MAHALAKSHMI...MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI-621213. QUESTION BANK DEPARTMENT: EEE...

SSathya Priyadharshini. Asst. Prof./EEE 1

MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI-621213.

QUESTION BANK

DEPARTMENT: EEE SEMESTER : III

SUBJECT CODE: EE2203 SUBJECT NAME: ELECTRONIC

DEVICES &CIRCUITS

UNIT 1-PN DIODE AND ITS APPLICATIONS

PART A (2 Marks)

1. What are the advantages and limitations of LCD displays. (AUC NOV 12)

LCD displays are cheaper than LED displays. The disadvantage is that the viewing

angle is small i.e., it can be viewed only in straight angle. Images viewed from cross

angles appear blurred.

2. What is meant by dynamic resistance of diode? (AUC DEC’11)

Dynamic resistance of a diode can be defined as the ratio of change in voltage

across the diode to the change in current through the diode.

r = V / I

Where

r - Dynamic resistance of a diode

V - change in voltage across the diode

I - change in current through the diode

3. Differentiate between zener breakdown and avalanche breakdown. (AUC DEC ‘11)

Zener breakdown occurs in a reverse biased junction which gives a constant output

voltage. Avalanche breakdown does not provide a constant output voltage. This

constant voltage from a zener diode can be used as a reference voltage for many

regulators.

4. What is meant by diffusion current in a semi conductor? (AUC MAY’10)

In a non uniformly doped semi conductor charge carriers have a tedency to move

from a region of higher concentration to a region of lower concentration. The flow of

current due to this process is called diffusion current.

5. Define Knee voltage of a diode. (AUC NOV’10)


Knee voltage of a diode is defined as a breakover voltage above which the forward

current increases abruptly. This voltage is also called as brweakdown voltage of a

diode. It is 0.3 for Ge and 0.7 for Si.

6. What is peak inverse voltage? (AUC NOV’10)

The maximum reverse-bias potential that can be applied before entering the Zener

region is called the peak inverse voltage (referred to simply as the PIV rating) or the

peak reverse voltage (denoted by PRV rating). Peak inverse voltage is defined as the

maximum reverse voltage that a diode can be subjected to operate in a reverse

region so that the diode does not get damaged due to this reverse voltage.

7. What is LED? Which material is used for LED? (AUC MAY’09)

LED is Light Emmiting Diode. Gallium arsenide, gallium phosphide, Gallium arsenide

phosphide are the materials used for monufacturing LED which emits infra res, res,

green light respectively.

8. What is meant by doping in a semiconductor? (AUC MAY’09)

The method of adding impurities to a semiconductor to make them conduct is called

doping. It is the process of adding trivalent, pentavalent impurity to the

semiconductor which gives rise to P type and N type semiconductor.

9. Give the equation for diode current under reverse bias. (AUC MAY’08)

ID=IO [eVD/nVT-1]

10. What is diffusion capacitance? (AUC MAY’08)

Charge carriers flowing out of a depletion region, which is widened when the junction

is reverse biased, there is a large reverse current at first and which slowly decreases

to the level of reverse saturation current. The effect is like a discharging of a

capacitor called as diffusion capacitance.

11. How do the transition region width and contact potential across a PN junction vary

with the applied bias voltage? (AUC DEC’07)

The width of the PN junction is widenened for a reverse biased junction and narrows

for a forward biased junction.

12 . What is an ideal diode?

An ideal diode is one which offers zero resistance when forward biased and infinite

resistance when reverse biased.

13. Compare ideal diode as a switch.

An ideal diode when forward biased is equivalent a closed (ON) switch and when

reverse biased, it is equivalent to an open (OFF) switch.


14. State the mathematical equation which relates voltage applied across the PN

junction diode and current flowing through it.

15. Define knee/cut-in/threshold voltage of a PN diode.

It is the forward voltage applied across the PN diode below which practically no

current flows.

16. What is the effect of junction temperature on cut-in voltage of a PN diode?

Cut-in voltage of a PN diode decreases as junction temperature increases.

17. What is the effect of junction temperature on forward current and reverse current of a

PN diode?

For the same forward voltage, the forward current of a PN diode increases and

reverse saturation current increases with increase in junction temperature.

18. Differentiate between breakdown voltage and PIV of a PN diode.

The breakdown voltage of a PN diode is the reverse voltage applied to it at which the

PN junction breaks down with sudden rise in reverse current. Whereas, the peak

inverse voltage (PIV) is the maximum reverse voltage that can be applied to the PN

junction without damage to the junction.

19. Differentiate avalanche and zener breakdowns. Avalanche Breakdown

1. Breakdown occurs due to heavily doped junction and applied strong electric field.

2. Doping level is high.

3. Breakdown occurs at lower voltage compared to avalanche breakdown Zener

Breakdown

20. Draw the V-I characteristics of an ideal diode.


21. Differentiate between drift and diffusion currents.

Drift Current

1. It is developed due to potential gradient.

2. This phenomenon is found both in metals and semiconductors

Diffusion Current

1. It is developed to charge concentration gradient.

2. It is found only in semiconductors.

22. Draw the V-I characteristics of a practical PN diode

23. List the PN diode parameters.

1. Bulk Resistance.

2. Static Resistance/Junction Resistance (or) DC Forward Resistance

3. Dynamic Resistance (or) AC Forward Resistance

4. Reverse Resistance

5. Knee Voltage

6. Breakdown Voltage

7. Reverse Current (or) Leakage Current

24. State the PN diode ratings.

Even PN-Junction has limiting values of maximum forward current, peak

inverse voltage and maximum power rating.


25. Define reverse recovery time.

It is maximum time taken by the device to switch from ON to OFF stage. 15. List the

PN diode switching times.

1. Recovery Time

2. Forward Recovery Time 3. Reverse Recovery Time 4. Reverse recovery time,

5. Storage and Transition Times

26. Define transition capacitance of a diode.

Transition Capacitance (CT) or Space-charge Capacitance: When a PN-junction

is reverse-biased, the depletion region acts like an insulator or as a dielectric.

The P- and N-regions on either side have low resistance and act as the plates.

Hence it is similar to a parallel-plate capacitor. This junction capacitance is called

transition or space-charge capacitance (CT).

It is given by

Where, A = Cross-sectional area of depletion region. D = Width (or) thickness of

depletion region.

Its typical value is 40 pF.

Since the thickness of depletion layer depends on the amount of reverse bias, CT

can be controlled with the help of applied bias.

This property of variable capacitance is used in varicap or varactor diode. This

capacitance is is voltage dependent and is given by

27. Define diffusion capacitance of a diode.

Diffusion or Storage Capacitance (CD): This capacitive effect is present when

the junction is forward-biased.

It is called diffusion capacitance due to the time delay in minority charges across the

junction by diffusion process. Due to this fact, this capacitance cannot be identified in

terms of a dielectric and plates. It varies directly with forward current. When a

forward-biased PN-junction is suddenly reverse biased, a reverse current flows

which is large initially, but gradually decreases to the level of saturation current, I0.


This effect can be likened to the discharging, of a capacitor and is, therefore

called diffusion capacitance, CD. Its typical value is 0.02 F

It is given by:

28. List some applications of zener diode.

Zener diode find wide commercial and industrial applications. Some of their

common applications are:

As voltage regulators.

As peak clippers or voltage limiters.

For wave shaping.

For meter protection against damage from accidental application of

excessive voltage.

As a fixed reference voltage in a network for biasing and comparison

purposes and for calibrating voltmeters.

29. State the principle of operation of an LED.

When a free electron from the higher energy level gets recombined with the hole , it

gives out the light output. Here, in case of LEDs, the supply of higher level electrons

is provided by the battery connection.

30. State any four advantages of LED.

They are small in size.

Light in weight.

Mechanically rugged.

Low operating temperature.

Switch on time is very small.

Available in different colours.

They have longer life compared to lamp Linearity is better.

Compatible with ICs


. Low cost.

31. State some disadvantages of LED.

Output power gets affected by the temperature radiation Quantum

efficiency is low.

Gets damaged due to over –voltage and over-current.

32. List the applications of LED.

They are used in various types of displays.

They are used as source in opto-couplers.

Used in infrared remote controls.

Used as indicator lamps.

Used as indicators in measuring devices.

33. State the principle of operation of an LCD.

Basically this type of display consists of liquid crystal molecules.

These molecules have a special property. The change their orientation when

an electrical signal is applied to them.

The display consists of two glass plates and liquid crystal molecules are

placed in between the glass plates.

When no electrical signal is applied to the liquid crystal cell, then all the

liquid crystal molecules have random orientation with respect to their axis.

The incoming light passes through the gap of molecules. So, the light also gets

twisted.

Now, when an electrical signal is applied to this structure, then all the

liquid crystal molecules gets oriented by 90° to the glass plate. In this case, this light

passes in straight way along the molecular arrangement

34. State any four advantages of LCD.

Less amount of power per digit is required.

LCDs have best contrast ratio.

No external interfacing circuitry is required.

They have low threshold voltage.

They can be driven directly.


LCDs and MOS compatible. Small size and low cost

35. State any four application of LCD.

LCDs are generally applicable in the field of medical, domestic and

industrial electronics. Some of the applications of LCD are:

Wrist watches.

Telephones and cellular phones.

Digital panel meters.

PCO monitors.

Calculators.

For space applications.

In digital clocks.

Televisions.

Automobiles, etc.

36. Compare LEDs and LCDs.

LEDs

LCDs

1. More power is required. 2. Fastest

displays

3. More life.

4. LED is light source.

5. More temperature range. 6. Mounting is

easy

1. Less power is required. 2. Slowest

displays.

3. Less life.

4. LCD is not light source. It is a light

reflector.

5. Less temperature range 6. Mounting is

difficult.

37. Define rectifier efficiency.

It is defined as the ratio of DC power output to the applied AC power in put Rectifier

efficiency.

38. Define ripple factor of a rectifier.

The purpose of a rectifier is to convert AC into DC. But the pulsating output of

a rectifier contains a DC component and an AC component, called ripple.

The ratio of RMS value of AC components to the DC component in the rectifier

output is called ‘ripple factor’.


The ripple factor is very important in deciding the effectiveness of a rectifier. It

indicates the purity of the DC power output. The smaller the ripple factor, the lesser

the effective AC component and hence more effective is the rectifier.

39. Define TUF of a rectifier.

Most of the rectifier circuits make use of transformer whose secondary feeds the

AC power. The transformer rating is necessary to design a power supply.

Transformer utilization factor (TF) id defined as the ratio of DC power delivered

to the load to the AC power rating of transformer secondary.

40. Give the advantages and disadvantages of HWR and FWR. Half Wave Rectifier

(HWR)

Advantages

Simple circuit Low cost.

Disadvantages.

Rectification efficiency is low (40.6%) Very high amount of ripple (γ = 1.21)

Low TUF (0.287)

Saturation of transformer core occurs.

41. What is the need for a filter in rectifier?

The output of a rectifier is pulsating and contains a steady DC component

with undesirable ripples. If such pulsating DC is given to the electronic

circuits, it produces disturbances and other interferences. Hence ripples have to

be kept far from the load.

42. What is voltage regulator? List some types.

A voltage regulator is a circuit which makes the rectifier-filter output

constant regardless of the variations in the input voltage or load.

PART B (16 Marks)

1. (i) How a PN junction diode is working? Draw and explain V-I characteristics of PN

diode with neat diagrams.

(ii) List out and explain the applications of LED and LCD (AUC NOV’12)

(i) PN junction diode

The semiconductor diode is formed by simply bringing these materials together

(constructed from the same base—Ge or Si). At the instant the two materials are

joined the electrons and holes in the region of the junction will combine, resulting in

a lack of carriers in the region near the junction. This region of uncovered positive

and negative ions is called the depletion region due to the depletion of carriers in


this region. Since the diode is a two-terminal device, the application of a voltage

across its terminals leaves three possibilities: no bias (VD = 0 V), forward bias (VD

>0 V), and reverse bias (VD< 0 V).

p-n junction with no external bias.

No Applied Bias (VD = 0 V)

Under no-bias (no applied voltage) conditions, any minority carriers (holes) in the n-

type material that find themselves within the depletion region will pass directly into

the p-type material. The closer the minority carrier is to the junction, the greater the

attraction for the layer of negative ions and the less the opposition of the positive ions

in the depletion region of the n-type material. Assume that all the minority carriers of

the n-type material that find themselves in the depletion region due to their random

motion will pass directly into the p-type material. Similar discussion can be applied to

the minority carriers (electrons) of the p-type material. This carrier flow has been

indicated in the above figure for the minority carriers of each material. The majority

carriers (electrons) of the n-type material must overcome the attractive forces of the

layer of positive ions in the n-type material and the shield of negative ions in the p-

type material to migrate into the area beyond the depletion region of the p-type

material. However, the number of majority carriers is so large in the n-type material

that there will invariably be a small number of majority carriers with sufficient kinetic

energy to pass through the depletion region into the p-type material. Again, the same

type of discussion can be applied to the majority carriers (holes) of the p-type

material. The resulting flow due to the majority carriers is also shown in the above

figure. In the absence of an applied bias voltage, the net flow of charge in any one

direction for a semiconductor diode is zero. The symbol for a diode is shown in the

below figure with the associated n- and p-type regions. Note that the arrow is

associated with the p-type component and the bar with the n-type region. As

indicated, for VD= 0 V, the current in any direction is 0 mA.


Diode Symbol

Reverse-Bias Condition (VD < 0 V)

If an external potential of V volts is applied across the p-n junction such that the

positive terminal is connected to the n-type material and the negative terminal is

connected to the p-type material as shown in the below figure. The number of

uncovered positive ions in the depletion region of the n-type material will increase

due to the large number of free electrons drawn to the positive potential of the

applied voltage. For similar reasons, the number of uncovered negative ions will

increase in the p-type material. The net effect, therefore, is a widening of the

depletion region. This widening of the depletion region will establish too great a

barrier for the majority carriers to overcome, effectively reducing the majority carrier

flow to zero as shown in the below figure.

Reverse-biased p-n junction.

The number of minority carriers, however, that find themselves entering the

depletion region will not change, resulting in minority-carrier flow vectors of the

same magnitude with no applied voltage the current that exists under reverse-bias

conditions is called the reverse saturation current and is represented by Io.

Forward-Bias Condition (VD > 0 V)

A forward-bias condition is established by applying the positive potential to the p-

type material and the negative potential to the n-type material as shown in the

below figure. A semiconductor diode is forward-biased when the association p-type


and positive and n-type and negative has been established.

Forward-biased p-n junction

The application of a forward-bias potential VD will pressure 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 as shown in the above

figure. The resulting minority-carrier flow of electrons from the p-type material to

the n-type material (and of holes from the n-type material to the p-type material)

has not changed in magnitude (since the conduction level is controlled primarily by

the limited number of impurities in the material), but the reduction in the width of

the depletion region has resulted 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.

Silicon semiconductor diode characteristics.


As the applied bias increases in magnitude the depletion region will continue to

decrease in width until a flood of electrons can pass through the junction, resulting in

an exponential rise in current as shown in the forward-bias region of the

characteristics curve shown before. Note that the vertical scale of characteristic

curve is measured in mill amperes and the horizontal scale in the forward-bias region

has a maximum of 1 V. typically, therefore, the voltage across a forward-biased

diode will be less than 1 V.

(ii) Applications of LED & LCD

LED:

Used in burglar alarm systems

For solid state video displays

In image sensing circuits used for picturephone

Optical fibre communication systems

Datalink & remote controllers

For numeric displays in hand held or pocket calculators.

LCD

Field effect LEDs are used in watches & portable instruments, where source

energy is prime consideration.

Thousands of tiny LCDs are used to form the picture elements (pixels) of the

screen in B&W pocket TV receiver.

Desktop monitors, notebook computer display

Cellular phone displays, display on personal Digital Assistants(PDA).

2. In a semiconductor at room temperature (300 degree Kelvin), the intrinsic carrier

concentration and resistivity are 1.5*106/cm3 and 2*103 Ωm respectively. It is

converted to an extrinsic semiconductor with a doping concentration of 1020/m3. For

the extrinsic semiconductor calculate the

(i) Minority carrier concentration

(ii) Resistivity

(iii) Shift in Fermi level due to doping

(iv) Minority carrier concentration when its temperature is increased to a value at

which the intrinsic carrier carrier concentration ‘n i’ doubles. Assume the

mobility of majority and minority carriers to be the same and kT= 20meV at

room temperature. (AUC NOV’12)


3. Draw the circuit diagram and explain the operation of full wave rectifier using center

tap transformer and using bridge rectifier without center tap transformer. Obtain the

expression for peak inverse voltage. (AUC DEC’11)

4. (i) With neat diagram explain the construction and working of LED.

(ii) Explain the working of LCD seven segment displays using square wave supply.

(AUC DEC’11)

5. With a neat diagram explain the working of a PN junction diode in forward bias and

reverse bias and show the effect of temperature on its V-I characteristics.

(AUC MAY’10)

Answer is same as Q1 in partB for working of diode in forward & reverse bias. The

temperature effect is alone explained here.

Temperature Effects of pn junction diode:

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

semiconductor diode as shown in the below figure. It has been found experimentally

that the reverse saturation current Io will just about double in magnitude for every

10°C increase in temperature.

Variation in diode characteristics with temperature change.

It is not uncommon for a germanium diode with an Io in the order of 1 or 2 A at 25°C

to have a leakage current of 100 A _ 0.1 mA at a temperature of 100°C. 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—a very important

reason that silicon devices enjoy a significantly higher level of development and

utilization in design. Fundamentally, the open-circuit equivalent in the reverse bias

region is better realized at any temperature with silicon than with germanium. The

increasing levels of Io with temperature account for the lower levels of threshold

voltage. Simply increase the level of Io in and not rise in diode current. Of course, the

level of TK also will be increase, but the increasing level of Io will overpower the

smaller percent change in TK. As the temperature increases the forward

characteristics are actually becoming more ideal.

6. Explain V-I characteristics of Zener diode. (AUC MAY’10)

Zener Region:

There is a point where the application of too negative a voltage will result in a sharp

change in the characteristics, as shown in the below figure. The current increases at

a very rapid rate in a direction opposite to that of the positive voltage region. The

reverse-bias potential that results in this dramatic change in characteristics is called

the Zener potential and is given the symbol VZ. As the voltage across the diode

increases in the reverse-bias region, the velocity of the minority carriers responsible

for the reverse saturation current Io will also increase. Eventually, their velocity and

associated kinetic energy will be sufficient to release additional carriers through

collisions with otherwise stable atomic structures. That is, an ionization process will

result whereby valence electrons absorb sufficient energy to leave the parent atom.

These additional carriers can then aid the ionization process to the point where a

high avalanche current is established and the avalanche breakdown region

determined. The avalanche region (VZ) can be brought closer to the vertical axis by

increasing the doping levels in the p- and n-type materials. However, as VZ

decreases to very low levels, such as -5 V, another mechanism, called Zener

breakdown, will contribute to the sharp change in the characteristic. It occurs

because there is a strong electric field in the region of the junction that can disrupt

the bonding forces within the atom and generate carriers. Although the Zener

breakdown mechanism is a significant contributor only at lower levels of VZ, this

sharp change in the characteristic at any level is called the Zener region and diodes

employing this unique portion of the characteristic of a p-n junction are called Zener

diodes.




peak reverse voltage (denoted by PRV rating). If an application requires a PIV rating

greater than that of a single unit, a number of diodes of the same characteristics can

be connected in series. Diodes are also connected in parallel to increase the current-

carrying capacity.

Zener breakdown region

ZENER DIODES

The characteristic drops in an almost vertical manner at a reverse-

bias potential denoted VZ. The fact that the curve drops down and

away from the horizontal axis rather than up and away for the

positive VD region reveals that the current in the Zener region has a

direction opposite to that of a forward-biased diode.

This region of unique characteristics is employed in the design of Zener diodes,


which have the graphic symbol appearing in below figure. Both the semiconductor

diode and zener diode are presented side by side in the below figure, to ensure

that the direction of conduction of each is clearly understood together with the

required polarity of the applied voltage. For the semiconductor diode the on state

will support a current in the direction of the arrow in the symbol. The location of the

Zener region can be controlled by varying the doping levels. An increase in doping,

producing an increase in the number of added impurities, will decrease the Zener

potential. Zener diodes are available having Zener potentials of 1.8 to 200 V with

power ratings from 14 to 50 W. Because of its higher temperature and current

capability, silicon is usually preferred in the manufacture of Zener diodes.

.

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, as shown in

below figure


7. Draw the circuit diagram and explain the working of full wave bridge rectifier and

derive the expression for average output current and rectification efficiency.

(AUC NOV’10, DEC 11)

A bridge rectifier makes use of four diodes in a bridge arrangement to achieve

full-wave rectification. This is a widely used configuration, both with individual

diodes wired as shown and with single component bridges where the diode

bridge is wired internally.

For both positive and negative swings of the transformer, there is a forward path

through the diode bridge. Both conduction paths cause current to flow in the same

direction through the load resistor, accomplishing full-wave rectification.

While one set of diodes is forward biased, the other set is reverse biased and

effectively eliminated from the circuit.


8. Explain the operation of FWR with centre tap transformer. Also derive the Following

for this transformer. (AUC APR09, NOV10, DEC11)

(i)Dc output voltage

(ii) dc output current

(iii) RMS output voltage.

Full wave Rectifier using a centre tapped transformer:

A full-wave rectifier converts an ac voltage into a pulsating dc voltage using both

half cycles of the applied ac voltage. In order to rectify both the half cycles of ac input,

two diodes are used in this circuit. The diodes feed a common load RL with the help of

a center-tap transformer.

A center-tap transformer is the one which produces two sinusoidal waveforms of

same magnitude and frequency but out of phase with respect to the ground in the


secondary winding of the transformer.

The full wave rectifier is shown in the figure below.

Voltage Waveforms of a center tapped full wave rectifier

Operation:

During the positive half cycle of the input, the anode of D1 becomes positive and the

anode of D2 becomes negative. Hence, D 1 conduct and D2 does not conducts. The

load current flows through D1 and the voltage drop across RL will be equal to the input

voltage.

During the negative half cycle of the input, the anode of D1 becomes

negative and the anode of D2 becomes positive. Hence, D 1 does not conduct and D2

conducts. The load current flows through D2 and the voltage drop across RL will be

equal to the input voltage.


It is noted that the load current flows in the both the half cycles of ac voltage

and in the same direction through the load resistance.


9. Explain the following regulator circuits: (AUC NOV’10)

(i)Transistor shunt regulator.

(ii) Zener diode shunt regulator.

10. (i)Distinguish between avalanche and zener mechanisms.

(ii)Explain how barrier potential is developed at the PN junction. (AUC APR’09)

(i) Avalanche & Zener mechanisms:

There is a point where the application of too negative a voltage will result in a sharp

change in the characteristics, as shown in the below figure. The current increases at

a very rapid rate in a direction opposite to that of the positive voltage region. The

reverse-bias potential that results in this dramatic change in characteristics is called

the Zener potential and is given the symbol VZ. As the voltage across the diode

increases in the reverse-bias region, the velocity of the minority carriers responsible

for the reverse saturation current Io will also increase. Eventually, their velocity and

associated kinetic energy will be sufficient to release additional carriers through

collisions with otherwise stable atomic structures. That is, an ionization process will

result whereby valence electrons absorb sufficient energy to leave the parent atom.

These additional carriers can then aid the ionization process to the point where a

high avalanche current is established and the avalanche breakdown region

determined. The avalanche region (VZ) can be brought closer to the vertical axis by

increasing the doping levels in the p- and n-type materials. However, as VZ

decreases to very low levels, such as -5 V, another mechanism, called Zener

breakdown, will contribute to the sharp change in the characteristic. It occurs

because there is a strong electric field in the region of the junction that can disrupt

the bonding forces within the atom and generate carriers. Although the Zener

breakdown mechanism is a significant contributor only at lower levels of VZ, this

sharp change in the characteristic at any level is called the Zener region and diodes

employing this unique portion of the characteristic of a p-n junction are called Zener

diodes.




peak reverse voltage (denoted by PRV rating). If an application requires a PIV rating

greater than that of a single unit, a number of diodes of the same characteristics can

be connected in series. Diodes are also connected in parallel to increase the current-

carrying capacity.

Zener breakdown region

(ii) Barrier potential development across pn junction:

The semiconductor diode is formed by simply bringing these materials together

(constructed from the same base—Ge or Si). At the instant the two materials are

joined the electrons and holes in the region of the junction will combine, resulting in

a lack of carriers in the region near the junction. This region of uncovered positive

and negative ions is called the depletion region due to the depletion of carriers in

this region. Since the diode is a two-terminal device, the application of a voltage

across its terminals leaves three possibilities: no bias (VD = 0 V), forward bias (VD

>0 V), and reverse bias (VD< 0 V).

p-n junction with no external bias.


Diode with no bias

Under no-bias (no applied voltage) conditions, any minority carriers (holes) in the n-

type material that find themselves within the depletion region will pass directly into

the p-type material. The closer the minority carrier is to the junction, the greater the

attraction for the layer of negative ions and the less the opposition of the positive ions

in the depletion region of the n-type material. Assume that all the minority carriers of

the n-type material that find themselves in the depletion region due to their random

motion will pass directly into the p-type material. Similar discussion can be applied to

the minority carriers (electrons) of the p-type material. This carrier flow has been

indicated in the above figure for the minority carriers of each material. The majority

carriers (electrons) of the n-type material must overcome the attractive forces of the

layer of positive ions in the n-type material and the shield of negative ions in the p-

type material to migrate into the area beyond the depletion region of the p-type

material. However, the number of majority carriers is so large in the n-type material

that there will invariably be a small number of majority carriers with sufficient kinetic

energy to pass through the depletion region into the p-type material. Again, the same

type of discussion can be applied to the majority carriers (holes) of the p-type

material. The resulting flow due to the majority carriers is also shown in the above

figure. In the absence of an applied bias voltage, the net flow of charge in any one

direction for a semiconductor diode is zero. The symbol for a diode is shown in the

below figure with the associated n- and p-type regions. Note that the arrow is

associated with the p-type component and the bar with the n-type region. As

indicated, for VD= 0 V, the current in any direction is 0 mA.

Reverse-Bias Condition (VD < 0 V)

If an external potential of V volts is applied across the p-n junction such that the

positive terminal is connected to the n-type material and the negative terminal is

connected to the p-type material as shown in the below figure. The number of

uncovered positive ions in the depletion region of the n-type material will increase

due to the large number of free electrons drawn to the positive potential of the

applied voltage. For similar reasons, the number of uncovered negative ions will

increase in the p-type material. The net effect, therefore, is a widening of the

depletion region. This widening of the depletion region will establish too great a

barrier for the majority carriers to overcome, effectively reducing the majority carrier

flow to zero as shown in the below figure.


Reverse-biased p-n junction.

The number of minority carriers, however, that find themselves entering the

depletion region will not change, resulting in minority-carrier flow vectors of the

same magnitude with no applied voltage the current that exists under reverse-bias

conditions is called the reverse saturation current and is represented by Io.

Forward-Bias Condition (VD > 0 V)

A forward-bias condition is established by applying the positive potential to the p-

type material and the negative potential to the n-type material as shown in the

below figure. A semiconductor diode is forward-biased when the association p-type

and positive and n-type and negative has been established.

Forward-biased p-n junction

The application of a forward-bias potential VD will pressure 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 as shown in the above

figure. The resulting minority-carrier flow of electrons from the p-type material to

the n-type material (and of holes from the n-type material to the p-type material)

has not changed in magnitude (since the conduction level is controlled primarily by

the limited number of impurities in the material), but the reduction in the width of

the depletion region has resulted 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.

11. Discuss the following: (i) Transition capacitance (ii) Diffusion capacitance. (AUC MAY’08)

Transition & Diffusion Capacitance:

Electronic devices are inherently sensitive to very high frequencies. Most shunt

capacitive effects that can be ignored at lower frequencies because the reactance

XC=1/2πfC is very large (open-circuit equivalent). This, however, cannot be ignored

at very high frequencies. XC will become sufficiently small due to the high value of f to

introduce a low-reactance ―shorting‖ path. 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. 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. Although the effect described above will also be present in the

forward-bias region, it is overshadowed by a capacitance effect directly dependent on

the rate at which charge is injected into the regions just outside the depletion region.

The capacitive effects described above are represented by a capacitor in parallel with

the ideal diode, as shown in the below figure. For low- or mid-frequency applications

(except in the power area), however, the capacitor is normally not included in the

diode symbol.

MAHALAKSHMI...MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI-621213. QUESTION BANK DEPARTMENT: EEE...

Documents

Transcript of MAHALAKSHMI...MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI-621213. QUESTION BANK DEPARTMENT: EEE...