Edc Unit-1(semiconductor Diode).pdf 464237

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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

Conductor

SemiConductor

Insulator

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.

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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 ing 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 ᶯ is unity for Ge and is 2 for Si. The symbol 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 ing through the P-N junction diode is almost zero. The voltage up to which no current ing 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 ing through P-N junction diode is almost constant. The voltage up to which constant current ing through P-N junction diode is called Peak Inverse Voltage (PIV) of a P-N junction. Beyond PIV,

the

junction breaks and enormous current ing 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 current in N side and

as indicated in the following figure.

represents the hole

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 ing 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

ᶯ But

ᶯ

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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

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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 =

ᶯ

is a constant

ᶯ = 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 ing 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 ᶯ 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 ing through P-N junction diode is almost zero. The voltage up to which no current ing 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 ing through p-N junction diode is almost constant. The voltage up to which constant current ing through P-N junction diode, is called Peak Inverse Voltage (PIV). Beyond PIV, the junction breaks and enormous current ing 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 10°C 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).

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