Electronic Devices — NEET Physics MCQs with answers

Electronic Devices is a Class 12 Physics chapter in the NEET (UG) syllabus. NEET720 has 611 reviewed practice questions on it, each with a quick answer and a step-by-step explanation. The 8 questions below are free and fixed, so you can bookmark this page; the full chapter, plus mistake tracking and spaced revision, is in the app.

Topics covered

Energy bands and classification of solids · Intrinsic and extrinsic semiconductors · Photodiode and LED · p-n junction and diode characteristics · Rectifiers · Special-purpose diodes · Logic gates · Diode characteristics · Photodiode/LED · Optoelectronic Devices · Energy bands in solids · Intrinsic semiconductors · Extrinsic semiconductors · Diode I-V characteristics · Junction transistor · Transistor as amplifier · Transistor as a switch · Zener and avalanche breakdown · p-n junction · Zener diode · Transistor · Rectifier · p-n junction diode · Transistor amplifier · Rectifier and filter · Photodiode and solar cell · Doping and conductivity · Silicon vs germanium diode · Half-wave vs full-wave rectifier · LED vs incandescent vs laser context · Zener regulator · p-n junction and transistor · Photodiode and band gap · p-n junction and rectification · Zener diode and band structure · Transistor and logic gates · Doping and p-n junction · Solar cell · Diode circuit · Logic gate

8 free Electronic Devices practice questions with answers

Choose an answer in your head before opening it. Each explanation says why the correct option is right and, where relevant, why the tempting wrong option is wrong.

1. Question 1 · hard · Energy bands and classification of solids

   A student argues: "Since diamond (an insulator) and silicon (a semiconductor) are both made of the same group-IV type of covalent bonding, their band gaps should be nearly equal." Which statement correctly evaluates this claim?

   • A.The claim is false; diamond's band gap (~5.5 eV) is much larger than silicon's (~1.1 eV) because the gap depends on the specific lattice spacing and bond strength, not merely on the bonding type being covalent
   • B.The claim is true; all covalently bonded group-IV crystals have essentially the same band gap by definition
   • C.The claim is false, but only because diamond is not actually a group-IV element
   • D.The claim is true for silicon and germanium but false only for diamond, because diamond is not crystalline
   Show answer and explanation

   Answer: A. The claim is false; diamond's band gap (~5.5 eV) is much larger than silicon's (~1.1 eV) because the gap depends on the specific lattice spacing and bond strength, not merely on the bonding type being covalent

   Band gap size is set by the specific crystal's lattice constant and bond strength, not just by 'being covalent group-IV'; diamond's tightly packed, strong C–C bonds give a much larger gap than silicon's.

   Carbon, silicon and germanium are all group-IV elements with covalently bonded diamond-cubic lattices, yet their band gaps differ hugely: diamond ≈ 5.5 eV (insulator), silicon ≈ 1.1 eV (semiconductor), germanium ≈ 0.7 eV (semiconductor). This is because the band gap arises from how strongly the atomic orbitals overlap and split into bands, which depends sensitively on the interatomic spacing and bond strength — diamond's very short, strong C–C bonds push the bands far apart, while silicon's longer, weaker bonds give a much narrower gap. So 'same bonding category' does not imply 'same gap'; the specific element and lattice matter. Option A captures this correctly; B is the false claim being tested; C and D invent incorrect justifications even though C's conclusion direction is coincidentally 'false'.

   Common mistake: Assuming materials in the same bonding family must share similar band gaps

2. Question 2 · medium · Intrinsic and extrinsic semiconductors

   Pure silicon (a group-14 element) is doped with a small amount of phosphorus (a group-15 element) to form n-type silicon. What role does the phosphorus atom play in the lattice?

   • A.It removes one electron from a neighbouring silicon atom, creating a hole as the majority carrier
   • B.It replaces a silicon atom without forming any covalent bonds, floating freely in the lattice as an ion
   • C.It contributes all five of its valence electrons to the conduction band immediately at 0 K
   • D.It forms four covalent bonds with neighbouring silicon atoms using four of its five valence electrons, leaving the fifth electron loosely bound and easily freed as a donor electron
   Show answer and explanation

   Answer: D. It forms four covalent bonds with neighbouring silicon atoms using four of its five valence electrons, leaving the fifth electron loosely bound and easily freed as a donor electron

   A pentavalent phosphorus atom substitutes for a silicon atom, uses four of its five valence electrons for covalent bonding, and the fifth extra electron is loosely bound, easily ionised to become a free (donor) electron, making the material n-type.

   Silicon has 4 valence electrons and forms 4 covalent bonds with its neighbours in the crystal lattice. When a pentavalent (group-15) impurity such as phosphorus, arsenic, or antimony is substituted at a silicon lattice site, it also forms 4 covalent bonds using 4 of its 5 valence electrons — but this leaves a fifth valence electron with no bonding role. This extra electron is only weakly bound to its parent atom (via a small ionisation energy, much smaller than the intrinsic band gap) and, at ordinary temperatures, is easily freed into the conduction band. Because the phosphorus atom 'donates' this free electron without creating a compensating hole, it is called a donor impurity, and the resulting material is n-type with electrons as majority carriers.

   Common mistake: Confusing donor (pentavalent, n-type) doping with acceptor (trivalent, p-type) doping mechanisms

3. Question 3 · medium · Intrinsic and extrinsic semiconductors

   A student claims: "An n-type semiconductor carries a net negative electric charge because it has an excess of electrons as majority carriers." Is this correct?

   • A.Yes; n-type material is always negatively charged overall because electrons outnumber holes
   • B.No; the donor atoms that release free electrons become fixed positive ions, so the crystal as a whole remains electrically neutral even though electrons are the majority mobile carrier
   • C.No, because n-type semiconductors actually have more holes than electrons
   • D.Yes, but only until the doping atoms are fully ionised, after which it becomes neutral
   Show answer and explanation

   Answer: B. No; the donor atoms that release free electrons become fixed positive ions, so the crystal as a whole remains electrically neutral even though electrons are the majority mobile carrier

   Even though electrons are the majority mobile carrier in n-type material, each donor atom that releases an electron becomes a fixed positive ion, so the crystal overall stays electrically neutral.

   Doping does not add or remove net charge from the crystal — it only redistributes which carriers are mobile. When a pentavalent donor atom (e.g. phosphorus) contributes its extra electron to the conduction band, the donor atom itself, having lost an electron, becomes a fixed positively charged ion embedded in the lattice. This positive ion core exactly balances the negative charge of the free electron it released. So while n-type material has electrons as its majority mobile charge carriers, the material as a whole (mobile carriers plus fixed ion cores) is electrically neutral, just as p-type material (holes as majority carriers, fixed negative acceptor ions) is also neutral overall. The common error is treating 'majority carrier type' as equivalent to 'net charge of the material', which is incorrect.

   Common mistake: Believing n-type material is net negatively charged and p-type net positively charged

4. Question 4 · medium · Intrinsic and extrinsic semiconductors

   Pure germanium is doped with a small amount of indium (a group-13, trivalent element) to make p-type germanium. What happens at the site of each indium atom in the lattice?

   • A.The indium atom donates an extra electron to the conduction band, just as phosphorus does in n-type doping
   • B.The indium atom forms five covalent bonds by borrowing electrons from four germanium neighbours simultaneously
   • C.The indium atom forms only three complete covalent bonds with neighbouring germanium atoms (having only 3 valence electrons), leaving one bond incomplete; this vacancy readily accepts an electron from a nearby bond, effectively creating a mobile hole
   • D.The indium atom does not bond at all and instead occupies interstitial space between lattice sites
   Show answer and explanation

   Answer: C. The indium atom forms only three complete covalent bonds with neighbouring germanium atoms (having only 3 valence electrons), leaving one bond incomplete; this vacancy readily accepts an electron from a nearby bond, effectively creating a mobile hole

   Trivalent indium can complete only 3 of the 4 covalent bonds needed at a germanium lattice site; the resulting vacancy readily accepts an electron from a neighbouring bond, generating a mobile hole and making the material p-type.

   Germanium, like silicon, has 4 valence electrons and needs 4 covalent bonds per atom in the crystal lattice. A trivalent (group-13) impurity such as indium, boron, or gallium has only 3 valence electrons, so when substituted into the lattice it can form only 3 complete covalent bonds; the fourth bond is left with a vacancy (missing electron). This vacancy readily 'accepts' an electron from a neighbouring Ge–Ge bond, which fills the gap here but creates a new vacancy (hole) at the neighbouring site — in effect, the hole becomes mobile as it can migrate from bond to bond. Because the impurity accepts an electron, it is called an acceptor impurity, and the doped material is p-type with holes as the majority carriers.

   Common mistake: Mixing up trivalent acceptor doping with pentavalent donor doping

5. Question 5 · medium · Intrinsic and extrinsic semiconductors

   Which of the following best describes what a 'hole' represents in semiconductor physics?

   • A.A vacancy left in the valence band bonding structure by a departed electron, which behaves for conduction purposes like a mobile positive charge carrier
   • B.An actual physical antiparticle of the electron (a positron) generated in the crystal
   • C.An empty physical cavity or pore in the crystal lattice where an atom is missing
   • D.A free proton that has become detached from a silicon nucleus
   Show answer and explanation

   Answer: A. A vacancy left in the valence band bonding structure by a departed electron, which behaves for conduction purposes like a mobile positive charge carrier

   A hole is not a real particle but the absence of a valence electron in a covalent bond; as neighbouring electrons hop to fill it, the vacancy appears to move like a positive charge, hence it behaves as a mobile positive carrier.

   When a covalent bond electron gains enough energy to break free (becoming a conduction electron), it leaves behind an empty spot in the bonding structure called a hole. This is a bookkeeping concept, not a real particle: when an adjacent bond's electron hops over to fill this vacancy, the hole effectively shifts to the neighbouring site. Repeated over many such hops, the vacancy appears to migrate through the crystal, and because the net effect is motion of positive charge (electron deficiency) in the direction opposite to the actual electron hopping, holes act as mobile positive charge carriers with their own effective mass and mobility. They are not antimatter positrons, not structural lattice vacancies (missing atoms), and certainly not free protons.

   Common mistake: Confusing electronic holes with positrons or with physical lattice defects

6. Question 6 · medium · Intrinsic and extrinsic semiconductors

   Silicon (group 14, 4 valence electrons) is doped with impurity atoms to change its carrier type. Why must the dopant be chosen from group 13 or group 15 of the periodic table, rather than another group-14 element?

   • A.Group-14 dopants are physically too large to fit into the silicon lattice at all, regardless of electronic structure
   • B.Group 13 and 15 elements are simply more abundant in nature, making them more practical, not because of any electronic requirement
   • C.A group-14 dopant would form exactly 4 covalent bonds just like silicon itself, leaving no extra electron and no incomplete bond, so it would not create any new majority carrier; only group 13 (3 valence electrons, creates a hole) or group 15 (5 valence electrons, creates a free electron) can introduce the needed valence mismatch
   • D.Any element from any group could be used equally well; the group number has no bearing on whether doping succeeds
   Show answer and explanation

   Answer: C. A group-14 dopant would form exactly 4 covalent bonds just like silicon itself, leaving no extra electron and no incomplete bond, so it would not create any new majority carrier; only group 13 (3 valence electrons, creates a hole) or group 15 (5 valence electrons, creates a free electron) can introduce the needed valence mismatch

   Doping works by creating a valence-electron mismatch relative to silicon's four: group 15 (5 electrons) leaves one extra, loosely-bound donor electron; group 13 (3 electrons) leaves one incomplete bond (an acceptor site). A group-14 dopant, having exactly 4 valence electrons like silicon, creates neither.

   Each silicon atom forms four covalent bonds using its four valence electrons. For doping to introduce a new type of mobile carrier, the substituted impurity atom must have a different number of valence electrons than silicon. A pentavalent (group 15) atom has one electron too many for the four bonds, leaving a loosely bound extra electron that becomes a donor carrier (n-type). A trivalent (group 13) atom has one electron too few, leaving an incomplete bond that readily accepts an electron from a neighbour, creating a hole (p-type acceptor). A group-14 dopant, however, would have exactly four valence electrons — identical to silicon's own requirement — so it would form four complete bonds just like a native silicon atom, contributing no extra carrier of either kind and having essentially no doping effect (aside from minor crystal-lattice/isotope-type differences). This is why only groups adjacent to silicon's own group (13 or 15) are used as effective dopants.

   Common mistake: Attributing dopant choice to atomic size or elemental abundance rather than valence-electron count

7. Question 7 · medium · Intrinsic and extrinsic semiconductors

   Silicon is separately doped with each of these elements: phosphorus (P, group 15), aluminium (Al, group 13), arsenic (As, group 15), and gallium (Ga, group 13). Which pair correctly identifies which elements act as donors and which act as acceptors?

   • A.Donors: aluminium and gallium; acceptors: phosphorus and arsenic
   • B.All four elements act as donors, since any impurity added to silicon increases the number of free electrons
   • C.Phosphorus and aluminium are donors; arsenic and gallium are acceptors
   • D.Donors: phosphorus and arsenic; acceptors: aluminium and gallium
   Show answer and explanation

   Answer: D. Donors: phosphorus and arsenic; acceptors: aluminium and gallium

   Phosphorus and arsenic are both group 15 (pentavalent) and act as donors, contributing free electrons; aluminium and gallium are both group 13 (trivalent) and act as acceptors, creating holes.

   Silicon is a group-14 (tetravalent) element forming four covalent bonds. Any dopant with 5 valence electrons (group 15: phosphorus, arsenic, antimony) forms four bonds and leaves one loosely bound extra electron, acting as a donor and producing n-type material. Any dopant with 3 valence electrons (group 13: aluminium, gallium, boron, indium) forms only three complete bonds, leaving an incomplete bond that readily accepts an electron, acting as an acceptor and producing p-type material. Correctly sorting the four given elements by their periodic table group therefore gives phosphorus and arsenic as donors, and aluminium and gallium as acceptors — this is a direct test of recognising the group-13/group-15 pattern rather than memorising a single named example.

   Common mistake: Reversing which periodic group corresponds to donor versus acceptor behaviour

8. Question 8 · medium · Logic gates

   A two-input logic gate is tested with all four possible input combinations, and the measured output Y is recorded as follows: (A=0,B=0) → Y=0; (A=0,B=1) → Y=0; (A=1,B=0) → Y=0; (A=1,B=1) → Y=1. Which logic gate does this table represent?

   • A.AND gate
   • B.OR gate
   • C.NAND gate
   • D.NOR gate
   Show answer and explanation

   Answer: A. AND gate

   The output is HIGH (1) only in the single case where both inputs are HIGH (A=1, B=1), and LOW in all other cases — this is exactly the truth table of an AND gate.

   Comparing the given table to standard two-input gate truth tables: an AND gate outputs 1 only when every input is 1, and 0 for all other combinations. The given data shows Y = 0 for (0,0), (0,1), and (1,0), and Y = 1 only for (1,1) — this matches the AND gate exactly. (For contrast: an OR gate would give Y = 1 for (0,1), (1,0), and (1,1); a NAND gate would give the exact complement of the AND table, i.e. Y = 0 only for (1,1); a NOR gate would give Y = 1 only for (0,0).)

   Common mistake: Mixing up which gate gives output 1 for a single row versus which gives output 1 for three rows.

Questions about Electronic Devices for NEET

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Questions are original NEET720 compositions reviewed for correctness, syllabus fit and option quality. Counts update as the bank grows (611 active practice questions in this chapter today).