Solution:

Ans. (i) Try yourself, Similar to Q. 3, Short Answer Type Questions-II. (ii) When the semiconductor is doped with an acceptor impurity there is an additional energy level which is little above the top of the valence band. Also, electrons from the valence band, easily jump over to the acceptor level, leaving ‘holes’ behind, so ‘holes’ become the majority charge carriers.

$1 \frac{1}{2}$

(iii) The donor impurity results in an additional energy level which is little below the bottom of the conduction band. Electrons from donor level, easily ‘jump over’ to the conduction band. Hence, electrons become the majority charge carriers. 11⁄2 [CBSE Marking Scheme 2016]

TOPIC-2

Semiconductor Diodes and Applications

Revision Notes

Diode

Diode is an electronic device consisting of a junction of semiconductors $p$-type and $n$-type. It is represented as :

Semiconductor diode

• Semiconductor diodes were first semiconductor electronic devices which is common type of diode that is made of crystalline piece of semiconductor material with $p-n$ junction across its terminals.

Dhen a $p$-type semieonductor material is suitably joined to $n$-type semiconductor, the contact surface is called a $p$ - $n$ junction.

It is an electrical device that allows current only in one direction The direction. of arrow is the direction of current when it is forward biased.

At junction, electrons and holes diffuse and form the diffusion current.

$p-n$ junction layer is also called the depletion layer. Potential barrier is created at junction due to diffusion current. It acts as barrier for majority carriers.

• The potential barrier helps the minority carrier to flow. Thus a drift current forms, which is opposite in direction of the diffusion current.
• Under equilibrium condition, diffusion current is equal to the drift current and hence net current is zero as both are in opposite direction.

There are many types of semiconductor diode such as :

Avalanche diodes, Gunn diodes, light emitting diodes (LED), Photodiodes etc.

• Semiconductor diode can be made either from Silicon or Germanium and each differs in size and properties.

Forward Bias

When an external voltage is applied, where negative of battery is connected to $n$ side while positive of battery is connected to $p$ side, then barrier potential will get reduced and more current can flow across the junction that decreases the $p-n$ junction width.

The positive terminal of battery repels majority carriers, holes in $p$-region while negative terminal repels electrons in $n$-region which pushes them towards the junction.

Here, an increase in concentration of charge carriers near the junction is observed, where recombination takes place thereby reducing width of depletion region.

Due to rise in forward bias voltage, depletion region will continue to reduce its width which results in more and more carriers recombination.

Reverse Bias

• If an external voltage is applied in reverse direction where positive battery is connected to $n$ side while negative of battery is connected to $p$ side, then barrier potential willincrease and minority charge carriers will flow across junction.

In this, the current will be quietsmall and is independent of external voltage.

Beyond certain voltage, diodeyill break down with Avalanche breakdown mechanism or Zener breakdown mechanism.

• Here, negative terminal of battery will attract majority carriers, holes in $p$-region and positive terminal attracts electrons in $n$-region which pulls them away from the junction.
• As a result of this, there will be decrease in concentration of charge carriers near junction which increases the width of depletion region.

A small amount of current flows due to minority carriers known as reverse bias current or leakage current and with rise in reverse bias voltage, depletion region continues to increase in width without any increase in flow of current.

I-V Characteristics of Diode

• In I-V characteristics of diode, on voltage axis, “Reverse Bias” is an external voltage potential that increases the potential barrier while external voltage that decreases the potential barrier is in “Forward Bias” direction.

Biasing of Diode can be Forward Biasing or Reverse Biasing.

Diode as rectifier

Rectifier is a circuit which converts AC supply into unidirectional DC supply.

With rectification, alternating current (AC) gets converted to directcurrent (DC) through a rectifier.

• The bridge rectifier circuits uses semiconductor diode for converting $\mathrm{AC}$ as it allows the current to flow in one direction only.

Half wave rectification

The half-wave rectifier with single diode, allows current to flow in one direction.

Here, AC power source $V_{\text {ac }}$ is conhected to primary side of transformer while secondary terminals of transformer are connected to diode and resistor in series.

If $\mathrm{V}_{a c}$ is in positive cycle, a positive voltage is produced on secondary side of transformer.

• The positive voltage will forward bias the diode and diode will start passing the current as a result, the voltage will drop across the load.

If $V_{a c}$ is in negative cycle, then secondary side will have negative voltage where diode is reverse biased and will not pass any current.

Voltage waveform across load resistor is shown below, where positive side of sinusoidal cycle is present while negative side sinusoidal cycle has been clamped off.

The output voltage $V_{d c}$ is similar to the output of battery which is always positive.

The positive waveform is bumpy as single diode is applied to produce half wave rectification where one half of AC wave is removed that will not pass through the diode.

Full wave rectification

For rectifying AC power for using both half cycles of sine wave, full wave rectification is used.

• A simple kind of full wave rectifier uses centre tap transformers with two diodes.

In full wave rectification, in first half-cycle, when source voltage polarity is positive (+) on top and negative (-) on bottom, then only top diode will conduct while bottom diode blocks the current When source voltage polarity is negative (-) on top and positive ( + ) on bottom then only bottom diode will conduct while the top diode blocks the current.

Special purpose $p-n$ junction diodes

Apart from simple $p-n$ junction diodes, there are many more types of diodes which are used in various specific applications that take advantage of the behaviour and features.

Light-emitting diode

Photo diode

LED

Schottky diode

Light Emitting Diode or LED is most widely used semiconductor diodes among all the different types of semiconductor diodes available today.

• It emits visible light or invisible infrared light when forward biased.

The LEDs which emit invisible infrared light are used for remote controls.

In this, diode in forward biased will make electrons and holes to move fast across the junction and helps in combining constantly by removing one another.

• Electrons which move from $n$-typeto $k$-type silicon will combine with holes and give energy in the form of light.

Recombination of electrons and holes in depletion region decreases the width of the region which allows more charge carriers to cross the $p-n$ junction.

Here, some of the charge carriers from $p$-side and $n$-side will cross the $p$ - $n$ junction before they recombine in depletion region.

Photodiode

Photodiode is a transducer which takes light energy and converts it to electrical energy.

It is a $p-n$ junction which consumes light energy to generate electric current.

It is referred to as photo-detector, photo-sensor or light detector.

It is specially designed to operate in reverse bias condition where $p$-side is connected to negative terminal of battery and $n$-side connected to positive terminal of battery.

It is sensitive to light as when light or photons fall on it, it easily converts light into electric current.

• In photodiode circuit, current flows from the cathode to anode when exposed to light.
• Photodiode is capable of converting light energy to electrical energy and can be expressed as a percentage known as Quantum Efficiency (Q.E.).

Solar cell

• Solar cell is an electronic device which absorbs sunlight and generates emf.

In this, there are $n$-type silicon and $p$-type silicon layers that generates electricity using sunlight to make electrons in order to jump across the junction between different types of silicon material.

When sunlight shines on solar cell, photons bombard the upper surface and generates electron-hole pairs.

They get separated due to voltage barrier at junction, electrons are swept to $n$ side & holes are swept to $p$-side.

Metal contacts hold these electron-hole pairs. Thus $p$ side becomes positive and $n$ side becomes negative and hence it act as photo voltage cell.

Zener diode and its characteristics

I-V characteristics

Zener Diode is an electronic component which can be used to make very simple voltage regulator circuit.

Symbol of Zener diode

They are special type of semiconductor diodes which allow the current to flow in one direction when exposed to high voltage.

It is a $p-n$ junction semiconductor device which is designed to operate in reverse breakdown region.

• The breakdown voltage of zener diode is set by controlling the doping level.

Zener Diode allows electric current in forward direction similar to normal diode and also allows electric current in reverse direction when applied reverse voltage is more than zener voltage.

• It is always connected in reverse direction since it is specifically designed to work in reverse direction.

Zener Diode circuit enables a fixed stable voltage to be taken from an unstable voltage source like battery that fluctuates as per state of charge of battery.

The circuit of Zener Diode has a resistor in series with diode which limits the output current.

In Zener Diode, there are two breakdown mechanisms : Zener Breakdown or Avalanche Breakdown mechanism.

I-V Characteristics Curve of zener diode shows current-voltage relationship.

• In $I-V$ curve, in right half side, zenerdiode receives forward voltage which is positive voltage across its anode to cathode terminals.

In right half side of zener diode characteristics curve, diode is forward biased and current is more.

• In left half side of $I-V$ curve, zener diode will receive positive voltage across its cathode to anode terminals where diode is reverse biased

At reverse voltage, current will be very small which is known as leakage current that flows through the diode.

$>$ After hitting breakdown voltage, avalanche current will sharply increase.

At breakdown voltage point, when voltage of zener diode reaches, it remains constant in spite of increase in current making zener diode suitable for voltage regulation.

Zener diode as voltage regulator

Voltage regulation is a measure of ability of circuit to maintain constant voltage output under variation either in input voltage or load current.

In zener diode voltage regulator circuit :

• resistor $R_{s}$ is used to limit reverse current through diode to safer value.
• $V_{s}$ and $R_{s}$ are selected such that diode operates in breakdown region.
• series resistor $R_{s}$ absorbs output voltage fluctuations to maintain voltage across load to constant value.

Zener diode maintains constant voltage across load as long as supply voltage is more than zener voltage.

When input voltage increases, current through Zener diode also increases keeping the voltage drop constant.

• Current in the circuit increases the voltage drop across the resistor which increases by an amount equal to difference between the input voltage and zener voltage of the diode.

Objective Type Questions