ELECTRONIC DEVICES AND CIRCUITS

SUBJECT CODE SEC1204

UNIT I

SEMICONDUCTOR DIODE

INTRINSIC AND EXTRINSIC SEMICONDUCTORS:

INTRINSIC SEMICONDUCTOR is an un-doped semiconductor, in which there is no

impurities added where as extrinsic semiconductor is a doped semiconductor, which has

impurities in it. Doping is a process, involving adding dopant atoms to the intrinsic

semiconductor. An intrinsic semiconductor is the purest form of semiconductor. Group 14

elements like Germanium and Silicon are typical examples of intrinsic semiconductors. These

intrinsic semiconductors are free from the presence of any doping agents.

An EXTRINSIC SEMICONDUCTOR is obtained by doping an intrinsic semiconductor with

other elements. When we dope group 14 element with a group 13 element, we get a p type

semiconductor where the majority charge carriers are holes. When the group 14 element, that is,

the intrinsic semi conductor is doped with a group 15 element we get a n type semiconductor.

Here the majority charge carriers are electrons.

Note:

THERMAL ENERGY: The power that is created by heat, or the increase in temperature.

Intrinsic semiconductor

A pure semiconductor free from any impurity is called intrinsic semiconductor. Here charge

carriers (electrons and holes) are created by thermal excitation. Si and Ge are examples for this.

Both Si and Ge are tetravalent, i.e. each has four valence electrons in the outermost shell.

Consider the case of Ge that has a total of 32 electrons. Out of these 32 electrons, 28 are tightly

bound to the nucleus, while the remaining 4 electrons (valence electrons) revolve in the

outermost orbit. In a solid, each atom shares its 4 valence electrons with its nearest neighbors to

form covalent bonds. The energy needed to liberate an electron from Ge atom is very small, of

the order of 0.7 eV. Thus even at room temperature, a few electrons can detach from its bonds by

thermal excitation. When the electron escapes from the covalent bond, an empty space or a hole

is created. The number of free electrons is always equal to the number of holes.

Extrinsic semiconductor:

Extrinsic semiconductors are formed by adding suitable impurities to the intrinsic

semiconductor. This process of adding impurities are called doping. Doping increases the

electrical conductivity in semiconductors. The added impurity is very small, of the order of one

atom per million atoms of the pure semiconductor. The added impurity may be pentavalent or

trivalent. Depending on the type of impurity added,

Extrinsic semiconductors can be divided into two classes: n-type and p-type.

n-type semiconductor

When pentavalent impurity is added to pure semiconductor, it results in n-type semiconductor.

Consider the case when pentavalent Arsenic is added to pure Silicon (Si) crystal. As shown in

the figure, four electrons of Arsenic atom form covalent bonds with the four valence electrons of

neighboring Si atoms. The fifth electron of Arsenic atom is not covalently bonded, but it is

loosely bound to the Arsenic atom. Now by increasing the thermal energy or by applying electric

field, this electron can be easily excited from the valence band to the conduction band. Thus

every Arsenic atom contributes one conduction electron without creating a positive hole. Hence

Arsenic is called donor element since it donates free electrons. Since current carriers are

negatively charged particles, this type of semiconductor is called n-type semiconductor.

P-type semiconductor

When trivalent impurity is added to pure semiconductor, it results in p-type semiconductor.

Consider the case when trivalent gallium is added to pure silicon (si) crystal. As shown in the

figure, three valence electrons of Gallium atom form covalent bonds with the three neighboring

Si atoms. There is a deficiency of one electron (hole) in the bonding with the fourth Si atom. The

Si atom will steal an electron from the neighboring Si atom to form a covalent bond. Due to this

stealing action, a hole is created in the adjacent atom.

P–N JUNCTION DIODE:

On the voltage axis above, "Reverse Bias" refers to an external voltage potential which increases

the potential barrier. An external voltage which decreases the potential barrier is said to act in the

"Forward Bias" direction.

Reverse Bias:

When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-

type material and a negative voltage is applied to the P-type material. The positive voltage

applied to the N-type material attracts electrons towards the positive electrode and away from the

junction, while the holes in the P-type end are also attracted away from the junction towards the

negative electrode.The net result is that the depletion layer grows wider due to a lack of electrons

and holes. This creates a high impedance path, which is almost an insulator. The result is that a

high potential barrier is created thus preventing current from flowing through the semiconductor

material.

Reverse Biased Junction Diode showing an Increase in the Depletion Layer

Forward Biased Junction Diode showing a Reduction in the Depletion Layer

This condition represents the low resistance path through the PN junction allowing very large

currents to flow through the diode with only a small increase in bias voltage. The actual potential

difference across the junction or diode is kept constant by the action of the depletion layer at

approximately 0.3v for germanium and approximately 0.7v for silicon junction diodes. Since the

diode can conduct "infinite" current above this knee point as it effectively becomes a short

circuit, therefore resistors are used in series with the diode to limit its current flow. Exceeding its

maximum forward current specification causes the device to dissipate more power in the form of

heat than it was designed for resulting in a very quick failure of the device.

There are two operating regions and three possible "biasing" conditions for the standard

Junction Diode and these are:

1. Zero Bias - No external voltage potential is applied to the PN-junction.

2. Reverse Bias - The voltage potential is connected negative, (-ve) to the P-type material

and positive, (+ve) to the N-type material across the diode which has the effect of

Increasing the PN-junction width.

3 . Forward Bias - The voltage potential is connected positive, (+ve) to the P-type material and

negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the

PN-junction width.

ENERGY BAND DIAGRAM &CURRENT COMPONENTS OF PN JUNCTION DIODE:

Current components in PN junction

1.UNDER FORWARD BIAS, FIG1

2.UNDER REVERSE BIAS, FIG2

3.UNDER EQUILIBRIUM, FIG3

Fig.1) Under forward bias conduction takes place and hence no depletion region.

Fig.2) Under reverse bias conduction does not takes place, depletion region forms.

Fig.3) Under equilibrium condition, zero voltage across diode &hence no current flow.

APPLICATION OF DIODE

1) DIODE SWITCH

2) CLIPPER

3) CLAMPER

4) VOLTAGE MULTIPLIERS

1) DIODE SWITCH

Diode Switch

Figure 1: Basic diode switch

In addition to their use as simple rectifiers, diodes are also used in circuits that mix signals

together (mixers), detect the presence of a signal (detector), and act as a switch “to open or close

a circuit”. Diodes used in these applications are commonly referred to as “signal diodes”. The

simplest application of a signal diode is the basic diode switch shown in figure 1.

When the input to this circuit is at zero potential, the diode is forward biased because of the zero

potential on the cathode and the positive voltage on the anode. In this condition, the diode

conducts and acts as a straight piece of wire because of its very low forward resistance. In effect,

the input is directly coupled to the output resulting in zero volts across the output terminals.

Therefore the diode, acts as a closed switch when its anode is positive with respect to its

cathode.

If we apply a positive input voltage to the diode's cathode, the diode will be reverse biased. In

this situation, the diode is cut off and acts as an open switch between the input and output

terminals. Consequently, with no current flow in the circuit, the positive voltage on the diode's

anode will be felt at the output terminal. Therefore, the diode acts as an open switch when it is

reverse biased.

2) CLIPPER

Clipper is a circuit that prevents the amplitude of a waveform from exceeding a specified value.

It is also called LIMITER where a circuit designed to limit the amplitude of an output signal to

a preset level.The basic components required for a clipping circuit are an ideal diode and a

resistor. In order to fix the clipping level to the desired amount, a dc battery must also be

included. When the diode is forward biased, it acts as a closed switch, and when it is reverse

biased, it acts as an open switch. Different levels of clipping can be obtained by varying the

amount of voltage of the battery and also interchanging the positions of the diode and resistor.

Depending on the features of the diode, the positive or negative region of the input signal is

“clipped” off and accordingly the diode clippers may be positive or negative clippers.

There are two general categories of clippers: series and parallel (or shunt). The series

configuration is defined as one where diode is in series with the load, while the shunt clipper has

the diode in a branch parallel to the load.

1. Positive Clipper and Negative Clipper Positive Diode Clipper

In a positive clipper, the positive half cycles of the input voltage will be removed. The circuit

arrangements for a positive clipper are illustrated in the figure given below.

As shown in the figure, the diode is kept in series with the load. During the positive half cycle of the input waveform, the diode ‘D’ is reverse biased, which maintains the output voltage at 0 volts. Thus causes the positive half cycle to be clipped off. During the negative half cycle of the input, the diode is forward biased and so the negative half cycle appears across the output. In Figure (b), the diode is kept in parallel with the load. This is the diagram of a positive shunt clipper circuit. During the positive half cycle, the diode ‘D’ is forward biased and the diode acts as a closed switch. This causes the diode to conduct heavily. This causes the voltage drop across the diode or across the load resistance RL to be zero. Thus output voltage during the positive half cycles is zero, as shown in the output waveform. During the negative half cycle of the input signal voltage, the diode D is reverse biased and behaves as an open switch. Consequently the entire input voltage appears across the diode or across the load resistance RL if R is much smaller than RL. Actually the circuit behaves as

a voltage divider with an output voltage of [RL / R+ RL] Vmax = -Vmax when RL >> R

Negative Diode Clipper The negative clipping circuit is almost same as the positive clipping circuit, with only one difference. If the diode in figures (a) and (b) is reconnected with reversed polarity, the circuits will become for a negative series clipper and negative shunt clipper respectively. The negative series and negative shunt clippers are shown in figures (a) and (b) as given below.

In all the above discussions, the diode is considered to be ideal one. In a practical diode, the breakdown voltage will exist (0.7 V for silicon and 0.3 V for Germanium). When this is taken into account, the output waveforms for positive and negative clippers will be of the shape shown in the figure below.

Negative and Positive Clipping Waveforms

APPLICATION:

The diode clipper can be used for the protection of different types of circuits.

3) CLAMPER

A clamping circuit is used to place either the positive or negative peak of a signal at a desired

level. For a clamping circuit the three components are diode, capacitor and resistor. The dc

component is simply added or subtracted to/from the input signal. The clamper is also referred to

as an IC restorer and ac signal level shifter.

In some cases, like a TV receiver, when the signal passes through the capacitive coupling

network, it loses its dc component. This is when the clamper circuit is used so as to re-establish

the dc component into the signal input. Though the dc component that is lost in transmission is

not the same as that introduced through a clamping circuit, the necessity to establish the

extremity of the positive or negative signal excursion at some reference level is important.

A clamp circuit adds the positive or negative dc component to the input signal so as to push it

either on the positive side, as illustrated in figure (a) or on the negative side, as illustrated in

figure (b).

The circuit will be called a positive clamper , when the signal is pushed upward by the circuit.

When the signal moves upward, as shown in figure (a), the negative peak of the signal coincides

with the zero level.

The circuit will be called a negative clamper, when the signal is pushed downward by the circuit.

When the signal is pushed on the negative side, as shown in figure (b), the positive peak of the

input signal coincides with the zero level.

ZENER DIODE:

A zener diode is a special kind of diode which allows current to flow in the forward direction

in the same manner as an ideal diode, also permit to flow in the reverse direction when the

voltage is above a certain value known as the breakdown voltage, "zener knee voltage" or

"zener voltage."

Fig) Symbol of zener diode Reverse current is shown.

(VOLTAGE CURRENT CHARACTERISTICS) OF ZENER DIODE:

The illustration above shows this phenomenon in a Current vs. Voltage graph. With a zener

diode connected in the forward direction, it behaves exactly the same as a standard diode. In the

reverse direction however there is a very small leakage current i.e. just a tiny amount of current

is able to flow. When the voltage reaches the breakdown voltage (Vz), suddenly current can flow

freely through it. In reverse-bias mode, they do not conduct until the applied voltage reaches or

exceeds the so-called zener voltage.

Zener breakdown & Avalanche breakdown

Zener breakdown occurs when the doping level in a semiconductor is high and the band gap is

narrow. When these conditions are satisfied tunneling of electrons from one level to other takes

place and conductivity increases. When the conditions of zener breakdown are not satisfied

avalanche breakdown takes place at higher temperature. In avalanche breakdown electrons

collide with the particles in the depletion region. This result in ionization and result in the

formation of a electron hole pair.This extra electron may result in further multiplication called

avalanche multiplication.

ZENER VOLTAGE REGULATOR

Zener Diode Voltage Regulator Circuit

• A voltage regulator is an electrical regulator designed to automatically maintain a

constant voltage level.

Resistor value (ohms) = (VIN - VOUT) / (Zener current + Load current) A zener diode can be used to make a simple voltage regulation circuit as pictured above. The output voltage is fixed at the zener voltage of the zener diode used and can be used to power devices requiring a fixed voltage. Applications of Zener Diodes are as follows:

1. Voltage Regulators (overvoltage protection)

2. Surge Suppressors i.e for device protection