Class 12 Physics CHAPTER 14 SEMICONDUCTOR ELECTRONICS
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NCERT Notes For Class 12 Physics CHAPTER 14 SEMICONDUCTOR ELECTRONICS
Class 12 Physics CHAPTER 14 SEMICONDUCTOR ELECTRONICS
CLASSIFICATION OF SOLIDS
On the basis of conductivity
- On the basis of the relative values of electrical conductivity (σ ) or resistivity (ρ = 1/σ ), the solids are broadly classified as:
- Metals: They possess very low resistivity (or high conductivity).
- Semiconductors: They have resistivity or conductivity intermediate to metals and insulators.
- Elemental semiconductors: Si and Ge
- Compound semiconductors:
Inorganic: CdS, GaAs, CdSe, Inp, etc. Organic: anthracene, doped pthalocyanines, etc. Organic polymers: polypyrrole, polyaniline, polythiophene, etc.
- Insulators: They have high resistivity (or low conductivity).
ENERGY BANDS IN SOLIDS
- In a solid each electron will have a different energy level.
- The range of energy possessed by the electrons in a solid is called the energy band.
- Valence band is the range of energy possessed by the valence electrons.
- It is the most occupied band.
- Conduction band is the range of energy possessed by the conduction electrons.
- It is the least occupied band.
Forbidden energy gap
• The energy difference between the bottom of the conduction band and the top of the valence band is called the forbidden energy gap.
Energy band in metals
- In certain metals the conduction band is partially filled and the valence band is partially empty.
- Then the electrons in the lower levels of valence band move to its higher levels making conduction possible.
- In some other metals the valence band and the conduction band overlap each other.
- Then the electrons in the valence band can easily move to the conduction band
- As the temperature increases in a metal the electrons in the valence band get enough energy to reach the conduction band.
- As the number of free electrons increases the number of collisions increases.
- So the resistance of a metal increases with temperature.
Energy band in Insulator
- The band gap energy is greater than 3 eV
- No free electrons are available in the conduction band.
Energy Band in Semi conductors
- The energy gap is small (<3 eV ) compared to insulators.
- At absolute zero of temperature there are no electrons in the conduction band of a semiconductor.
- As the temperature increases electrons in the valence band get enough energy to reach the conduction band.
- So the resistance of semiconductors decreases with temperature.
- At low temperatures, semiconductor behaves as an insulator.
- At room temperature some electrons leaves valence band and reaches conduction band and helps in conduction process.
- The vacancy of electrons (deficiency of electrons) in valence band is called hole.
- A hole is equivalent to a positive charge.
- In a semiconductor both electrons and holes are charge carriers.
Commonly used semiconductors
- Commonly used semiconductors are Si and Ge.
- Every Si or Ge atom tends to share one of its four valence electrons with each of its four nearest neighbor atoms, and also to take share of one electron from each such neighbor.
- As the temperature increases, more thermal energy becomes available to
these electrons and some of these electrons may break–away and holes are created.
Classification of semiconductors
• In general semiconductors can be classified as intrinsic semiconductors and extrinsic semi conductors.
- Semiconductor in its pure form are called intrinsic semiconductors.
- In intrinsic semiconductors, the number of free electrons, ne is equal to the number of holes, nh.
- Under the action of an electric field, these holes move towards negative potential giving the hole current, Ih.
- The total current, I is thus the sum of the electron current Ie and the hole current Ih:
Energy band in an intrinsic semiconductor at T = 0
Energy band in an intrinsic semiconductor at T > 0
• The methods to increase conductivity of an intrinsic semiconductor are heating and doping.
• The process of adding impurities to an intrinsic semiconductor so as to increase its conductivity is called doping.
- Doped semiconductors are called extrinsic semiconductors.
- The semiconductor crystal maintains an overall charge neutrality as the charge of additional charge carriers is just equal and opposite to that of the ionised cores in the lattice.
Energy band in extrinsic semiconductor Dopants
- The impurity atoms used for doping are called dopants.
- There are two types of dopants used in doping the tetravalent Si or Ge:
- Pentavalent (valency 5); like Arsenic (As), Antimony (Sb), Phosphorous (P), etc
- Trivalent (valency 3); like Indium (In), Boron (B), Aluminium (Al), etc.
Types of extrinsic semiconductors
- n- type semiconductors
- p-type semiconductors n-type semiconductor
- When a pentavalent impurity is added to an intrinsic semiconductor Ge or Si crystal, an n-type semiconductor is formed.
- Corresponding to each impurity atom added a free electron is created in the crystal.
- In an n-type semiconductor electrons are the majority charge carriers and holes are the minority charge carriers
- Thus, the pentavalent dopant is donating one extra electron for conduction and hence is known as donor impurity.
- As, Sb, P, Bi etc. are examples of pentavalent impurities.
- For n-type semiconductors,
Donor energy level in the energy band diagram
- When a trivalent impurity added to an intrinsic semiconductor Ge or Si crystal, an p-type semiconductor is formed.
- Corresponding to each impurity atom added a free hole is created in the crystal.
- In a p -type semiconductor holes are the majority charge carriers and electrons are the minority charge carriers.
- The trivalent impurity atom which accepts the electron is called an acceptor atom.
- The 13thgroup elements like In, B, Al, Ga etc. are examples of trivalent impurities.
- For a p-type semiconductor
Acceptor energy level in the energy band diagram
• A junction formed when a p-type semiconductor and n-type conductor are brought together is called a p-n junction.
Formation of a p-n junction
• Two important processes occur during the formation of a p-n junction: diffusion and drift
- The holes diffuse from p-side to n-side (p → n) and electrons diffuse from n-side to p-side (n → p).
- Due to diffusion, a layer of positive charge (or positive space-charge region) is developed on n-side of the junction and a layer of negative charge (or negative space-charge region) is developed on the p-side of the junction .
Depletion region (Depletion layer)
- The space-charge region on either side of the junction together is known as depletion region.
- In depletion region there are no free electrons and holes.
- The positive charge on n-side of the junction and negative charge on p-side of the junction develops an electric field.
- Due to this field, an electron on p-side of the junction moves to n-side and a hole on n- side of the junction moves to p-side.
- The motion of charge carriers due to the electric field is called drift.
- As the diffusion process continues, drift current increases.
- This process continues until the diffusion current equals the drift current.
- In a p-n junction under equilibrium there is no net current.
- The potential difference produced due to the diffusion of charge carriers across a pn junction is called barrier potential.
- The barrier potential limits further diffusion of holes and electrons.
SEMICONDUCTOR DIODE (p-n junction Diode)
- A semiconductor diode is a p-n junction with metallic contacts provided at the ends for the application of an external voltage.
- It is a two terminal device.
• The barrier voltage of a Ge diode is 0.2V and that of a Si diode is 0.7V.
p-n junction diode under forward bias
- In forward biasing the p-side is connected to the positive terminal of the battery and n-side to the negative terminal.
- Due to the applied voltage, electrons from n-side cross the depletion region and reach p-side and holes from p-side cross the junction and reach the n-side.
- This process under forward bias is known as minority carrier injection.
• In forward bias
- The height of the barrier potential reduces for majority carriers.
- The junction offers a very low resistance to the flow of current.
- The current increases sharply with forward voltage
- The width of depletion layer decreases.
p-n junction diode under reverse bias
- In reverse biasing n-side is connected to positive of the battery and p-side to negative of the battery.
- The barrier height increases and the depletion region widens due to the change in the electric field.
- This suppresses the flow of electrons from n → p and holes from p → n.
- The current under reverse bias is essentially voltage independent up to a critical reverse bias voltage, known as breakdown voltage (Vbr ).
In reverse bias
- Height of barrier potential increases
- Junction resistance is very high for current flow
- The reverse current is very low and is due to minority carriers.
- The width of depletion layer increases. Forward characteristics of a Diode
Circuit diagram for studying Forward characteristics
- Forward characteristics are the graph between voltage and current of a forward biased diode.
- After the characteristic voltage, the diode current increases significantly (exponentially), even for a very small increase in the diode bias voltage.
- This voltage is called the threshold voltage or cut-in voltage (~0.2V for germanium diode and ~0.7 V for silicon diode).
• The forward voltage after which the current through a diode increases linearly with voltage is called knee voltage.
Reverse characteristics of a diode Circuit diagram
Reverse saturation current( Leakage current)
• In the reverse bias of a diode if a voltage less than the breakdown voltage is applied very small constant current flows through the diode due to the minority charge carriers. This current is called reverse saturation current.
- The reverse voltage at which the current increases sharply is called reverse breakdown voltage.
- the phenomenon in which reverse current increases sharply at break down voltage is called Zener effect.
- The breakdown of the diode at the critical reverse voltage due to the increased production of electron-hole pair is called avalanche breakdown .
• It is the ratio of small change in voltage ΔV to a small change in current ΔI:
APPLICATION OF JUNCTION DIODE – RECTIFIER
- The process of conversion of ac current to dc current is called rectification.
- Device used for rectification is called rectifier.
Half wave Rectifier:
- It uses only one diode.
- The diode becomes forward biased only in the positive half cycle of ac.
- Efficiency is only 40.6%.
Full wave rectifier
- A simple full wave rectifier consists of two diodes.
- A centre tapped transformer is used in the circuit.
- During the positive half cycle first diode conducts current and second diode during negative half cycle.
- The circuits used to filter out the ac ripples from the rectifier output are called filters.
- The capacitor input filters use large capacitors.
SPECIAL PURPOSE p-n JUNCTION DIODES Zener diode
- It is developed by C. Zener
- Zener diode is used in the reverse bias, in the breakdown region.
- Zener diode has a sharp break down voltage called Zener voltage.
- A zener diode is used as a voltage regulator.
- Zener diode is fabricated by heavily doping both p, and n- sides of the junction.
- When the applied reverse bias voltage (V) reaches the breakdown voltage (Vz) of the Zener diode, there is a large change in the current.
- Zener voltage remains constant, even though current through the Zener diode varies over a wide range. This property of the Zener diode is used for regulating supply voltages so that they are constant.
Zener diode as a voltage regulator(Stabiliser):
- The unregulated dc voltage (filtered output of a rectifier) is connected to the Zener diode through a series resistance Rs such that the Zener diode is reverse biased.
- Any increase/ decrease in the input voltage results in, increase/ decrease of the voltage drop across Rs without any change in voltage across the Zener diode. regulator.
- Thus the Zener diode acts as a voltage
Optoelectronic junction devices:
- Devices in which conductivity changes due to photo-excitation
- A heavily doped p-n junction diode with a transparent window.
- Used as a photo detector.
- Operated under reverse bias
- When light (photons) with energy (hν) greater than the energy gap (Eg) of the semiconductor falls at the junction electron-hole pairs are generated.
- Due to the reverse bias the electrons move to n- region and holes to p-region giving rise a current in the external circuit.
- The current increases with intensity of light.
- A photodiode can be used as a photo detector to detect optical signals.
Light emitting diode (LED)
• It is a heavily doped p-n junction in forward bias.
• The diode is encapsulated with a transparent cover
- An LED converts electrical energy to light energy.
- When an electron makes a transition from conduction band to valance band photons with energy equal to or slightly less than the band gap are emitted.
- As the forward current increases, intensity of light increases and reaches a maximum.
- The V-I characteristics of a LED is similar to that of a Si junction diode.
- LEDs that can emit red, yellow, green and blue light are commercially orange, available.
- The compound semiconductor Gallium Arsenide – Phosphide is used for making LEDs of different colors.
- The LEDs are used in remote controls, burglar alarm systems, optical communication, etc.
LEDs have the following advantages over conventional incandescent low power lamps:
- Low operational voltage and less power.
- Fast action and no warm-up time required.
- The light emitted is nearly monochromatic.
- Long life.
- Fast on-off switching capability.
- Solar cell is a junction diode used to convert solar energy into electrical energy.
- The n- region is thin and transparent and the p – region is thick.
- The p-end is coated with a metal (back contact).
- A metal finger electrode (metallic grid) is deposited at the n-end. This acts as a front contact.
- The generation of emf by a solar cell, has three basic processes: generation, separation and collection.
- Generation of e-h pairs due to light (with hν > Eg) close to the junction.
- Separation of electrons and holes due to electric field of the depletion region.
- Electrons are swept to n-side and holes to p-side;
- The electrons reaching the n-side are collected by the front contact and holes reaching p-side are collected by the back contact.
- Thus p-side becomes positive and n-side becomes negative giving rise to photo voltage.
- Solar cells are made with semiconductors like Si , Ga As ,CdTe Cu In Se2 ,etc.
- The important criteria for the selection of a material for solar cell fabrication are electrical conductivity, availability of the band gap, high optical absorption, raw material, and cost.
- Invented by J. Bardeen and W.H. Brattain
- A transistor has three doped regions hence called bipolar junction transistor. forming two p-n junctions between them- transistor.
- Two types are- n p n transistor and p n p transistor
- Moderate size and heavily doped.
- It supplies a large number of majority carriers for the current flow through the transistor.
- This is the central segment.
- It is very thin and lightly doped.
- This segment collects a major portion of the majority carriers supplied by the emitter.
- It is moderately doped and larger in size as compared to the emitter
Transistor action n-p-n Transistor
- The electrons from the emitter diffuse into the base (emitter current) and holes from the base diffuse into emitter.
- Since base is thin most of the electrons reach the collector and produces collector current.
- The recombination of electrons with holes at base constitutes base current which is very small.
- The emitter current is the sum of collector current and base current:
- This is the transistor equation.
Basic transistor circuit configurations
• The transistor can be connected in three configurations:
- Common Emitter (CE),
- Common Base (CB),
- Common Collector (CC)
Transistor characteristics Common emitter transistor characteristics
- The emitter is common to both input and output.
- The input is between the base and the emitter and the output is between the collector and the emitter.
• The variation of the base current IB with the base-emitter voltage VBE at constant collector emitter voltage is called the input characteristic.
ac Input resistance (ri)
- This is defined as the ratio of change in base emitter voltage (ΔVBE) to the resulting change in base current (ΔIB) at constant collector-emitter voltage (VCE).
- The reciprocal of the slope of the graph gives the input resistance.
- Its value changes with the operating current in the transistor.
- The value of input resistance can be a few hundreds to a few thousand ohms.
• The variation of the collector current IC with the collector-emitter voltage VCE at constant base current is called the output characteristic.
ac output resistance (ro)
- This is defined as the ratio of change in collector-emitter voltage (ΔVCE) to the change in collector current (ΔIC) at a constant base current IB .
- The reciprocal of the slope of the linear part of the output characteristics gives the output resistance.
- The output resistance is of the order of 100 kΩ.
Current amplification factor
• It is the ratio of output current to input current.
- The base is common to both input and output.
- Current amplification factor α = IC/IE
- value of α ranges from 0.9-0.99
- Collector is common to both input and output
- Current amplification factor γ = IE/IB
- Its value is greater than that of β
- In CE configuration it is the ratio of the change in collector current to the change in base current at a constant collectoremitter voltage (VCE).
- This is also known as small signal current gain and its value is very large.
- The dc β of the transistor is given by
- The value of β ranges from 20-500
Relation between α and β
Transistor as a device
• The transistor can be used as a device application depending on the configuration used , the biasing of the E-B and B-C junction and the operation region namely cutoff, active region and saturation.
- Applying Kirchhoff’s voltage rule to the input and output sides of the circuit, we get,
- VBB is the d.c input voltage Vi and VCE is the d.c output voltage V0. So, we have
Variation of output voltage with input – transfer characteristics of a transistor
- A graph connecting input voltage and output voltage of a transistor is called transfer characteristics.
- When a transistor is used in the cut off or saturation state, it acts as a switch.
- If it is operated in the active region, it acts as an amplifier.
Cut off region
- In the case of Si transistor, as long as input Vi is less than 0.6 V, the transistor will be in cut off state and current IC will be zero.
- Hence Vo = Vcc
- When Vi becomes greater than 0.6 V the transistor is in active state with some current IC in the output path and the output Vo decreases.
- With increase of Vi, IC increases almost linearly and so Vo decreases linearly till its value becomes less than about 1.0 V.
• When Vo becomes 1V the change becomes non linear and transistor goes into saturation state and further increase in Vi , Vo decreases towards zero.
Transistor as a switch
- When the transistor is used in the cutoff or saturation state it acts as a switch.
- At cut off state, transistor doesn’t conduct-switched off.
- At saturation state the transistor conducts – switched on.
- Thus a low input switches the transistor off and a high input switches it on.
Transistor as an amplifier
- Amplifier is a device used to increase the amplitude of electrical signals.
- A transistor in active region acts as an amplifier.
- The operating point of a transistor amplifier is in the middle of the active region.
- When the small ac signal is applied in series with VBB, the base current will have sinusoidal variations superimposed on the value of IB.
- We have ΔIC = β ΔIB, so the changes in the base current are amplified and we get
amplified sinusoidal variations superimposed on the value of IC.
- Large capacitor is used at the output to block dc voltages.
- The output is taken between the collector and the ground
- Applying Kirchhoff’s voltage rule, to the input loop in the absence of signal ,we get
- When a signal vi is applied, the changes in the voltages VBE and IBRB are respectively ΔIBri and ΔIBRB.
- Therefore, the ac input signal voltage can be written as
- Applying Kirchhoff’s voltage rule to the output loop, we get,
- The change in IC due to change in IB causes a change in the output voltage given by,
∆VCE =∆ −∆VCC I RC C
- The negative sign represents that output voltage is opposite with phase with the input voltage.
• The power gain Ap can be expressed as the product of the current gain and voltage gain.
- Oscillator is a device used to convert dc voltage into ac voltage.
- When a portion of the output power of an amplifier is returned back (feedback) to the input in phase with the starting power it acts as an oscillator.
- The feedback can be achieved by inductive coupling (through mutual inductance) or LC or RC networks.
- Some of the oscillators are , Colpitt’s oscillator, Hartley oscillator, RC-oscillator etc.
• An oscillator has three parts
- Oscillatory circuit or tank circuit or tuned circuit.
- Feedback circuit
- Transistor Amplifier
Oscillatory circuit or tank circuit or tuned circuit
- A parallel combination of an inductor L and a capacitor C acts as the tank circuit.
- This LC circuit produces electrical oscillations of frequency,
- The oscillations produced by an LC circuit are damped oscillations.
- The feedback circuit compensates for the damping.
- A portion of the output power is returned back (fed back) in to the input in phase with the input power. This is called positive feedback.
- The feedback can be achieved by inductive coupling.
- A transistor in CE configuration is used as amplifier.
- The transistor amplifies the fed back voltage.
- When VCC is switched on, the collector current starts increasing through the coil L.
- Due to mutual induction emf is induced in the coils T1 and T2′.
- The emf across T2′ is the output.
- The emf across T1 is fed back to the input.
- Due to continuous positive feedback the collector current goes on increasing and reaches the maximum value.
- Since the collector current has a constant value, no mutual induction occurs.
- Without continued feedback the collector current begins to fall.
- The decrease in collector current through the coil L induces emf in T1.
- But this emf is in the opposite direction and it decreases the emitter current. So the collector current again decreases.
- This causes a further decrease in the emitter current and transistor goes to the cut-off state. Then IE and IC both become zero.
- The transistor now reached back to the original state.
- The whole process now repeats itself.
- In digital circuits only two values output voltage are permissible. (represented by 0 or 1) of the input and
- The continuous, time-varying voltage or current signals are called continuous or analogue signals.
- A waveform in which only discrete values of voltages are possible is a digital signal,
- A logic gate is a digital circuit that follows curtain logical relationship between the input and output voltages.
- The five common logic gates used are NOT, AND, OR, NAND, NOR.
- NOT, OR, and AND gates are fundamental or basic gates.
- NAND and NOR gates are called universal gates.
- This is the most basic gate, with one input and one output.
- It produces an inverted version of the input at its output.
- It is also known as an inverter.
- The table which describes the input output relationship is known as truth table.
• It can have one output and any number of inputs.
• It can have one output and any number of inputs.
• It is a combination of AND and NOT Gate
• It is a combination of OR gate and NOT gate.
INTEGRATED CIRCUITS ( IC )
- An entire circuit fabricated (consisting of many passive components like R and C and active devices like diode and transistor) on a small single block (or chip) of a semiconductor is called integrated circuit.
- Depending on nature of input signals, IC’s can be grouped in two categories: linear or analogue IC’s and digital IC’s
- Depending upon the level of integration (i.e., the number of circuit components or logic gates), the IC’s are termed as
- Small Scale Integration, SSI (logic gates <
- Medium Scale Integration, MSI (logic gates < 100)
- Large Scale Integration, LSI (logic gates < 1000)
- Very Large Scale Integration, VLSI (logic gates > 1000).
- The most widely used IC technology is the Monolithic Integrated Circuit.