The majority charge carriers in a p-type semiconductor are
- A.free electrons
- B.conduction electrons
- C.ions
- D.holes
Correct answer
Explanation
The correct answer is D, holes. A p-type semiconductor is made by adding a small amount of a trivalent impurity, such as boron, aluminium, gallium or indium, to pure silicon or germanium. Each impurity atom has only three valence electrons, so one of the four bonds with the neighbouring silicon atoms is left one electron short. That vacancy is a hole, which behaves like a positive charge and moves through the crystal as electrons jump in to fill it. Holes therefore far outnumber free electrons and are the majority carriers, while electrons are the minority carriers. The impurity is called an acceptor because it accepts an electron. Option A is wrong because free electrons are the majority carriers in n-type material, made with pentavalent donors such as phosphorus or arsenic. Option B is wrong because conduction electrons are the same free electrons and are the minority carriers in p-type material. Option C is wrong because the dopant ions are fixed in the crystal lattice and do not carry current. Exam tip: p-type = trivalent acceptor, holes majority; n-type = pentavalent donor, electrons majority.
Practice Questions
View allChlorophyll in photosynthetic prokaryotic bacteria is associated with
- A.plastids
- B.membranous vesicles
- C.nucleoids
- D.chromosomes
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Photosynthetic bacteria have no chloroplasts; their pigments sit on infoldings of the cell membrane that form vesicles or sacs, often called chromatophores.
Read the full explanation →Fat in human body is stored in
- A.Adipose tissue
- B.Red blood cells
- C.Muscle tissue
- D.Epithelial tissue
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Adipose tissue is a loose connective tissue whose cells, called adipocytes, are built to store fat. Each cell holds a large droplet of fat that pushes the nucleus and cytoplasm to one side.
Read the full explanation →Which one of the following is a metalloid ?
- A.Germanium
- B.Coke
- C.Iodine
- D.Sugar
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Germanium is a metalloid, an element whose properties lie between those of metals and non-metals. It is a hard, shiny, grey-white solid like a metal, yet it is brittle and conducts electricity far less well than a metal, which makes it a semiconductor.
Read the full explanation →A photon of X-ray has energy of 1 keV. A photon of visible radiation has energy of 3 eV. In this context, which one of the following statements is not correct?
- A.The wavelength of X-ray photon is less than the wavelength of visible radiation photon.
- B.Both the photons have different energies.
- C.The speeds of both the photons in vacuum are different.
- D.The frequency of X-ray photon is higher than the frequency of visible radiation photon.
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This statement is wrong because every photon, whatever its energy, travels in vacuum at the same speed of light, about 3 × 10^8 metres per second.
Read the full explanation →Which of the following substances cause temporary hardness in water?
1. Mg(HCO3)2
2. Ca(HCO3)2
3. CaCl2
4. MgSO4
Select the correct answer using the code given below.
- A.3 and 4
- B.2 and 3
- C.1 and 4
- D.1 and 2
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Temporary hardness is caused by the bicarbonates of magnesium and calcium, Mg(HCO3)2 and Ca(HCO3)2. It is called temporary because simple boiling removes it: on heating, the soluble bicarbonates break down into insoluble carbonate or hydroxide, carbon dioxide and water, and the solid settles out.
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