Semiconductor and electronics

• The substances having electrical conductivity intermediate between conductors and insulators are called semiconductors. Examples: silicon, germanium, etc.
• If any electron is released from the bond of an atom, the deficit of election appears at that position is known as a hole. It’s effective charge is +e, although it is not a real particle.
• Innumerable energy levels which remain very close to each other, form an energy band.
• The separation between two consecutive energy bands in band solid is called forbidden band or forbidden zone. No electrons can stay in the forbidden zone.

• Electrons residing at the highest energy band in an atom are called valence electrons. The energy band that is formed by the energy levels in which the valence electrons of a substance can reside, is called valence band.
• The energy levels possessed by the free electrons or conduction electrons of a substance constitute the band known as conduction band.
• The energy difference between conduction band and valence band is called energy gap or band gap. If energy of the conduction band be E_C, energy of the valence band be E_V and band gap be E_g, then, E_g = E_C – E_V.
• In case of insulators, the energy gap between valence band and conduction band is very large.
• In case of conductors, the upper portion of valence band overlaps with the lower portion of the conduction band.
• In case of semiconductors, the energy gap between valence band and conduction band is small.
• In case of intrinsic semiconductor, the number of electrons in the conduction band and the number of holes in the valence band are equal.
• If some special type of impurities be mixed with intrinsic semiconductor in a controlled manner, the conduction of the semiconductor increases many fold. This type of semiconductor is known as extrinsic semiconductor. The method of mixing impurities, is called doping. The impurities thus mixed are called dopants.
• If pentavalent (group V) elements (like arsenic or phosphorus) be doped as impurities in the crystal of an intrinsic semiconductor (like Si or Ge) in controlled manner, the crystal thus formed is called n-type semiconductor. Its majority carriers are electrons.
  • Phosphorus or arsenic supplies free electrons to the intrinsic semiconductor crystal and hence they are called donors.
• If trivalent (group III) elements (like boron or aluminium) be doped as impurities in the crystal of an intrinsic semiconductor in a proper well-controlled manner, the crystal thus formed is called p-type semiconductor. Its majority carriers are holes.
  • Boron or aluminum when mixed in the pure crystal, produces holes in their bonding and can accept electrons. Hence, they are called acceptors.
• By opposite kind of doping, if one part of a semiconductor crystal be made of p-type and the other part of n-type, then that crystal is called a p–n junction or semiconductor diode.
• The connection of the electrical components like diode, transistor, etc., with an external source of electricity (e.g., a battery), is called biasing. To apply forward bias to a p-n junction, its p-end and n-end is connected with the positive and negative terminal of the external electric source respectively. Reverse bias is applied to a p-n junction by connecting the n -end of the junction with the positive terminal of the external source and the p-end with the negative terminal.
• The variation of current with potential difference applied to a p-n junction diode in its forward or reverse biased condition is known as I-V characteristics or simply characteristic curve of a p–n junction.
• The arrangement which converts an alternating waveform into unidirectional waveform, e.g., an alternating current into unidirectional current is called a rectifier.
  • For the rectification of an alternating current, p-n junction diodes are widely used.
• The mostly used form of a transistor is a bipolar junction transistor (BJT). It is a semiconductor device containing three terminals or connecting points (base, emitter and collector).
• In a p-n-p and an n-p-n transistor, the majority charge carriers are holes and electrons respectively.
• Three kinds of circuits can be made by using transistors: (i) common-base (CB), (ii) common-emitter (CE) and (iii) common-collector (CC)
  • As an amplifier circuit, CE configuration is widely used.
• In using a transistor in the CE mode, (i) the circuit connecting the base and the emitter is the input circuit. (ii) the circuit connecting the collector and the emitter is the output circuit. (iii) emitter is common and in an ac circuit the emitter is grounded.
• In the CE mode, (i) by keeping the emitter grounded, the base is forward biased in the input circuit. (ii) by keeping the emitter grounded, the collector is reverse biased in the output circuit.
• The output characteristic curves of a transistor has three regions: (i) active region, (ii) cut-off region and (iii) saturation region.
• The system which can convert a de or unregulated ac signal to ac signal of a certain frequency is called an oscillator.

• The circuit of a feedback amplifier is designed in such a way so that the effective amplification of the amplifier reaches infinity.

Useful Relations for Solving Numerical Problems

• In a semiconductor, current density, J = e (nv_e + pv_h)
  and conductivity, \sigma= e(n\mu_e + p\mu_h) where, n = number density of electrons; p= number density of holes; v_e= drift velocity of electrons; v_h = drift velocity of holes; \mu_e = mobility of electrons; \mu_h = mobility of holes.
• For n -type semiconductor, n > p and for p -type, p > n.
• Energy of the forbidden gap, E_g=\frac{hc}{\lambda_{max}}
• If V (volt) – P (watt) is the rating of a Zener diode, then maximum safe current through the Zener diode as a voltage regulator is I_{max}=\frac{P}{V} ampere
• If the emitter current be I_E , base current be I_B, and collector be I_C, then in case of CE mode of a transistor, I_E=I_B + I_C .
  • I_C = f(I_B,V_{CE})
  • \alpha= \frac{\Delta I_C}{\Delta I_E} current amplification factor, \beta = \frac{\delta I_C}{\delta I_B}=\frac{\alpha}{1-\alpha}
• In a feedback oscillator, if an LC circuit is used as a feed- back circuit, then output frequency of the oscillator, f_0=\frac{1}{2\pi\sqrt{LC}}

MULTIPLE CHOICE QUESTIONS

Choose the correct answer

Semiconductor

• If potential difference is applied, the value of electric current
  1. becomes infinite for an insulator kept at 0 K
  2. becomes zero for a semiconductor kept at 0 K
  3. becomes finite for a metal kept at 0 K
  4. becomes infinite in a forward biased p- junction diode kept at 300 K
• Two pieces of copper and germanium are cooled from room temperature to 77 K. As a result,
  1. resistance of both the pieces will increase
  2. resistance of both the pieces will decrease
  3. resistance of copper will decrease but that of germanium will increase
  4. resistance of copper will increase but that of germanium will decrease
• For in intrinsic semiconductor, the energy gap of forbidden zone is approximately
  1. 0.01 eV
  2. 0.1 eV
  3. 1 eV
  4. 10 eV
• In a semiconducting material the mobilities of electrons and holes are \mu_e and \mu_h respectively. Which of the following is true?
  1. \mu_e>\mu_h
  2. \mu_e<\mu_h
  3. \mu_e = \mu_h
  4. \mu_e<0; \mu_h>0
• In some substances, charge can flow at ordinary temperature, but not at very low temperatures. These are called
  1. conductors
  2. insulators
  3. semiconductors
  4. dielectrics

Semiconductor diode

• A rectifier converts
  1. mechanical energy into electrical energy
  2. light energy into electrical energy
  3. ac to dc
  4. dc to ac
• If a p-n junction diode is not connected in a circuit, then
  1. potential is same everywhere
  2. potential of p -end is more than n-end
  3. an electric field acts from the n -end to p -end at the junction
  4. an electric field acts from the p -end to n -end at the junction
• In the circuit shown in Fig. , the forward bias resistances of both the diodes are 50 2. and reverse bias resistances are infinite. If the battery voltage be 6 V, the current through 100 12 resistance in the unit A [image]
  1. zero
  2. 0.02
  3. 0.03
  4. 0.036
• Two identical p-n junctions are connected in series with a battery in three different ways [image]. The potential drop across the two p-n junctions will be equal image
  1. in the circuits 1 and 2
  2. in the circuits 2 and 3
  3. in the circuits 3 and 1
  4. in the circuit 1 only
• When a p-n diode is reverse biased, then
  1. no current flows
  2. the depletion region is increased
  3. the depletion region is reduced
  4. the height of the potential barrier is reduced.
• The current through the given circuit of Fig. 1.49. image image
  1. \frac{3}{40} A
  2. \frac1{10} A
  3. \frac3{50} A
  4. \frac3{10} A
• The impurity atoms that should be added to germenium to make it n -type semiconductor is
  1. iodine
  2. arsenic
  3. indium
  4. aluminium
• The circuit has two oppositely connected ideal diodes in parallel. What is the current in the circuit ? image
  1. 1.33 A
  2. 1.71 A
  3. 2.00 A
  4. 2.31 A
• When a p-n junction is reverse biased, then
  1. no current flows
  2. the depletion layer is increased
  3. the depletion layer is decreased
  4. height of potential barriers is reduced

Transistor

• If the rate of doping in the emitter, base and collector of a transistor be D_e, D_y and D_c, respectively, then
  1. D_e= D_b = D_e
  2. D_e< D_b = D_e
  3. D_e> D_b > D_e
  4. D_e= D_c = D_b
• In the active region of a transistor, the biasing at the emitter-base junction and collector-base junction are respectively
  1. forward, forward
  2. forward, reverse
  3. reverse, forward
  4. reverse, reverse
• A transistor is used as
  1. a rectifier
  2. an amplifier
  3. an oscillator
  4. a source of electrons and holes
• In an n-p-n transistor circuit, collector current is 10 mA. If 90% of electrons from the emitter enter into the collector, then,
  1. emitter current = 11.11 mA
  2. emitter current = 9 mA
  3. base current = 2.1 mA
  4. base current = 0.9 mA
• For a transistor, if \frac{\delta I_C}{\delta I_E} = 0.96, then the current gain, \beta is
  1. 6
  2. 12
  3. 24
  4. 48

Oscillator

• If open loop gain and feedback ratio of a positive feedback oscillator are A are r respectively, then closed loop gain, A_f is
  1. 1
  2. <A
  3. <Ar
  4. infinity

VERY SHORT ANSWER TYPE QUESTIONS

Answer in one or two words

Semiconductor

• What type of impurity is required to prepare an n -type semiconductor? [ pentavalent element ]
• What type of impurity is required to prepare a p -type semiconductor? [ trivalent element ]
• What kind of semiconductor will be produced if it is doped with a donor element? [ n -type ]

• What is the effective electric charge of a hole? [ +e ]
• The total number of negative charge carriers in an intrinsic semiconductor is n. What is the total number of positive charge carriers in this semiconductor? [ n, because total number of positive and negative charge carriers are equal ]
• What change in the energy band gap of a pure semiconductor is occurs due to increase of temperature? (remains same)
• What change in the energy band gap of a semiconductor occurs due to increase of doping? [ decrease ]
• At which temperature is a semiconductor completely transformed into an insulator? [ 0 K ]
• What kind of semiconductor will be produced if a silicon crystal is doped with arsenic? [ n-type ]

Semiconductor diode

• If a full-wave rectifier draws an input from a 50 Hz mains, what will be the ripple frequency of the output? [ 100 Hz ]
• What will be the change in the thickness of depletion region, if a p-n junction is forward biased? [ thickness will decrease ]
• In which condition, does a semiconductor diode behave like an open switch? [ In reverse biasing ]
• What kind of biasing is required to use a Zener diode as a voltage regulator? [ reverse biasing ]
• What type of biasing gives a semiconductor diode very high resistance? [ reverse biasing ]
• Mention the practical importance of a Zener diode in the laboratory. [ as voltage regulator ]
• Under what condition does a p- junction diode work as an open switch? [ at reverse bias ]

Transistor

• How many effective regions are found in the CB mode of a transistor?
• In which region of the output characteristics is a transistor generally used? [ active region ]
• What is the majority charge carrier in a p-n-p transistor? [ hole ]
• What is the majority charge carrier in an n-p-n transistor? [ electron ]
• What is the relation among emitter current (I_E) , collector current (I_C) and base current (I_B) for a transistor? [ I_E = I_C+ I_B]
• How will you recognise from a transistor circuit whether the transistor is n-p-n or p-n-p? [ for n-p-n transistor, the arrow head is along the emitter from the base whereas for p-n-p, the arrow head is along the base from the emitter ]
• Calculate the change in collector current due to the change of 10 \muA in base current of a transistor having \beta= 150. [ 1.5 mA ]

Oscillator

• What is the product of open loop gain and feedback ratio of the amplifier in a feedback oscillator. [ 1 ]
• What is the natural frequency of an LC oscillator? [ \frac1{2\pi\sqrt{LC}} ]

SHORT ANSWER TYPE QUESTIONS-I

• How will you identify a conductor and a semiconductor from two given rods of same resistance?
• What change in energy band gap of a semiconductor due to doping of pentavalent element?
• What is the effect of doping on electric conductivity of a semiconductor?
• How is sample of an -type semiconductor electrically neutral though it has an excess of negative change carriers?
• Explain the law of mass action ( np = n_i² ) .
• Draw V-I characteristic curve of a p–n junction and compare it with the characteristic of a resistance.
• What type of biasing has been given to the diode of ? image
• In following diodes, which one of the two diodes is forward biased and which is reverse biased? image
• When breakdown voltage of a Zener diode is around 6 V, then the value of this voltage does not depend upon temperature-Explain.
• What is the relation between the energy band gap of an LED and maximum possible wavelength of emitted light from it?
• In case of solar diode, what is the significance of volt-ampere relation of characteristic curve in fourth quadrant?
• Why is the base of a transistor slightly doped?
• In which region will a transistor act if both emitter-base and collector-base junctions are reverse biased?
• In which region will a transistor act if both emitter-base and collector-base junctions are forward biased?
• Without any input signal, a feedback oscillator gives output signal. Explain how the conservation of energy is not violated.
• What is the significance of Barkhausen criterion?
• Open loop gain of an amplifier is A. If a feedback oscillator is made by this amplifier, then what feedback ratio is to be taken?

SHORT ANSWER TYPE QUESTIONS-II

• The current in the forward bias is known to be more (\simmA) than the current in the reverse bias (\sim\muA). What is the reason then to operate the photodiode in reverse bias?
• Explain why the p-n junction contact potential cannot be measured by a voltmeter across the diode terminals.
• Why two diodes connected back to back cannot behave as a transistor?
• Why is the base region of transistor usually made thin?
• Explain why Si and Ge are not used in LED.

PROBLEM SET-I

Semiconductor

• A semiconductor has the electron concentration of 8\times{10}^{13} cm^{-3} and hole concentration 4\times{10}{12} cm^{-3}. Is this semiconductor n -type or p -type? Also calculate resistivity of its material. Given : Electron mobility =24000 cm^2.V^{-1}.s^{-1} and hole mobility =200 cm^2.V^{-1}.s^{-1} [ n type, 3.25\Omega . cm ]
• Pure silicon at 300K has same electron and hole concentrations of 1.5\times{10}^{16} m^{-3}. Doping by indium, the concentration of holes Increases to 4.5\times{10}^{22} m^{-3}. Calculate electron concentration in doped silicon. [ 5\times{10}^{9} m^{-3} |
• A semiconductor has the electron concentration of 4\times{10}^{12}cm^{-3} and the hole concentration of 7\times{10}^{13}cm^{-3} . Is this semiconductor n -type or p -type? Also calculate conductivity of its material. Given : Electron mobility = 22000 cm^2.V^{-1}.s^{-1} and hole mobility = 150 cm^2.V^{-1}.s^{-1} [ p type, 1.576 mho.m^-1 ]
• Calculate the conductivity of a semiconductor having electron and hole concentration concentration . \mu_e =2.3V^{-1}.s^{-1}.m^2 and \mu_e =0.1V^{-1}.s^{-1}.m^2 ] [1.968 mho/m]
• The energy of a photon of light ( \lambda=590 nm ) equals the band gap of a semiconducting material. Find the minimum energy required to create a hole-electron pair. [ 2.09 eV ]

Semiconductor diode

• If an unregulated voltage of 10 V is regulated by a Zener diode of rating 4.7V-1 W, calculate minimum resistance to be connected to the circuit for safe current. [ 25 \Omega (approx.)]

Transistor

• A transistor has \beta = 100 and I_B = 20 \muA. Find the emitter current. [ 2.02 mA ]
• Current gain of a common-emitter transistor amplifier is 50. If the emitter current is 6.6 mA, calculate collector current and base current. [ 6.47 mA, 0.13 mA ]
• In CE circuit, if base current of the transistor is increased from 100 \muA to 150 \muA, then collector current increases to 10 mA from 5 mA. Calculate current gain B of the transistor. [ 100 ]

Oscillator

• In LC circuit of a feedback oscillator, L = 36 mH. If the oscillator is used as an alternating source of different frequencies in the range 100 Hz to 1000 Hz, then find the range in which the capacitance of C should be kept. [ 0.07 \mu F to 70\mu F ]

PROBLEM SET-II

Semiconductor

• The I- V characteristic of a silicon diode is shown in the Fig. 1.53. Calculate the resistance of the diode at (i) I_D = 15 A , (ii) V_D=-10 V [ 10\Omega and 1.0\times 10^7 ] image

• Assume that the silicon diode in the circuit requires a minimum current of 1 mA to be above the knee point (0.7 V) of its I-V characteristics. Also assume that the voltage across the diode is independent of current above the knee point. (i) If V_B = 5 V, what should be the maximum value of R so that the voltage is above the knee point? (ii) If V_B = 5 V, what should be the maximum value of R to establish the current of 5 mA in the circuit? (iii) What is the power dissipated in the resistance R and in the diode, when a current of 5 mA flows in the circuit at V_B=6V? (iv) If R = 1k\Omega, what is the minimum voltage V required to keep the diode above the knee point? [(i) 4.3 k\Omega ; (il) 860 k\Omega : (¡i) 26.5 mW in resistance, 3.5 mW in diode; (iv) 1.7 V ] image
• An LED is constructed from a p-n junction based on a certain Ga- As-P semiconducting material whose energy is 1.9 eV. What is the wavelength of the emitted light? [ 650 nm ]