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UNIT-I

P-N JUNCTION DIODE

Classification of materials (Exp Q. What is an insulator, a semiconductor and a

metal? Explain with the help of energy band diagrams)

Materials are broadly classified into

1. Conductors

2. Insulators

3. Semiconductors

CONDUCTOR:

Conductor is one in which Conduction band and valence band are overlapped with

each other (i.e.) no energy gap between Conduction band and valence band. Ex: Copper,

Aluminium

INSULATOR:

An insulator is one in which large energy gap between conduction band and valance

band. In this material, Forbidden energy gap is large (EG 6e.V). Practically it is not possible to

jump electrons from valance band to conduction band. Ex: Diamond is a perfect insulator.

Conductor

Semi-

Conductor

Insulator

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SEMI CONDUCTOR:

A semiconductor material is one in which the forbidden energy gap is greater than

conductors but less than insulators. Ex: Germanium, Silicon (Energy gap of Germanium is

about 0.785 eV and for silicon it is 1.21ev).

TYPES OF SEMICONDUCTORS (Exp Q. What is meant by doping? Explain about

intrinsic & extrinsic semiconductors, N-type material & P-type material)

Doping

Adding impurities to a semiconductor is called Doping. Doping is mainly used to increase the

conductivity. Pure semiconductor is called intrinsic semiconductor. Semiconductor with

impurities added is called extrinsic semiconductor.

EXTRINSIC SEMICONDUCTOR MATERIALS:

The characteristics of semiconductor materials can be altered significantly by the

addition of certain impurity atoms into the relatively pure semiconductor material. These

impurities, although only added to perhaps 1 part in 108, can alter the band structure

sufficiently to totally change the electrical properties of the material. A semiconductor

material that has been subjected to the doping process is called an extrinsic material. There

are two extrinsic materials of immeasurable importance to semiconductor device fabrication:

n-type and p-type.

N-Type Material: both the n- and p-type materials are formed by adding a predetermined

number of impurity atoms into a germanium or silicon base. A small amount of pentavalent

impurity such as antimony, arsenic or phosphorus is added to the pure semiconductor

(Germanium or Silicon) to get N type Semiconductor. Ge atom has four valence electrons

and antimony has five valence electrons. The effect of such impurity element is indicated in

the following figure. Each antimony atom forms a covalent bond with surrounding four Ge

atoms. Thus four valence electrons of antimony atom form covalent bond with four valance

electrons of individual Ge atoms and fifth valance electron is left free which is loosely bound

to the antimony atom. This loosely bound electron can be easily excited from the valence

band to the conduction band by the application of external energy. Thus every antimony atom

donates one electron without creating a hole. Such pentavalent impurities are called donor

impurities because it donates one electron to the conduction band. The donor atom becomes

positively charged ion after giving an electron to the conduction band.

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Thus the addition of pentavalent impurity increases the number of electrons in the conduction

band thereby increasing the conductivity of N type semiconductor. So free electrons are

majority charge carries and holes are minority charge carriers.

P-Type Material:

A small amount of trivalent impurity such as boron is added to the pure semiconductor

(Germanium or Silicon) to get P type Semiconductor. Ge atom has four valence electrons and

boron has three valence electrons. The effect of such impurity element is indicated in the

above figure. Each boron atom forms a covalent bond with surrounding four Ge atoms

leaving one bond incomplete which gives rise to a hole. Thus trivalent impurity when added

to the intrinsic semiconductor (Ge) introduces large number of holes in the valence band.

Such trivalent impurities are called acceptor impurities because it accepts free electron in the

place of hole. The acceptor atom becomes negatively charged ion after accepting electron to

the hole.

Thus the addition of trivalent impurity increases the number of holes in the valence

band thereby increasing the conductivity of P type semiconductor. So holes are majority

charge carriers and free electrons are minority charge carries.

Concept of Majority and Minority Carriers (Exp Q. Explain the concept of majority

and minority carriers)

Majority and Minority Carriers

In the intrinsic state, the number of free electrons in Ge or Si is due only to those few

electrons in the valence band that has acquired sufficient energy from thermal or light sources

to break the covalent bond or to the few impurities that could not be removed. The vacancies

left behind in the covalent bonding structure represent our very limited supply of holes. In an

n-type material, the number of holes has not changed significantly from this intrinsic level.

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The net result, therefore, is that the number of electrons far outweighs the number of holes.

For this reason:

In an n-type material the electron is called the majority carrier and the hole the minority

carrier.

For the p-type material the number of holes far outweighs the number of electrons, as shown

in Therefore: In a p-type material the hole is the majority carrier and the electron is the

minority carrier. When the fifth electron of a donor atom leaves the parent atom, the atom

remaining acquires a net positive charge: hence the positive sign in the donor-ion

representation. For similar reasons, the negative sign appears in the acceptor ion. In N type

material Fermi level is just below the conduction band. In P type material Fermi level is just

above the valence band

Open Circuited P-N Junction:

If donor impurities are introduced into one side and acceptor impurities into other side of a

single crystal of a semiconductor, a P-N junction is formed. Such a system is shown in the

following figure. The donor ion is indicated by a plus sign because after this impurity atom

donates an electron, it becomes a positive ion. The acceptor ion is indicated by a negative

sign, because after this atom accepts an electron, it becomes a negative ion. Every acceptor

atom accept one electron from valence band, resulting one immobile negatively charged

acceptor ion and one mobile hole in the valence band occurred. Similarly every donor atom

donates one free electron to the conduction band, resulting one immobile positively charged

donor ion and one mobile electron in the conduction band occurred.

Due to concentration gradient, electrons diffuse across the junction from N region to the P

region while holes diffuse across the junction from p region to N region. The holes on

diffusing from P region to the N region combine with electrons in the vicinity of the junction.

Similarly the electrons on diffusing from N side the P side combines with holes in the vicinity

of the junction. As a result of displacement of these charges, a potential will appear across the

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junction. This is called barrier potential. This value is equal to 0.2 Volts for Ge and 0.6 Volts

for Si. The region of the junction is depleted of mobile charges, it is called depletion region.

It is also called as space charge region or transition region. The thickness of the region is of

the order of one micro meter.

P-N Junction Diode (Exp Q.Explain PN diode characteristics in forward bias and

reverse bias regions)

The P-N junction diode permits the easy flow of current in one direction but restricts

the flow current in the opposite direction. In order to understand the working of P-N junction

diode, we shall consider the effect of forward bias and Reverse bias across the junction.

REVERSE BIAS: If an external voltage is applied in such a way that positive terminal to n

side and negative terminal to P side of a P-N junction as shown below. The junction is said to

be reverse biased. In this arrangement electrons from the N side are attracted towards the

positive terminal and holes from the P side are attracted towards the negative terminal. Thus,

both the holes in the p type and electrons in the N type to move away from the junction.

Resultant, the region of negative charge density on the P side and region of positive charge

density on the N side become wider (i.e.) the width of depletion layer increases. As depletion

region widens, barrier potential across the junction also increases. This increased barrier

potential, reduce the flow of majority charge carriers to the other side(i.e) holes from the P

side to the N side and electrons from N side to the P side. However the flow of minority

charge carriers remains uninfluenced by the increased barrier potential. But there are very

few minority charge carriers (i.e.) hole in the N region and an electron in P region crosses the

junction. Thus a small amount of current flows through the diode which is called Reverse

Saturation Current (I0) order of micro amperes. This reverse saturation current will increase

with increasing temperature.

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FORWARD BIAS:

If an external voltage is applied in such a way that positive terminal to P side and negative

terminal to N side of a P-N junction as shown above. The junction is said to be forward

biased. In this arrangement electrons on the N side are repelled from the negative terminal

and driven towards the junction. Similarly holes on the P side are repelled from the positive

terminal and driven towards the junction. Resultant, the width of depletion layer decreases

and the barrier potential also decreased. Thus majority charge carriers crossing the junction

(i.e.) electrons flow from n side to p side and holes flow from P side to N side. Since the

barrier potential is very small, a small forward voltage is sufficient to eliminate the barrier

completely. Once the barrier is eliminated by the application of forward voltage, junction

resistance becomes almost zero. Resultant large current flows through the diode which is

called forward current.

VOLT AMPERE CHARACTERISTICS OF P-N JUNCTION DIODE:

The current passing through a P-N junction diode is given by:

The diode is forward biased if V is positive (i.e.) P side of the junction is positive and N side

of the junction is negative. The symbol is unity for Ge and is 2 for Si. The symbol stands

for volt equivalent of temperature and is given by:

At room temperature, T=27+273= ,

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The volt ampere characteristics are shown in the following figure. In order to display forward

and reverse volt ampere characteristics of a P-N junction on a single graph, it is necessary to

two different current scales. Forward current scale is in milli amperes and reverse current

scale is in micro amperes.

In forward characteristics, up to some applied voltage, the current passing through the P-N

junction diode is almost zero. The voltage up to which no current passing through the P-N

junction diode is called cut in voltage. It is denoted by . It is equal to 0.2 V for Ge and 0.6

V for Si.

In reverse characteristics, up to some applied voltage, current passing through P-N junction

diode is almost constant. The voltage up to which constant current passing through P-N

junction diode is called Peak Inverse Voltage (PIV) of a P-N junction. Beyond PIV, the

junction breaks and enormous current passing through P-N junction diode.

CURRENT COMPONENTS IN A P-N JUNCTION DIODE:

When a forward bias is applied to a P-N junction diode, holes are injected in to the N side and

electrons in to the P side. After injecting, these are minority charge carries move away from

the junction exponentially decreases with the distance as shown below. There are two

minority currents as indicated in the following figure. represents the hole

current in N side and represents the electron current in the P side as a function of x.

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Electrons crossing the junction at x=0 from right to left constitute a current in the same

direction as holes crossing the junction from left to right. Hence the total current I at x =0 is

given by:

Since current is same throughout series circuit, I independent of x and is shown as a

horizontal line in the following figure. Consequently in the P side, there must be a second

component of current which combining with gives the total current I. hence the hole

current in the P side is given by:

Similarly the electron current in the N side is given by:

For away from the junction in P side, the current is a drift current ( ) of holes. As the holes

approaching the junction, some of them recombine with electrons which are injected in to the

P side from N side. Thus the current decreases towards the junction. The rate of decrease

is such that the total current remains constant independent of distance. The decreased hole

current at the junction now enters the N side and becomes the hole diffusion current ( ).

Similar remarks can be made with respect to current ( ). Hence in a forward biased P-N

junction diode, the current enters in to the P side as a hole current and leaves from the N side

as an electron current of the same magnitude.

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DIODE RESISTANCE:

Static resistance and dynamic resistance (Exp Q. Explain static resistance and dynamic resistance)

DC or Static Resistance:

The static resistance of a diode is defined as the ratio of voltage and current. Any

point on the V-I characteristics of the diode, the static resistance is equal to reciprocal of the

slope of a line joining the operating point to the origin. The static resistance varies with V and

I and is not a useful parameter.

Dynamic resistance:

It is defined as the reciprocal of slope of the V-I characteristics of a diode.

But we know that

But

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Law of Junction:

hole concentration in P side

hole concentration in N side

thermal equilibrium hole concentration in P side

thermal equilibrium hole concentration in N side

Total hole concentration in N side (i.e.) thermal equilibrium

hole concentration + Injected hole concentration in N side

Barrier Potential

Volt temperature equivalent =

At room temperature, T =27+273 = ,

According to Boltzmann relationship of kinetic gas theory,

In the case of open circuited P-N junction,

In the case of forward biased P_N junction diode by an applied voltage V,

But equations (1) and (2) are equal.

This boundary condition is the law of junction.

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This law states that for a forward biased P-N junction diode, the injected hole concentration

at the junction increases over thermal equilibrium value . It is given by:

Diode current equation:

Let us derive the expression for the total current as a function of applied voltage assuming

that the width of the depletion region is zero. When a forward bias is applied to a P-N

junction diode, holes are injected from the P side to N side. Due to this, the concentration of

holes in the N side ( ) is increased to thermal equilibrium hole concentration in N side

( ) plus injected hole concentration in N side [ ].

(1)

Where is the diffusion length for holes in the N side.

At the junction (i.e.) x=0

(2)

Where is the injected hole concentration at the junction.

But according to law of junction

Substitute this value in equation (2)

(3)

The diffusion hole current in the N side is given by:

From equation (1),

Where diffusion coefficient for holes in

A Cross sectional area of a semiconductor

D Charge of an electron

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The diffusion hole current at x=0 is given by:

But from(3)

(4)

Similarly, the diffusion electron current crossing the junction in to P side with x=0 is given

by:

(5)

The total diode current is sum of equations (4) and (5)

[

]

But [

] = reverse saturation current

The general equation of the P-N junction diode current equation is given by:

(6)

But correct equation for the P-N junction diode current equation is given by:

(7)

Where V external applied voltage

volt temperature equivalent =

is a constant

= 1 for Ge and 2 for Si.

Temperature dependence of V-I characteristics:

Electron hole pairs are generated in semiconductors whenever temperature increases as a

result conductivity increases. Thus, current passing through the diode increases with

temperature as given by the diode current equation

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The reverse saturation current becomes doubles for every rise in temperature. Hence,

the temperature increased at constant voltage, current I increases. The reverse saturation

current increases with temperature according to the following equation.

Where Reverse saturation current of the diode at temperature

Reverse saturation current of the diode at temperature

Volt-Ampere Characteristics of P-N junction diode:

The total current as a function of applied voltage for a P-N junction diode is given by:

The diode is forward biased if V is positive indicating that the P side of the junction is

positive with respect to N side. The symbol is unity for Ge and 2 for Si. The symbol

stands for volt equivalent of temperature and is given by:

At room temperature, T= 27+273= ,

The V-I characteristics of P-N junction diode is shown below. When diode is reverse biased,

V is negative.

In order to display forward and reverse V=I characteristics of a P-N junction diode on a

single graph, it is necessary to use two different current scales. Forward current scale is in

milli amperes and reverse current scale is in micro amperes.

In forward biased characteristics, up to some applied voltage, the current passing through P-N

junction diode is almost zero. The voltage up to which no current passing through the diode is

called Cut-in voltage and is denoted by . is approximately equal to 0.2 V for Ge and

0.6 V for Si. In reverse biased characteristics, up to some applied voltage, the current passing

through p-N junction diode is almost constant. The voltage up to which constant current

passing through P-N junction diode, is called Peak Inverse Voltage (PIV). Beyond PIV, the

junction breaks and enormous current passing through the diode.

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TRANSITION (CT) AND DIFFUSION CAPACITANCE (CD) (Exp Q. Explain

Transition capacitance and Diffusion capacitances)

In the p-n semiconductor diode, there are two capacitive effects to be considered. In

the reverse bias region we have the transition- or depletion-region capacitance (CT), while in

the forward bias region we have the diffusion (CD) or storage capacitance. Recall that the

basic equation for the capacitance of a parallel-plate capacitor is defined by C=A/d, where

is the permittivity of the dielectric (insulator) between the plates of area A separated by a

distance d. In the reverse bias region there is a depletion region (free of carriers) that behaves

essentially like an insulator between the layers of opposite charge. Since the depletion width

(d) will increase with increased reverse-bias potential, the resulting transition capacitance will

decrease. The fact that the capacitance is dependent on the applied reverse-bias potential has

application in a number of electronic systems. The capacitive effects described above are

represented by a capacitor in parallel with the ideal diode, as shown below.

Temperature effects on p-n diode (Exp Q. Explain temperature effects on p-n diode characteristics)

Temperature can have a marked effect on the characteristics of a silicon

semiconductor diode. It has been found experimentally that the reverse saturation current Io

will just about double in magnitude for every 10C increase in temperature. Typical values of

Io for silicon are much lower than that of germanium for similar power and current levels. The

result is that even at high temperatures the levels of Io for silicon diodes do not reach the same

high levels obtained for germanium. As the temperature increases the forward characteristics

are actually becoming more ideal.

Zener Diode (Exp Q.Explain break down mechanisms in semiconductor diodes)

Zener diode is a reverse biased heavily doped whose doping concentration is one part of

impurity is added to every 105 parts of pure semiconductor material P-N junction diode.

Zener Diode operates in the breakdown region. When a Zener Diode is operated in forword

biased, its characteristics are same as that of ordinary P-N junction diode.

Following are two mechanisms of Zener breakdown.

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1) Zener Breakdown: Zener breakdown generally occur in very thin junctions

(i.e.) when both sides of the junction are heavily doped and resultant the depletion layer is

narrow. When a small reverse bias is applied, a very strong electric field is set up across the

junction. This field is enough to break the covalent bonds. Now large number of free

electrons and holes are produced which constitute the reverse saturation current.

2) Avalanche breakdown: This type of breakdown occurs when both sides of

junction are lightly doped and resultant the depletion layer is large. In this case, the electric

field across the junction is not so strong to produce Zener breakdown. In this case, free

electrons acquire sufficient energy from applied potential and collide with the semiconductor

atoms in the depletion region. Due to the collision with valance electrons, covalent bonds are

broken and electron hole pairs are generated. These new charge carriers again acquire

sufficient energy from the applied potential and collide with semiconductor atoms. In turn

produce additional charge carriers. This forms cumulative process called an avalanche

multiplication. The breakdown is called avalanche breakdown.

In general, at reverse voltages less than 6 volts, Zener breakdown occurs while greater

6 volts, Avalanche breakdown occurs. When the breakdown voltage is reached in Zener

diode, the current increases rapidly with small change in voltage. The V-I characteristics of

Zener Diode is shown below. The complete equivalent circuit of the Zener diode in the Zener

region includes a small dynamic resistance and dc battery equal to the Zener potential

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Tunnel Diode:

A Tunnel diode also called Esaki diode is a heavily heavily doped P-N junction diode whose

doping concentration is one part in 103.

In the case of lightly doped P-N junction diode, the

Fermi level lies inside the forbidden energy gap. But in the case of heavily heavily doped P-N

junction diode, the Fermi level lies outside forbidden energy gap. In heavily heavily doped n

type material, the Fermi level lies in the conduction band. In heavily heavily doped P type

semiconductor, the Fermi level lies in the valence band.

Under open circuit conditions, the energy band diagram of a heavily heavily doped P-N

junction diode is shown in the following figure (a). The Fermi level in the P side is at same

energy as the Fermi level in the N side. Above the Fermi level in the valence band in P side

indicates completely filled with holes. Below the Fermi level in the conduction band in N

side indicates completely filled with free electrons.

Reverse Biased Condition: Let us consider that the P type semiconductor is grounded and

that a voltage applied across a diode shift the N side with respect to the P side. If a reverse

bias voltage is applied across the tunnel diode, the height of barrier is increased above its

open circuit value . Hence N side levels must shift downward with respect to the P side

levels as shown in the following figure (b). Hence, electrons will tunnel from the P side to the

N side, giving rise to a reverse diode current. As the magnitude of the reverse bias increases,

the reverse current also increases as shown in the following figure (c).

Forward Bias Condition: If a forward bias voltage is applied across a tunnel diode, the

height of barrier is decreased below its open circuit value . Hence N side levels must shift

upward with respect to the P side levels as shown in the following figure (D). Resultant,

electrons will tunnel from the N side to the P side, giving rise to a forward diode current as

shown below figure (C).

As the forward bias is increased further, the condition is shown in the following figure (E).

Now maximum number of electrons will tunnel from the N side to the P side, giving rise to

the peak current as shown in the following figure (C). If still more forward bias voltage is

applied, the condition is shown in the following figure (F). Now number of electrons will

tunnel from the N side to the P side decreases. Resultant, the tunnelling current decreases as

shown in the following figure (C). On further increasing forward bias voltage, the condition

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is shown in the following figure (G). Now electrons will not tunnel from the N side to the P

side, giving rise to zero current as shown in the following figure (C).

The solid curve gives the tunnelling current which is shown in the following figure (C). But

in addition to this, the P-N junction diode current also flows and is shown by dotted lines in

the following figure (c0. The sum these two currents is the tunnel diode current which is

shown in the following figure (H). The symbol for tunnel diode is shown in the following

figure (I).