Solids and Semiconductor devices Revision Notes - IIT JEE/NEET Preparation | Nucleon

Solids and Semiconductor devices

  • SOLIDS AND SEMICONDUCTOR DEVICES
  • Electronic instruments are being utilized in various fields like telecommunication, entertainment,
    computers, nuclear physics and many more. Although the history started with the advent of vacuum tubes,
    however the rapid advancement in electronics which we see today is due to the valuable contributions of
    semiconductor devices

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  • ENERGY LEVELS AND ENERGY BANDS IN SOLIDS
  • The electrons of an isolated atom are restricted to well defined energy levels. The maximum number of electrons
    which can be accommodated in any level is determined by the Pauli exclusion principle. The electrons belonging
    to the outermost energy level are called valence electrons. For example, the electronic configuration of sodium

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  • Energy Bands
  • This theory is based on the Pauli exclusion principle.
    In isolated atom the valence electrons can exist only in one of the allowed orbitals each of a sharply defined
    energy called energy levels. But when two atoms are brought nearer to each other, there are alterations in energy
    levels and they spread in the form of bands

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  • ENERGY BAND DESCRIPTION OF CONDUCTOR, INSULATOR AND SEMICONDUCTOR
  • ENERGY BAND DESCRIPTION OF CONDUCTOR,
    INSULATOR AND SEMICONDUCTOR

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  • Conductors
  • These are solids in which either the energy band containing valence band is partially filled or the energy band
    containing valence electrons overlaps with next higher band to give a new band which is partially filled too. For
    both these situations there are enough free levels available for electrons to which they can be excited by receiving
    energy from an applied electric field.

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  • Insulator
  • It is a solid in which the energy band formation takes place in such a manner, that the valence band is completely
    filled while the conduction band is completely empty. In addition to this, these two bands are separated by a
    large energy gap called forbidden energy gap or band gap.

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  • Semiconductors
  • In case of semiconductors, the band structure is essentially of the same type as that for insulators with the only
    difference that of a relatively smaller forbidden gap. In case of a semiconductor this is typically of the order of
    1eV. At absolute zero temperature, the valence band is completely filled and the conduction band is completely
    empty and consequently no electrical conduction can result. This is the same behaviour as observed in insulators.
    i.e at absolute zero a semiconductor behaves like an insulator

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  • INTRINSIC SEMICONDUCTORS
  • A semiconductor free from impurities is called an intrinsic semiconductor. Ideally an intrinsic
    semiconductor crystal should contain atoms of this semiconductor only but it is not possible in practice to
    obtain crystals with such purities. However if the impurity is less than 1 in 108 part of semiconductor it can be
    treated as intrinsic. For describing the properties of intrinsic semiconductor we are taking
    examples of silicon and germanium. Both silicon and germanium are members of the group IV of periodic table
    of elements and are tetravalent. Their electronic configuration is as follows:

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  • Electrical conductivity of intrinsic semiconductor
  • A semiconductor, at room temperature, contains electrons in the conduction band and holes in the valence band.
    When an external electric field is applied, the electrons move opposite to the field and the holes move in the
    direction of the field, thus constituting current in the same direction. The total current is the sum of the electron
    and hole currents

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  • Electrical conductivity of intrinsic semiconductor
  • A semiconductor, at room temperature, contains electrons in the conduction band and holes in the valence band.
    When an external electric field is applied, the electrons move opposite to the field and the holes move in the
    direction of the field, thus constituting current in the same direction. The total current is the sum of the electron
    and hole currents.

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  • Extrinsic semiconductor are of two types
  • When a pentavalent impurity atom (antimony, phosphorus
    or arsenic) is added to a Ge(or Si) crystal, it replaces a
    Ge (or Si) atom in the crystal lattice. Four of the five valence

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  • p–type semiconductor
  • When a trivalent impurity atom (boron, aluminium,
    gallium or indium) is added to a Ge (or Si) crystal, it
    also replaces one of the Ge (or Si) atoms in the crystal
    lattice . Its three valence electrons form covalent bonds
    with one each valence electron of these Ge (or Si) atoms

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  • Electrical conductivity of extrinsic semiconductors
  • A semiconductor, at room temperature, contains electrons
    in the conduction band and holes in the valence band.
    When an external electric field is applied, the electrons
    move opposite to the field and the hole move in the direction
    of the field, thus constituting current in the same direction.
    The total current is the sum of the electron and hole
    currents.

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  • Junction Diode
  • A junction diode is a basic semiconductor device. It is a semiconductor crystal having acceptor impurities in one
    region (P – type crystal) and donor impurities in the other region (n–type crystal). The boundary between the two
    regions is called ‘p–n junction

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  • Diffusion & Drift Current
  • Due to concontration difference hole try to diffuse from p side to n side but due to depletion
    layer only those hole are able to diffuse from p to n side which have high kinetic energy.
    Similarly electron of high kinetic energy also diffuse from n to p so diffusion current flow from p
    to n side

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  • Forward and Reverse Biasing of Junction Diode
  • The junction diode can be connected to an external battery in two ways, called 'forward biasing’ and 'reverse
    biasing' of the diode.It means the way of connecting emf source to P-N junction diode. It is of following two types

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  • Reverse Biasing
  • A junction diode is said to be reverse-biased when the positive terminal of the external battery is connected to the
    n -region and the negative terminal to the p -region of the diode

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  • Avalanche Breakdown
  • The avalanche breakdown occurs in lightly doped junction. If the reverse bias is made very
    high, the minority-carriers acquire kinetic energy enough to break the covalent bonds near the
    junction, thus liberating electron-hole pairs. These charge-carriers are accelerated and produce,
    in the same way, other electron-hole pairs

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  • Avalanche Breakdown
  • The avalanche breakdown occurs in lightly doped junction. If the reverse bias is made very
    high, the minority-carriers acquire kinetic energy enough to break the covalent bonds near the
    junction, thus liberating electron-hole pairs. These charge-carriers are accelerated and produce,
    in the same way, other electron-hole pairs. The process is cumulative and an avalanche of
    electron-hole pairs is produced. The reverse current then increases abruptly to a relatively
    large value (part CD of the characteristic).

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  • Dynamic Resistance of a Junction Diode
  • The current-voltage curve of junction diode shows that the current does not vary linearly with the
    voltage, that is, Ohm's law is not obeyed. In such situation, a quantity known as 'dynamic resistance'
    (or a.c. resistance) is defined

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  • p-n Junction Diode as a Rectifier
  • An electronic device which converts alternating current / voltage into direct current

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  • p-n Junction Diode as Half·wave Rectifier
  • The half-wave rectifier circuit is shown in Fig. (a) and the input and output wave forms in Fig. (b). The alternating
    input voltage is applied across the primary P1P2 of a transformer. S1S2 is the secondary coil of the same transformer.
    S1 is connected to the p -type crystal of the junction diode and S2 is connected to the n -type crystal through a
    load resistance RL.

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  • p-n Junction Diode as Full-wave Rectifier
  • In a full-wave rectifier, a unidirectional, pulsating output current is obtained for both halves of the a.c. input voltage.
    Essentially, it requires two junction diodes so connected that one diode rectifies one half and the second diode
    rectifies the second half of the input

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  • p-n Junction Diode as Full-wave Rectifier
  • In a full-wave rectifier, a unidirectional, pulsating output current is obtained for both halves of the alternating input
    voltage. Essentially, it requires two junction diodes so connected that one diode rectifies one half and the second
    diode rectifies the second half of the input

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  • Filter
  • The rectified voltage is in the form of pulses of the shape of half sinusoids. Though it is unidirectional it does not
    have a steady value. To get steady dc output from the pulsating voltage normally a capacitor is connected across
    the output terminals (parallel to the load RL). One can also use an inductor in series with RL for the same
    purpose. Since these additional circuits appear to filter out the ac ripple and give a pure dc voltage, so they are
    called filters

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  • Full wave bridge rectifier
  • Full wave bridge rectifier

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  • Different Types of Junction Diode
  • The junction diodes are of many types. The important types are Zener diode, photodiode, light-emitting diode
    (LED) and solar cell

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  • JUNCTION TRANSISTOR
  • Transistor structure and action :
    A transistor has three doped regions forming two p–n junctions between them. There are two types of transistors,
    as shown in figure

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  • JUNCTION TRANSISTOR
  • JUNCTION TRANSISTOR :
    Transistor structure and action :
    A transistor has three doped regions forming two p–n junctions between them. There are two types of transistors,
    as shown in figure.

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  • Working of Transistor
  • There are four possible ways of biasing the two P-N junctions (emitter junction and collector junction) of
    transistor.

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  • Transistor Configurations
  • A transistor can be connected in a circuit in the following three different configurations.
    Common base (CB), Common emitter (CE) and Common collector (CC) configuration.
    (1) CB configurations : Base is common to both emitter and collector

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  • COMMON EMITTER
  • The transistor is most widely used in the CE configuration.
    When a transistor is used in CE configuration, 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 ?B with the base–emitter voltage VBE is
    called the input characteristic. The output characteristics are controlled by the input characteristics. This implies
    that the collector current changes with the base current

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  • Transistor as a device
  • The transistor can be used as a device application depending on the configuration used (namely CB, CC and
    CE), the biasing of the E-B and B-C junction and the operation region namely cutoff, active region and saturation.
    When the transistor is used in the cutoff or saturation state it acts as a switch. On the other hand for using the
    transistor as an amplifier, it has to operate in the active region.

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  • Transistor as an Amplifier
  • To operate the transistor as an amplifier it is necessary to fix its operating point somewhere in the middle of its
    active region. If we fix the value of VBB corresponding to a point in the middle of the linear part of the transfer curve
    then the dc base current IB would be constant and corresponding collector current IC will be constant The dc

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  • FEEDBACK AMPLIFIER AND TRANSISTOR OSCILLATOR
  • In an oscillator, we get ac output without any external input singnal. A portion of the output power is returned back
    (feedback) to the input in phase with the starting power (this process is termed positive feedback) as shown in
    figure(a). The feedback can be achieved by inductive coupling (through mutual inductance) or LC or RC networks.

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  • ANALOGUE CIRCUITS AND DIGITAL CIRCUITS AND SIGNAL
  • There are two types of electronic circuits : analogue circuits and digital circuits :
    In analogue circuits, the voltage (or current) varies continuously with time (figure a). Such a voltage
    (or current) signal is called an ‘analogue signal’.Figure shows a typical voltage analogue signal varying
    sinusoidally between 0 and 5V.

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  • Logic Gates
  • A logic gate is a digital circuit which works according to some logical relationship between input and output
    voltages. It either allows a signal to pass through or stops it.

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  • Combinations of gates
  • Various combinations of the three basic gates, namely, OR, AND and NOT, produce complicated digital circuits,
    which are also called ‘gates’. The commonly used combinations of basic gates are NAND gate, NOR, gate.
    These are also called universal gates.

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