Plus Two Physics Notes Chapter 14 Semiconductor Electronics: Materials, Devices and Simple Circuits is part of Plus Two Physics Notes. Here we have given Plus Two Physics Notes Chapter 14 Semiconductor Electronics: Materials, Devices and Simple Circuits.
|Text Book||NCERT Based|
|Chapter Name||Semiconductor Electronics: Materials, Devices and Simple Circuits|
|Category||Plus Two Kerala|
Kerala Plus Two Physics Notes Chapter 14 Semiconductor Electronics: Materials, Devices and Simple Circuits
Simplified Detailed Notes
Electronic device: A device whose action is based on controlled flow of electrons through it.
Classification of solids based on their resistivities:
- Metals have low resistivty (10-1 to 10-8 Ω m)
- Insulators have high resisitvity (>108 Ω m).
- Semiconductors have intermediate resisti¬vities (105 to 10° Ω m).
Due to interatomic interactions in a circuit, the electrons of the outer shells are forced to have energies different from those in isolated atoms. Each energy level splits into a number of energy levels forming a continuous band. The collection of these closely spaced energy levels are called an energy band.
- Valence band: It is the highest energy band occupied by the valence electrons.
- Conduction band: It is the energy band above the valence band.
On the basis of purity semiconductors are of two types:
Intrinsic Semiconductors: The pure semiconuctors in which ne = nh = ni
ne = free electron denisty in conduction band,
nh = nhole density in the valence band.
ni = intrinsic carrier concentration.
Extrinsic semiconductors: A semiconductor doped with the impurity so as to increase its conductivity is called an extrinsic semiconductor. They are two types, n-type semiconductor and p-type semiconductor.
Difference between conductors, insulators and semiconductors
|Conduction band is filled. The valence and conduction band partialy overlap. This makes available large number of free electrons for conduction. So metals have high con- ductivity.||Conduction band is empty. The valence band is filled. The forbidden energy gap is large (Eg > 3 eV). Electrons cannot be exited from the valence band to conduction band. Therefore no conduction is possible.||The empty conduction band is separated from the filled valence band by a small energygap (Eg < 3 eV). Some electrons of the valence band easily get exited from the conduction band and can conduct electricity. So semiconductors at-tain small conductivity even at room temperature.|
Note: Both the type of semicoductors are electrically neutral.
p-type semiconductor: In this, majority charge carriers are holes and minority charge carriers are electrons, i.e., nh > ne Doping is done with atoms which have three valence electrons in their valence shell (trivalent).
n-type semiconductor: In this, majority charge carriers are electron and minority charge carriers are holes, ie., ne > nh. Doping is done with atoms which have five valence electrons in their valence shell (pentavalent).
Note: If doping cone is small → depletion layer width will be large, barrier field will be weak.
Note: At equilibrium condition, nenh = ni2
p-n junction: A p-n junction is an arrangement made by a close contact of n-type semiconductor and p-type semiconductor.
Formation of Depletion Layer and Potential Barrier:
- During the formation of p-n junction, holes diffuse from p-side to n-side (p → n) and electrons diffuse from n-side to p-side (n → p).
- Thus near the junction there is an excess of positively charged ions in the n-region and negatively charged ions in p-region. This region which is depleted of free charge carriers is called depletion layer.
- This sets up a potential difference across the junction called potential barrier and hence an internal electric field from n-side to p-side. This field stops further diffusion of charge carriers.
- Also under this field minority charge carriers drift across the junction in a direction opposite to the diffusion current (i.e., electrons from p to n and holes from n to p) till drift current becomes equal to diffusion current (thus zero net current).
Forward biasing of p-n junction: A p-n junction is said to be forward biased when p side is maintained at a higher potenital with respect to the n side.
When forward biased, majority charge carriers in both the sides are pushed through the junction. The depletion region’s width decreases and the junction offers low resistance.
Reverse biasing of p-n Junction: A p-n junction is said to be reverse biased when its p side is maintained at a lower potenital with respect to its n side.
When reverse biased, majority charge carriers in both the regions are puhsed away from the junction. The depletion region’s width increases. The minority charge carriers are pushed through the junction causing a small current. The p-n junction offers a very high resisitance when reverse biased.
p-n junction Diode as Rectifier
1. Half wave rectifier
- it is based on the principle that p-n junction conducts only when it is forward biased.
- During the + ve half cycle of a.c the end A is positive and B is negative. The diode is forward biased and conducts. Therefore current flows through RL.
- During the -ve half cycle of a.c, the end A becomes negative and B positive, The diode is reverse biased and does not conduct. Hence no current flows through RL.
- The output current is unidirectional but pulsating.
2. Full wave rectifier
- During +ve half cycle of a.c, the e.ncl A is positive and end B is negative with respect to the centre tap. Diode D1, gets forward biased and conducts current while D2 is reverse biased and does not conduct.
- During -ve half cycle, the end A becomes negative and end B becomes positive with re spect to the centre tap. The diode D1 gets re verse biased and does not conduct. The diode D2 gets forward biased and conducts current.
- The output current is unidirectional and pul sating which may be filtered by connecting a capacitoc paralle’ to load RL.
Zener Diode: A device specially designed to operate only in the reverse breakdown region without getting damaged is called Zener diode.
Zener diode as a Voltage Regulator
Principle: When operated in the reverse break down region, the voltage across it remains practically constant (= VZ) for a large change in the reverse current.
- When input voltage increases, the current through RS and Zener diode increases. This increases voltage drop across RS without any change in voltage across the Zener diode.
- Similarily if input voltage decreases the voltage across RS decreases without any change in the voltage across the Zener diode.
- Thus any increase/decrease of input voltage does not increase/decrease voltage across Zener diode. Hence Zener diode acts as a voltage regulator.
It is operated in reverse bias. It is used to detect optical signals. When light photons of energy greater than energy gap of the semi conductor (hν > Eg ) are incident on it electron pairs are generated due to the absorption of photons. These charge carriers occurs in or near the depletion region. Due to the junction field (in reverse bias) electrons are collected on n-side and holes on p-side setting up an emf. This results in a current in the load.
Note: In reverse bias we can easily observe the change in photocurrent with change in radiation intensity.
Light EmItting DIode (LED): When p-n junction is forward biased, electrons are sent from n to p-side and holes aré sent from p to n-si-de. Near the junction, the concentration of minority carriers increases as compared to the equilibrium concentration. On either side near junction, the excess minority carriers combine with majority carriers. On recombination photons with energy equal to or slightly less than the band gap are emitted.
it is unbiased. It converts solar energy into electricity. When light photons with energy, hν > Eg reach the junction, electron hole pairs are generated close to the junction. Then the electrons move to n skie and’holes to p side due to electric field in the depletion region. Thus pside becomes positive and n side becomes negative giving rise to photovoltage hence a photo current in the external resistance.
Note: Semiconductors with band gap close to 1.5 e Vare ideal for this. eg., Si, GaAs, etc.
Junction transistor: A transistor is a three terminal semiconductor device consisting of two p-n junctions fromed by placing a thin layer of doped semiconductor (p or n-type) between two thick similar opposite type.
Two types of transistors are:
(i) n-p-n transistor: Two segments of n-type semiconductor (emitter and collector) are separated by a segment of p-type semiconductor (base).
(ii) p-n-p transIstor: Two segments of p-type (emitter and collector) are separated by a segment of n-type semiconductor (base).
Action of n-p-n transistor
The forward bias of the emitter-base circuit repels electrons of emitter towards base setting up emitter current IE , As the base is very thin and lightly doped, a very electrons from emitter combines with the holes of base, giving rise to base current IB and the remaining electrons are pulled by the collector which is at high +ve potential. The electrons get collected at the +ve terminai of battery VCB . giving rise to collector current IC .
IE = IB + IC
Action of p-n-p transistor: The emitter-base junction is given small forward bias, while base collector junction is given a large reverse bias.
Forward bias in emitter-base circuit repelsholes of emitter towards base and electrons to
wards emitter: As base is very thin and lightly doped, most of the holes entering it pass on to collector while a few of them recombine with electrons of the base region.
IE = IB + IC
Common Emitter Characteristics
Input characteristic: It is a graph showing the variation of base current IB with base-emitter voltage VBE at constant collector-emitter voltage VCE .
Output characteristic: It is a graph showing the variation of collector current IC with collector-emitter voltage VCE at constant base-current IB.
Note: As IB increases IC also increases for a given VCE.
Current amplification factor
Transistor as a switch
Cut-off region: When V is so small that it can not give forward bias, IB = O and IC = O. From equation (i), V0 = VCC. Transistor IS OFF.
Active region: When Vi increases above 0.6V for (for Si transistor), IC increases, V0 decreases.
IC ∝ B and transistor is used as amplifier in active region.
Saturation region: When Vi is high, IC is large and V0 approaches zero. IC cannot increase anymore(saturated). Transistor is ON.
Transisitor as an Amplifier (CE configuration)
v0 = VCE = VCC – ICRC
During +ve half cycle, forward bias of the circuit increases, IE and IC lincreases. This increases the potential drop across RC which makes output voltage less positive (more negative).
So as the input signal goes through its posi tive half cycle, the amplified output goes through a negative half cycle. Similarily as the input signal goes through ts negative half cycle, the amplified output signal goes through its positive half cycle. Therefore in a CE amplifier, the input and output voltages are 180° out of phase.
Current gain (a.c and d.c):
a.c voltage gain :
Transistor as an Oscillator
Principle: A portion of the output power is returned back (fedback) in to the Input in phase with the starting power (+ve feedback).
Working: When switch S1 is closed, collector current increases. This increases current through T2. The inductive coupling between T2 and T1 causes a current to flow the emitter circuit (+ve feedback) which increases. When current in T2 becomes saturated, collector current does not increase anymore and thus emitter current starts to decrease. This process repeats (oscillates).
1. NOT GATE (Y = Ā)
It can invert the functions.
2. OR GATE (Y = A+B)
3. AND GATE (Y=A.B)
4. NAND GATE (Y = )
which can perform OR, AND and inverted logical functions.
5. NOR GATE (Y = )
The NOR gate is second type of universal hardware building block, It can be used to perform OR function and AND function. The NOR gate is inverted OR gate.
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