NAVODAYA VIDYALAYA SAMITI,�HYDERABAD REGION

NAVODAYA VIDYALAYA SAMITI

e-Content

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Class: XII

Subject: Physics

Chapter: Semiconductor Electronics: Materials, Devices and Simple circuits

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Prepared by Manjusha M Nair, PGT Physics,

JNV Wayanad, Kerala.

What are Semiconductors?�

• Semiconductors are the materials which have a conductivity between conductors  and non-conductors or insulators .
• Resistivity: 10^-5 to 10⁶ Ωm
• Conductivity: 10⁵ to 10^-6 mho/m
• Temperature coefficient of resistance: Negative
• Eg: Elemental form- Silicon and germanium
• Compound form-GaAs, CdS, InP etc.

Why are semiconductors?

• Semiconductors can conduct electricity under preferable conditions or circumstances. This unique property makes it an excellent material to conduct electricity in a controlled manner as required.
• They consume low power, are small in size, operate at low voltages and have a long life and high reliability.

ENERGY BANDS IN SOLIDS

• The electrons in an atom are present in different energy level. When we try to assemble a lattice of a solid with N atoms, then each level of an atom must split up into N levels in the solid. This splitting up of sharp and tightly packed energy levels forms Energy Bands.

ENERGY BANDS IN SOLIDS

• In a free atom the energies of electrons can have only some definite (quantized) values.
• If an atom belongs to a crystal, then the energy levels are modified.
• This modification is not appreciable in the case of energy levels of electrons in the inner shells (completely filled).
• But in the outermost shells, modification is appreciable because the electrons are shared by many neighbouring atoms.
• Due to influence of high electric field between the core of the atoms and the shared electrons, energy levels are split-up or spread out forming energy bands.

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1s²

2s²

2p⁶

3p²

3s²

Inter atomic spacing (r)

Energy

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b

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d

Conduction Band

Valence Band

Forbidden Energy Gap

Ion core state

Formation of Energy Bands in Solids:

Each of N atoms has its own energy levels. The energy levels are identical, sharp, discrete and distinct.

(ii) Oc < r < Od:

There is no visible splitting of energy levels but there develops a tendency for the splitting of energy levels.

(iii) r = Oc:

The interaction between the outermost shell electrons of neighbouring silicon atoms becomes appreciable and the splitting of the energy levels commences.

1. r = Od (>> Oa):

(iv) Ob < r < Oc:

The energy corresponding to the s and p levels of each atom gets slightly changed. Corresponding to a single s level of an isolated atom, we get 2N levels. Similarly, there are 6N levels for a single p level of an isolated atom.

Formation of Energy Bands in Solids:

The collection of very closely spaced energy levels is called an energy band.

(v) r = Ob:

The energy gap disappears completely. 8N levels are distributed continuously. We can only say that 4N levels are filled and 4N levels are empty.

(vi) r = Oa:

The band of 4N filled energy levels is separated from the band of 4N unfilled energy levels by an energy gap called forbidden gap or energy gap or band gap.

The lower completely filled band (with valence electrons) is called the valence band and the upper unfilled band is called the conduction band.

Formation of Energy Bands in Solids:

Valance Band & Conduction Band

• Valence and Conduction band are the two different energy levels separated by a certain amount of energy.
• The main difference between the valence band and conduction band is that valence band specifies the energy level of electrons present in the valence shell of an atomic structure.
• A conduction band holds those electrons that are responsible for conduction.

Forbidden Energy Gap

• Forbidden energy gap, also known as band gap refers to the energy difference (eV) between the top energy level of valence band and the bottom energy level of the conduction band in materials.

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• Band gap in silicon and germanium is  1.11eV   and   0.67eV    respectively.

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ENERGY BANDS OF METALS, SEMICONDUCTORS AND INSULATORS

Charge carriers in Semiconductors:

• Holes and electrons are the types of charge carriers accountable for the flow of current in semiconductors. 
• Holes are the positively charged electric charge carrier whereas electrons are the negatively charged particles. Both electrons and holes are equal in magnitude but opposite in polarity.

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Types of Semiconductors�

• Semiconductors can be classified as:
• Intrinsic Semiconductor is made to be very pure chemically. It is made up of only a single type of element.
• Eg: Germanium (Ge) and Silicon (Si).
• Extrinsic Semiconductor-A semiconductor to which an impurity at controlled rate is added to make it conductive .

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Intrinsic semiconductors

• The pure form of the semiconductor is known as the intrinsic semiconductor.
• Electronic Configuration of Silicon and Germanium
• Silicon --- 1s² 2s²2p⁶ 3s² 3p²
• Germanium --- 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p² 
• Silicon and germanium are tetravalent.

Intrinsic semiconductors

• In its crystalline structure, Si and Ge tends to share one of its valence electrons and also take share of one electron from its neighbour atoms.

Concept of hole:

• As temperature is increased, some of the electrons break free and behave as conduction electrons. This creates a vacancy in the bond and creates a positive charge also known as a hole.

Electrons and Holes:�

• On receiving an additional energy, one of the electrons from a covalent bond breaks and is free to move in the crystal lattice.
• While coming out of the covalent bond, it leaves behind a vacancy named ‘hole’.
• An electron from the neighbouring atom can break away from the bond and can come to the place of the missing electron (or hole) completing the covalent bond and creating a hole at another place.
• The holes move randomly in a crystal lattice.

Concept of hole:

• Semiconductors possess the unique property that apart from electrons the holes also move.
• These free electrons and holes contribute to the conduction of electricity in the semiconductor.
• The negative and positive charge carriers are equal in number.

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Concept of hole:

• A hole behaves as an apparent free charge similar to an electron and contributes to conduction.
• Holes act as charge carriers in the sense that electrons from nearby sites can move into the hole.

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Intrinsic or Pure Semiconductor:

C.B

V.B

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0.74 eV

Heat Energy

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Carrier concentration in intrinsic semiconductors:

• In intrinsic semiconductors, current flows due to the motion of free electrons as well as holes. The total current is the sum of the electron current I_e due to thermally generated electrons and the hole current I_h
• Total Current (I) = I_e + I_h
• If ne and nh are the concentration of electrons and holes respectively, then ne = nh.
• The quantity ne or nh is referred to as the ‘intrinsic carrier concentration’.

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Crystal structure of intrinsic semiconductor at T=0K.

Energy Band Diagram of Intrinsic Semiconductor�

1. Intrinsic Semiconductor at T = 0 Kelvin, behaves like an insulator

(b) At T>0, four thermally generated electrons

Doping a Semiconductor:

• The process by which an impurity is added to a semiconductor is known as Doping. 
• The purpose of adding impurity in the semiconductor crystal is to increase the number of free electrons or holes to make it conductive.
• The impurity atoms are called ‘dopants’.
• Depending upon the type of impurity added the extrinsic semiconductor may be classified as

 n type semiconductor and p type semiconductor.

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Extrinsic semiconductor:

N – Type semiconductorstors:

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When a semiconductor of Group IV (tetra valent) such as Si or Ge is doped with a penta valent impurity (Group V elements such as P, As or Sb), N – type semiconductor is formed.

When germanium (Ge) is doped with arsenic (As), the four valence electrons of As form covalent bonds with four Ge atoms and the fifth electron of As atom is loosely bound.

The energy state corresponding to the fifth electron is in the forbidden gap and slightly below the lower level of the conduction band. This energy level is called ‘donor level’.

Energy band diagram of n type semiconductor

Carrier Concentration in N - Type Semiconductors:

• If n and p represent the electron and hole concentrations respectively in N-type semiconductor, then

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When the concentration of electrons is increased above the intrinsic value by the addition of donor impurities, the concentration of holes falls below its intrinsic value, making the product np a constant, equal to n_i².

n p = ne nh = n_i²

P - Type Semiconductors:

When a semiconductor of Group IV (tetravalent) such as Si or Ge is doped with a trivalent impurity (Group III elements such as In, B or Ga), P – type semiconductor is formed.

When silicon (Si) is doped with indium (In), the three valence electrons of In form three covalent bonds with three Si atoms. The vacancy that exists with the fourth covalent bond with fourth Si atom constitutes a hole.

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Energy band diagram of p type semiconductor

• The acceptor impurity produces an energy level just above the valence band.
• This energy level is called ‘acceptor level’.

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Charge carriers in extrinsic semiconductors

• In p type semiconductor, holes are majority charge carriers whereas electrons are minority charge carriers. i.e nh>>ne
• In n type semiconductor, electrons are majority charge carriers whereas holes are minority charge carriers. . i.e nh<<ne
• But p type or n type semiconductors are electrically neutral.

Charge carriers in extrinsic semiconductors

Distinction between Intrinsic and Extrinsic Semiconductor:

PN Junction Diode:

Mobile Hole (Majority Carrier)

Immobile Negative Impurity Ion

Mobile Electron (Majority Carrier)

Immobile Positive Impurity Ion

When a P-type semiconductor is joined to a N-type semiconductor such that the crystal structure remains continuous at the boundary, the resulting arrangement is called a PN junction diode or a semiconductor diode or a crystal diode.

Formation of pn junction diode:

• Two important processes take place during the formation of a p-n Junction:
• Diffusion
• Drift

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Formation of pn junction diode:

• Diffusion is the process of movement of charge carriers due to concentration gradient along the semiconductor. In a p-n junction, n-side has excess of electrons and hence electrons diffuse from n-side to p-side. Similarly, holes diffuse from p-side to n-side.
• Drift is the process of movement of charge carriers due to the net electric field. In a pn-junction with no external source, electric field is from n-side to p-side and hence electrons drift from p-side to n-side. 

Depletion Region & Potential Barrier

• The region containing the immobile acceptor and donor ions is called ‘depletion region’ because this region is devoid of mobile charges.
• Since the region is having only immobile charges, therefore, this region is also called ‘space charge region’.
• The difference in potential between P and N regions across the junction makes it difficult for the holes and electrons to move across the junction. This acts as a barrier and hence called ‘potential barrier’ or ‘height of the barrier’.

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Forward bias

• When the positive terminal of the battery is connected to P-region and negative terminal is connected to N-region, then the PN junction diode is said to be forward-biased.

• Here the applied voltage is opposite to the barrier potential.

Forward bias

• Due to the applied voltage, holes from p side and free electrons from n side enter into the depletion region- minority carrier injection.
• The potential barrier and the width of the depletion region decrease.
• Therefore, a large number of majority carriers diffuse across the junction.
• Hole current and electronic current are in the same direction and add up.

• If the applied potential is increased, the potential barrier further decreases. As a result, a large number of majority carriers diffuse through the junction and a larger current flows.
• In forward biasing, the resistance of pn junction reduces considerably.
• Here diode act as a conductor and there is current. flow through the circuit

Forward bias

Forward characteristics of pn junction diode.

The minimum forward bias voltage required for a diode to conduct is called threshold voltage. Threshold voltage is the voltage above which current increases very rapidly with applied voltage.

Reverse Bias

• When the negative terminal of the battery is connected to P-region and positive terminal is connected to N-region, then the PN junction diode is said to be reverse-biased.

• Here the applied voltage is in the same direction as the barrier potential.

Reverse Bias

• The majority carriers are pulled away from the junction.
• The potential barrier and the width of the depletion region increase.
• Therefore, it becomes more difficult for majority carriers to diffuse across the junction

• In reverse biasing, the resistance of the junction increases due to the applied voltage, hence no diffusion current.
• Thus pn junction diode act as an insulator in reverse bias mode.
• But the minority carriers from both the regions drift towards the junction and reach the majority zone giving rise to drift current.
• This drift current is of the order of a few microamperes.

Reverse Bias

Reverse characteristics of pn junction diode.

The breakdown voltage is the minimum reverse voltage that makes the diode conduct appreciably in reverse bias mode.

JUNCTION DIODE AS A RECTIFIER

• Rectifier is a device which is used for converting alternating current or voltage into direct current or voltage.
• The resistance of a p-n junction diode becomes low when forward biased and becomes high when reverse biased. This is the principle of the working of rectifier.

HALF WAVE RECTIFIER

• In a half-wave rectifier, one half of each a.c input cycle is rectified. When the p-n junction diode is forward biased, it gives little resistance and when it is reversing biased it provides high resistance.

INPUT OUTPUT WAVEFORM FOR A HALF WAVE RECTIFIER

• The rectified output of the circuit is only for half of the input ac wave, hence it is called half-wave rectifier.
• Here the frequency of input and output waveforms are the same.

FULL WAVE RECIFIER

• Here the circuit uses two diodes giving output rectified voltage corresponding to both the positive and negative half of ac cycle. Hence it is known as full wave rectifier.

INPUT OUTPUT WAVEFORMS FOR FULL WAVE RECTIFIER

• Here the frequency of output wave form is twice that of input waveform.

ROLE OF CAPACITOR FILTER

• The main function of this filter is to allow the ac components and blocks the dc components of the load. The filter circuit output will be a stable dc voltage.

ROLE OF CAPACITOR FILTER

•  By controlling the charging and discharging rate of the capacitor the pure DC can be obtained from the pulsating DC. In simple the capacitor allows AC and blocks DC, so the capacitor can connect parallel to the power supply so that the AC is filtered out and DC will reach the load.

ZENER DIODE

• A heavily doped semiconductor diode which is designed to operate in reverse direction is known as the Zener diode.
• The symbolic representation of Zener diode is shown in the figure below.

ZENER DIODE-Characteristics

• Heavily doped
• Depletion Region is < 10^-6 m
• Electric Field is very high (5x10⁶ V/m)
• Reverse biased
• Internal Field emission or field ionisation

ZENER DIODE-WORKING PRINCIPLE

• Due to the thinner depletion layer, electric field strength across the depletion layer is quite high. If the reverse voltage is continued to increase, after a certain applied voltage, the electrons from the covalent bonds within the depletion region come out and make the depletion region conductive. This breakdown is called Zener breakdown. The voltage at which this breakdown occurs is called Zener voltage.

ZENER DIODE AS VOLTAGE REGULATOR

• The voltage across the zener diode in the breakdown region is almost constant.
• Resistor, R_S is connected in series with the zener diode to limit the current flow through the diode with the voltage source, V_S being connected across the combination. The stabilised output voltage V_out is taken from across the zener diode.

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Optoelectronic Junction Devices

• Optoelectronic junction devices are p-n junction devices in which, charge carriers are generated by photons.
• Photodiodes, light emitting diodes (LEDs) and solar cells are examples of optoelectronic devices

PHOTODIODE

• It is also called as Photodetector, photo sensor or light detector.
• The photo diode accepts light energy as input to generate electric current.
• Photo diode operates in reverse bias condition

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WORKING OF PHOTODIODE

• The junction of the device is illuminated with light. This causes the electron and hole to get separated from each other.
• With the rise in the light intensity, more charge carriers are generated and flow through the device. Thereby, producing a large electric current through the device.

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CHARACTERISTICS OF PHOTODIODE�

• The figure below shows the VI characteristic curve of a photodiode:

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LIGHT EMITTING DIODE

• It is a heavily doped p-n junction which under forward bias emits spontaneous radiation.
• When a suitable voltage is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor.

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COLOUR OF LEDs:

• The colour of the light depends upon the types of material used in making the semiconductor diode.
• The semiconductor used for design of visible LEDs must have a bandgap from about 3eV to 1.8eV .
• Eg: Gallium – Arsenide (Ga-As) – Infrared radiation
• Eg: Gallium – Arsenide – Phosphide (GaAsP) – LEDs of different colours.

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V-I Characteristics of LED

• Here the threshold voltages are much higher than that of Si junction diode and slightly different for each colour.

ADVANTAGES OF LEDs

• LEDs have the following advantages over the conventional lamps:
• Low operational voltage and less power
• Fast action and no warm-up time required
• Long life
• Fast on-off switching capacity

��Solar cell� �

• Principle–These photo voltaic devices convert the optical radiation into electricity.
• When solar light falls on a p-n junction, it generates emf
• As the solar radiation is incident at the junction, the junction area is kept much larger for more power generation

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WORKING OF SOLAR CELL

• The generation of emf by the solar cell, when light falls on, is due to the following three basic processes:
• Generation
• Separation
• Collection

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SOLAR CELL-VI CURVE

• The graph showing the VI characteristics, with V along the X-axis and I along the Y-axis is as given above
• The graph is indicated in the fourth quadrant as solar cell does not draw current but supplies the same to the load

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��Solar cell� �

• Application
• Solar cells are used in power electronic devices in satellites and space vehicles
• They are also used as power supply in calculators
• Criteria for material selection of material for solar cell
• Band gap between 1.0 and 1.8 eV
• High optical absorption
• Electrical conductivity
• Availability of raw material
• Cost effective

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THANK YOU