Which of the following elements is NOT suitable for the fabrication of a light emitting diode structure?
- A.Gallium phosphide
- B.Indium gallium nitride
- C.Germanium
- D.Gallium arsenide
Correct answer
Explanation
The correct answer is C, Germanium. Germanium is not used to build light emitting diodes because it is an indirect band gap semiconductor, so an electron falling back across the gap gives up most of its energy as heat in the crystal rather than as light. Its band gap is also very small, about 0.66 electron volt, which is far too little to produce visible light. Light emitting diodes are therefore made from direct band gap compounds, whose gap can be tuned to the colour wanted. A is wrong because gallium phosphide is used for green and yellow diodes. B is wrong because indium gallium nitride gives the blue and white diodes that made modern lighting possible and won the Nobel Prize in Physics in 2014. D is wrong because gallium arsenide is the standard material for red and infrared diodes, including the ones inside television remote controls. Exam tip: light emitting diodes need direct band gap materials such as gallium arsenide, gallium phosphide and indium gallium nitride; silicon and germanium are indirect and do not emit light.
Practice Questions
View allThe fact that the planets move around the Sun, not in circles but in ellipses, was first demonstrated by
- A.Galileo
- B.Martin Luther
- C.Johannes Kepler
- D.Copernicus
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In 1609 Kepler showed that the planets move around the Sun in elliptical orbits, with the Sun at one focus of the ellipse.
Read the full explanation →Which one of the following statements is correct?
- A.Urea is produced in liver.
- B.Urea is produced in blood.
- C.Urea is produced from digestion of starch.
- D.Urea is produced in lung and kidney.
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Liver cells turn the toxic ammonia released from the breakdown of amino acids into urea through the ornithine cycle, also called the urea cycle.
Read the full explanation →Which one of the following is the correct sequence of passage of light in a compound microscope?
- A.Condenser-Objective lens-Eyepiece-Body tube
- B.Objective lens-Condenser-Body tube-Eyepiece
- C.Condenser-Objective lens-Body tube-Eyepiece
- D.Eyepiece-Objective lens-Body tube-Mirror
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Light passes through the condenser to the specimen, is picked up by the objective lens, travels up the body tube and reaches the eye through the eyepiece.
Read the full explanation →Which one of the following statements regarding haemoglobin is correct?
- A.Haemoglobin present in RBC can carry only oxygen but not carbon dioxide.
- B.Haemoglobin of RBC can carry both oxygen and carbon dioxide.
- C.Haemoglobin of RBC can carry only carbon dioxide.
- D.Haemoglobin is only used for blood clotting and not for carrying gases.
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It binds oxygen at its iron-containing haem groups and carries some carbon dioxide on its protein chains. Each haemoglobin molecule has four polypeptide chains and four haem groups, and each haem iron can hold one oxygen molecule, forming oxyhaemoglobin in the lungs.
Read the full explanation →Which one of the following statements regarding histone proteins is correct?
- A.Histones are proteins that are present in mitochondrial membrane.
- B.Histones are proteins that are present in nucleus in association with DNA.
- C.Histones are proteins associated with lipids in the cytosol.
- D.Histones are proteins associated with carbohydrates in the cytosol.
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They are the proteins around which DNA winds to form chromatin in the nucleus of eukaryotic cells. Histones are small basic proteins, rich in the positively charged amino acids lysine and arginine, which attract the negatively charged phosphate groups of DNA.
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