductor with tri-
valent (3 valent) impurities: Boron, Indium, Aluminium, and Gallium

One bond is incomplete and the position of the missing electron i.e. the hole
behaves like a positive change since it can attract an electron from a nearby
silicon atom, so forming another hole.

The impurity atoms then accept another electron and become negative ions
and as such they are called acceptor atoms

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At and close to the junction RECOMBINATION
occurs resulting in a depletion layer results.

The region on either side of the junction is

depleted of majority carriers

A pd known as junction voltage acts across the

depletion layer from n- to p-type being 0.7V for
silicon and 0.3V for germanium.

If there is charge on opposite sides of the junction with an associated

voltage, then there must be capacitance, Junction capacitance or Transition
capacitance
• or Depletion region capacitance.

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No bias conditions

(a) an internal distribution of charge;

(b) a diode symbol, with the defined polarity and the current direction

Any minority carriers (holes) in the n -type material that find themselves within the depletion
region for any reason whatsoever will pass quickly into the p -type material.

Any minority carriers of the n -type material that find themselves in the depletion region will
pass directly into the p -type material. This carrier flow is indicated as the minority carriers of
each material.

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 Reverse bias conditions
Resulting in the widening of the depletion region, and reducing the
majority carrier flow to zero.

The current, Is, that exists under reverse bias conditions is called
the reverse saturation current and is the same minority carrier
flow.

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 Forward Biased conditions
The electrons in the n-type material and holes in the p-type material to recombine with
the ions near the boundary and reduce the width of the depletion region.

The minority carrier flow does not change in magnitude, but the reduction in the width of
the depletion region results in a heavy majority flow across the junction.

An electron of the n-type material now “sees” a reduced barrier at the junction due to
the reduced depletion region and a strong attraction for the positive potential applied to
the p-type material. Increasing bias the depletion region decreases in width until a flood
of electrons pass through the junction resulting in an exponential rise in current

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Definitions of the Shockley’s Equation

IS is the reverse saturation current

VD is the applied forward-bias voltage across the diode
n is an ideality factor, which is a function of the
operating conditions and physical construction; it has a
range between 1 and 2 depending on a wide variety of
factors ( n = 1) in this course

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 Forward and Reverse Bias
I – V Characteristics

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Variation in Si diode characteristics with temperature change

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 Diode Equivalent Circuits (Models)

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 Approximate and Ideal Semiconductor Diode Models.

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TRANSITION AND DIFFUSION CAPACITANCES
1. The capacitance at the junction is called the transition
(CT), barriers, or depletion region capacitance.
2. Under forward bias condition there exists the
diffusion capacitance (CD)

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Semiconductor diode notation.

Commercially available semiconductor diodes

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Diode Checking Function
Checking a diode in the forward-bias state.
The meter has an internal constant-current
source (about 2 mA) that will define the voltage
level

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Checking a diode with an ohmmeter.
The resulting ohmmeter indication will be a function of the
current established through the diode by the internal
battery (often 1.5 V) of the ohmmeter circuit. The higher
the current, the lower is the resistance level.

For the reverse-bias situation the reading should be quite

high, requiring a high resistance scale on the meter. A high
resistance reading in both directions indicates an open
(defective-device) condition, whereas a very low resistance
reading in both directions will probably indicate a shorted
device.

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 BREAK
03/07/2025

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For the series diode configuration, determine VD, VR, and ID .

Repeat the above with the diode reversed.

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Determine VO and ID for the series circuit

KVL clockwise

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Determine I, V1, V2, and VO for the series dc configuration

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 Determine VO , I1 , ID1, and ID2 for the parallel diode configuration

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Two LEDs that can be used as a polarity detector are shown.

Apply a positive source voltage and a green light results. Negative supplies
result in a red light. Packages of such combinations are commercially
available.

Find the resistor R to ensure a current of 20 mA through the “on” diode

for the configuration. Both diodes have a reverse breakdown voltage of
3 V and an average turn-on voltage of 2 V.

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 Determine the currents I1 , I2 , and ID2 for the network

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 Other Types of Diodes
ZENER DIODE
A zener diode is a silicon junction diode designed for
stabilizing i.e. keeping steady the output voltage of a
power supply.

In forward bias the zener behaves likes an ordinary

silicon diode. However it is normally used in reverse bias.
Then, the reverse current is negligible until a reverse pd
VR reaches a certain value VZ, called the zener or
reference voltage.

Zener diodes are available having Zener potentials of 1.8

V to 200 V with power ratings from 1⁄4 W to 50 W.

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Conduction direction: (a) Zener diode; (b) semiconductor diode;
(c) resistive element.

Zener region

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 Zener diode characteristics with the equivalent model for each region.

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 Zener Diode

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Basic Zener Diode Regulator

Substituting the Zener equivalent for the “on” situation.

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•

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•

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 The value of the current limiting resistor should ensure
that the power rating of the diode is not exceeded.

E.g. a 400mW zener diode for which Vz =10V can pass

a maximum current Imax given by

Power 400mW
I max    40mA
Voltage 10V
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 LIGHT EMITTING DIODE (LED)

A LED is a PN junction diode made from the

semiconductor gallium arsenide phosphide. When
forward biased it conducts and emits red, yellow or
green light depending on its composition. No light
emission occurs on reverse bias which if it exceeds
5V may damage the LED.

The recombination of holes and electrons across the

junction releases some energy in the form of
photonic energy. In gallium arsenide phosphide some
of this energy is emitted as light.

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 Light Emitting Diodes

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Standard response curve of the human eye, showing the
eye’s response to light energy peaks at green and falls
off for blue and red.

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 External Resistor
The voltage drop Vf is greater across a conducting
LED than across an ordinary diode and it’s about
2V. Therefore R can be calculated from.

Since the typical forward current is 10mA{0.01A}

Uses
LED is used as indicator lamps especially in digital
electronic circuits and seven segment displays. The
advantages of LEDs are small size, reliability, long
life and high speed.

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 PHOTO DIODE
It is a PN junction device that converts light energy into electrical voltage
or current. The photo diode accepts light energy as input to generate
electric current.

It is also called as Photo detector, photo sensor or light detector. Photo

diode operates in reverse bias condition i.e. the p – side of the photodiode is
connected with negative terminal of battery (or the power supply) and n –
side to the positive terminal of battery.

Typical photodiode materials are Silicon, Germanium, Indium Gallium

Arsenide Phosphide and Indium gallium arsenide.

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LASER

It is PN junction device

The letters in the word laser stand

for Light Amplification by Stimulated Emission
of Radiation.

Laser knives and laser welders

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 SOLAR CELL / PHOTOVOLTAIC CELL

It converts light energy directly into electrical energy

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 THE END

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