Energy Bands & Semiconductors — Practice Questions

Free NEET Physics multiple-choice questions on Energy Bands & Semiconductors. Attempt each question and reveal the answer with a full explanation.

In an n-type semiconductor, which of the following statements is true? Electrons are majority carriers and trivalent atoms are the dopants. Electrons are minority carriers and pentavalent atoms are the dopants. Holes are minority carriers and pentavalent atoms are the dopants. Holes are majority carriers and trivalent atoms are the dopants. The energy band gap is maximum in: Insulators Superconductors Metals Semiconductors For a p-type semiconductor, which of the following statements is true? Holes are majority carriers and trivalent atoms are the dopants. Electrons are majority carriers and trivalent atoms are the dopants. Holes are majority carriers and pentavalent atoms are the dopants. Electrons are majority carriers and pentavalent atoms are the dopants. If the current gain β of a transistor is 50 , then the value of α is: 0.98 0.50 0.96 1.02 Increase in the width of the depletion layer in a p-n junction diode is due to: Reverse bias only Forward bias only Both forward and reverse bias Increase in forward current Which of the following materials has a negative temperature coefficient of resistance? Germanium Copper Aluminum Silver In a semiconductor, the separation between the conduction band and valence band is of the order of: 1 eV 10 eV 50 eV 0 eV If n e and n h are the number of electrons and holes in a semiconductor, for an n-type semiconductor: n e >> n h n h >> n e n e = n h n e = 0 The conductivity of an intrinsic semiconductor at 0 K is: Zero Infinite Equal to a conductor Equal to a p-type semiconductor Which of the following doped semiconductors is a p-type? Silicon doped with Indium Silicon doped with Phosphorus Germanium doped with Antimony Germanium doped with Arsenic The energy band gap of a material is 0 eV . The material is a/an: Conductor Insulator Semiconductor Superconductor A semiconductor has an electron concentration of 0.45 10 12 m -3 and a hole concentration of 5 10 20 m -3 . This semiconductor is: p-type n-type Intrinsic Insulator The majority charge carriers in a p-type semiconductor are: Holes Electrons Protons Neutrons An n-type semiconductor is formed when pure Silicon is doped with: Phosphorus Boron Aluminum Indium The energy level of the donor atoms in an n-type semiconductor lies: Just below the conduction band Just above the valence band At the center of the energy gap Inside the conduction band If the energy gap of a semiconductor is 1.1 eV , what is the maximum wavelength of a photon that can excite an electron from the valence band to the conduction band? 1127 nm 950 nm 1240 nm 800 nm In a semiconductor, the law of mass action states that: n e n h = n i 2 n e + n h = n i n e / n h = n i n e n h = 2n i When a small amount of Phosphorus is added to a Silicon crystal, the number of: Free electrons increases Holes increases Free electrons decreases Protons increases In the energy band diagram of a material, if the conduction band is empty and the valence band is completely filled, with an energy gap of 6 eV , the material is a/an: Insulator Semiconductor Conductor Superconductor The forbidden energy gap of Carbon (in diamond form) is approximately: 6.0 eV 1.1 eV 0.7 eV 0.1 eV Choose the only false statement from the following- The resistivity of a semiconductor increases with increase in temperature Substances with energy gap of the order of 10 eV are insulators. In conductors the valence and conduction bands may overlap The conductivity of a semiconductor increases with increases in temperature. Carbon, Silicon and Germanium atoms have four valence electrons each. Their valence and conduction bands are separated by energy band gaps represented by (E g) C , (E g) Si and (E g) Ge respectively. Which one of the following relationships is true in their case (E g) C<(E g) Ge (E g) C>(E g) Si (E g) C=(E g) Si (E g) C<(E g) Si Mass action law for semiconductors is given by: n e n h = n i 2 n e + n h = n i n e / n h = n i n e n h = n i Application of a forward bias to a p-n junction - Widens the depletion zone Increases the number of donors on the n side Increases the potential difference across the depletion zone Decreases the electric field in the depletion zone In a n -type semiconductor, which of the following statement is true? Electrons are majority carriers and trivalent atoms are dopants Electron are minority carriers and pentavalent atoms are dopants Holes are minority carriers and pentavalent atoms are dopants Holes are majority carriers and trivalent atoms are dopants The given graph represents V - I characteristic for a semiconductor device. Which of the following statement is correct ? It is V - I characteristic for solar cell where, point A represents open circuit voltage and point B short circuit current. It is a for a solar cell and point A and B represent open circuit voltage and current, respectively. It is for a photodiode and points A and B represent open circuit voltage and current, respectively. It is for a LED and points A and B represent open circuit voltage and short circuit current, respectively. The barrier potential of a p-n junction depends on: (a) type of semi conductor material (g) amount of doping (c) temperature Which one of the following is correct ? (a) and (b) only (b) only (b) and (c) only (a), (b) and (c) Consider the junction diode as ideal. The value of current flowing through AB is : 0 A 10 -2 A 10 -1 A 10 -3 A In a p-n junction diode, change in temperature due to heating Does not affect resistance of p-n junction Affects only forward resistance Affects only reverse resistance Affects the overall V - I characteristics of p-n junction For a p-type semiconductor, which of the following statements is true ? Electrons are the majority carriers and pentavalent atoms are the dopants. Electrons are the majority carriers and trivalent atoms are the dopants. Holes are the majority carriers and trivalent atoms are the dopants. Holes are the majority carriers and pentavalent atoms are the dopants. The increase in the width of the depletion region in a p-n junction diode is due to : reverse bias only both forward bias and reverse bias increase in forward current forward bias only The solids which have the negative temperature coefficient of resistance are: insulators only semiconductors only insulators and semiconductors metals As the temperature increases, the electrical resistance Decreases for both conductors and semiconductors Increases for conductors but decreases for semiconductors Decreases for conductors but increases for semiconductors Increases for both conductors and semiconductors In the energy band diagram of a material, the 'Fermi Level' represents the energy level with a probability of occupancy by an electron equal to: 0.5 0 1 0.25 Two statements are given below: A. When the forward bias voltage across a p-n junction diode increases above a certain threshold voltage, the diode current increases significantly. B. This current is called reverse saturation current. Choose the correct answer from the options given below: Both Statements A and B are true Both Statements A and B are false Statement A is true, but Statement B is false Statement A is false, but Statement B is true The energy band gap of Si is 1.14 eV . The maximum wavelength at which silicon will begin absorbing energy is: 10888 00c5 1088.8 00c5 10.888 00c5 108.88 00c5 The barrier potential of a p-n junction depends on: (i) type of semiconductor material (ii) amount of doping (iii) temperature. Which of the following is correct? (i), (ii) and (iii) (i) and (ii) only (ii) only (ii) and (iii) only Carbon, Silicon and Germanium have four valence electrons each. At room temperature, which of the following is true? The number of free electrons for conduction is very significant in C but small in Si and Ge. The number of free electrons for conduction is significant only in Si and Ge but small in C. The number of free electrons for conduction is small in all three. The number of free electrons for conduction is significant in all three. Pure Silicon at 500 K has equal number of electron and hole concentrations of 1.5 10 16 m -3 . Doping by Indium increases hole concentration n h to 4.5 10 22 m -3 . The doped semiconductor is: p-type with n e = 5 10 9 m -3 n-type with n h = 5 10 9 m -3 p-type with n e = 2.25 10 22 m -3 n-type with n e = 1.5 10 16 m -3 For an intrinsic semiconductor at T > 0 K , the probability of an electron occupying a state at the Fermi level E F is: 0.5 1.0 0 Dependent on the band gap An intrinsic semiconductor with energy gap E g has n i carrier concentration at temperature T . If the temperature is increased to T' , the carrier concentration n i' will: Increase exponentially Decrease exponentially Increase linearly Remain unchanged The intrinsic carrier concentration of silicon at 300 K is 1.5 10 16 m -3 . If it is doped with phosphorus such that the electron concentration becomes 4.5 10 22 m -3 , the hole concentration will be: 5 10 9 m -3 5 10 10 m -3 3 10 6 m -3 9 10 6 m -3 The forbidden energy gap in Ge is 0.72 eV . Given hc = 12400 eV , the maximum wavelength of radiation that will generate electron-hole pairs is approximately: 17222 15000 10000 5000 In an intrinsic semiconductor, the Fermi level lies: Exactly at the center of the band gap Near the conduction band Near the valence band Inside the valence band In a semiconductor, the displacement of electrons from the valence band to the conduction band results in the formation of: Electron-hole pairs Free electrons only Positive ions Negative ions The barrier potential of a p-n junction diode does NOT depend on which of the following? Forward bias voltage applied Temperature Doping density Nature of semiconductor In the case of a p-n junction, if the doping concentration is increased, the width of the depletion layer: Decreases Increases Remains same First increases then decreases Which of the following is true for a transistor working in the saturation region? Both junctions are forward biased Emitter junction is forward biased and collector junction is reverse biased Both junctions are reverse biased Emitter junction is reverse biased and collector junction is forward biased The ratio of the width of the depletion layer in a forward-biased junction to that in a reverse-biased junction is: Less than 1 Greater than 1 Equal to 1 Zero A transistor is operating in the 'saturation region' when: Both emitter-base and collector-base junctions are forward biased Both junctions are reverse biased Emitter-base is forward biased and collector-base is reverse biased The collector current is zero In a p-n junction diode, the current I in the forward bias increases exponentially with voltage V . If the temperature of the junction is increased, for a constant forward current, the required forward voltage will: Decrease Increase Remain unchanged First increase then decrease In an n-type semiconductor, the Fermi level E F lies: Just below the conduction band Just above the valence band Exactly in the middle of the forbidden gap Inside the conduction band In a semiconductor, the displacement of holes is due to: Movement of electrons in the valence band Movement of electrons in the conduction band Movement of ions in the lattice Movement of protons In the context of the V-I characteristics of a p-n junction, the 'dynamic resistance' r d is usually smaller than the 'static resistance' R dc during: Forward bias Reverse bias Breakdown region Zero bias The conductivity of a semiconductor increases with temperature because: The number density of carriers increases The relaxation time increases Both number density and relaxation time increase Number density increases more than relaxation time decreases If a small amount of Antimony (Sb) is added to a Germanium crystal: It becomes an n-type semiconductor The resistance of the crystal increases It becomes a p-type semiconductor The Germanium lattice is destroyed If the base-emitter junction of a transistor is reverse biased and the collector-base junction is also reverse biased, the transistor is in the: Cut-off region Active region Saturation region Inverted region For a p-n junction, the diffusion current is from: p-side to n-side n-side to p-side n-side to p-side for electrons and p-side to n-side for holes Does not flow unless biased If the ratio of the concentration of electrons to that of holes in a semiconductor is 7/5 and the ratio of currents is 7/4 , then what is the ratio of their drift velocities? 5/4 4/5 1/1 7/5 In a semiconductor, the mobility of electrons is higher than that of holes because electrons: have smaller effective mass are negatively charged experience fewer collisions are in the conduction band In a p-n junction, if the temperature is increased from 300 K to 310 K , the reverse saturation current approximately: Doubles Triples Remains same Halves The dynamic resistance of a p-n junction diode is 25 at a certain forward bias. If the thermal voltage is 25 mV , the current flowing through the diode is approximately: 1 mA 10 mA 25 mA 0.1 mA The transition capacitance C T of a p-n junction diode varies with the reverse bias voltage V R as: C T V R -1/2 C T V R 1/2 C T V R -1 C T V R In a p-n junction, the 'built-in potential' ( V bi ) increases if: The doping concentration in both regions is increased The temperature is increased The depletion width is decreased by external bias The intrinsic carrier concentration is increased In the context of BJT, the 'Early Effect' or base-width modulation is caused by: Variation of the depletion width of the collector-base junction with V CB High temperature in the emitter region Excessive doping in the collector region Recombination of carriers in the base In a p-type semiconductor, at room temperature, the concentration of holes is 10 17 cm -3 . If the intrinsic carrier concentration n i is 1.5 10 10 cm -3 , the electron concentration is: 2.25 10 3 cm -3 1.5 10 7 cm -3 2.25 10 10 cm -3 10 17 cm -3 In an intrinsic semiconductor, the number of electrons in the conduction band is n e and the number of holes in the valence band is n h . If n i is the intrinsic carrier concentration, then: n e = n h = n i n e > n h n e < n h n e n h = n i In a p-n junction, the 'Diffusion Capacitance' is primarily significant during which biasing condition? Forward bias Reverse bias Zero bias Breakdown region Which of the following physical quantities can be determined for a semiconductor using the Hall Effect? Sign and concentration of charge carriers Magnetic permeability of the material The exact energy band gap The lattice constant of the crystal In a p-n junction diode, the space charge density in the depletion region is: Positive on the n-side and negative on the p-side Negative on the n-side and positive on the p-side Zero throughout Uniformly positive throughout In the energy band diagram of a material shown below, the open circles and filled circles denote holes and electrons respectively. The material is an insulator a metal an n-type semiconductor a p-type semiconductor A p-n photodiode is fabricated from a semiconductor with a band gap of 2.5 eV . It can detect a signal of wavelength : 496 6000 4000 nm 6000 nm Two sides of a semiconductor germanium crystal A and B are doped with arsenic and indium, respectively. They are connected to a battery as shown in figure The correct graph between current and voltage for the arrangement is The electron concentration in an n -type semiconductor is the same as hole concentration in a p -type semiconductor. An external field (electric) is applied across each of them. Compare the currents in them. Current in p -type > current in n -type Current in n -type > current in p -type. No current will flow in p -type, current will only flow in n -type Current in n -type = current in p -type In the given circuits (a), (b) and (c), the potential drop across the two p - n junctions are equal in Circuit (b) only Circuit (c) only Both circuits (a) and (c) Circuit (a) only Consider the following statements A and B and identify the correct answer: A. For a solar-cell, the I-V characteristics lies in the IV quadrant of the given graph. B. In a reverse biased pn junction diode, the current measured in ( A ) , is due to majority charge carriers. A is correct but B is incorrect A is incorrect but B is correct Both A and B are correct Both A and B are incorrect In the circuit shown below, the voltage appearing across the diode D will be of the form: The unit of mobility of charge carriers in a semiconductor is: m 2 V -1 s -1 V m s -1 m V -1 s -1 m 2 V s -2 The current I in the circuit shown below is: (All diodes are ideal and identical) 1 3 A 15 2 A 5 3 A 5 9 A In a p-n junction diode, the current I can be expressed as I = I 0 [e eV/ k B T - 1] . If a diode has a reverse saturation current of 10 -12 A at 300 K , what is the forward current for a forward voltage of 0.6 V ? (Take e 23.2 10 -12 approximately 1.2 10 -2 ) 12 mA 1.2 mA 120 mA 0.12 mA In a semiconductor, the ratio of mobility of electrons to holes is found to be 3 . If the ratio of their concentrations is 2/3 , then the ratio of current carried by electrons to holes is: 2 4.5 0.5 1.5 Which of the following describes the 'Early Effect' (base-width modulation) in a transistor? Decrease in effective base width as collector-base reverse bias increases Increase in effective base width as emitter-base forward bias increases Decrease in collector current as collector voltage increases Increase in base current as temperature increases If the lattice constant of a semiconductor is decreased, the energy band gap usually: Increases Decreases Remains unchanged Becomes zero In an intrinsic semiconductor, the density of electrons ( n e ) and holes ( n h ) varies with temperature T as: n i ∝ e -E g / 2k B T n i ∝ e -E g / k B T n i ∝ T 2 n i ∝ 1/T In a p-type semiconductor, the Fermi level at T = 0 K lies: Between the acceptor level and the valence band Exactly in the middle of the energy gap Inside the conduction band At the donor level Which of the following transitions in a hydrogen-like atom would produce a photon capable of creating an electron-hole pair in a semiconductor with E g = 1.1 eV ? n=3 to n=2 n=4 to n=3 n=5 to n=4 n=2 to n=1 As the temperature of an intrinsic semiconductor increases, the position of the Fermi level E F : Remains essentially in the middle of the energy gap Shifts towards the conduction band Shifts towards the valence band Disappears as the material becomes a conductor A semiconductor material has a band gap of 0.05 eV . This material is likely to be a: Semimetal or narrow-gap semiconductor Insulator Wide-bandgap semiconductor Superconductor Which of the following statements about the mobility of electrons and holes in a semiconductor is true? Electron mobility is higher than hole mobility because electrons have smaller effective mass. Hole mobility is higher than electron mobility. Both mobilities are always equal. Mobility is independent of the type of charge carrier. For a given semiconductor, if the mobility of electrons is e and the mobility of holes is h , the Einstein relation relates these to their respective diffusion constants ( D e and D h ) as: D e e = D h h = k B T e D e e = h D h = k B T e D e e = D h h D e e = e k B T In a semiconductor, the intrinsic carrier concentration n i at temperature T is proportional to: T 3/2 e -E g / 2k B T T e -E g / k B T T 2 e -E g / 2k B T e -E g / k B T For an intrinsic semiconductor, the position of the Fermi level E F depends on the effective masses of electrons ( m e ) and holes ( m h ). If m h > m e , as temperature increases, the Fermi level: Moves slightly upward towards the conduction band Moves slightly downward towards the valence band Remains exactly at the center of the band gap Merges with the conduction band