226. Which of the following statements are true:
     1. LEDs fabricated from GaAs emit radiation in the ultraviolet region.
     2. LEDs fabricated from GaAs emit radiation in the infra red region.
     3. When a semiconductor is doped with pentavalent impurity, the resulting material is p-type.
     4. A n-type semiconductor is electrically neutral.

1. 1. 1 and 2 only

   
   
   

   2. 3 and 4 only

   
   
   

   3. 2 and 3 only

   
   
   

   4. 1, 2, 3 and 4

   
   
   

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   NA

   Correct Option: A

   NA

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227. Based on the cohesive energy of an ionic solid, which of the following is not a property of the ionic solid?

1. 1. It forms relatively stable and hard crystals

   
   
   

   2. It is a poor electric conductor

   
   
   

   3. It has a high vaporization temperature

   
   
   

   4. None of the above

   
   
   

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   NA

   Correct Option: D

   NA

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228. Which one of the theories given below is related to the theory of the thermocouple?

1. 1. Piezoelectric effect

   
   
   

   2. Skin effect

   
   
   

   3. Seebeck effect

   
   
   

   4. Faraday’s Law

   
   
   

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   NA

   Correct Option: C

   NA

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229. With respect to a completely filled band, which one of the following statements is correct?

1. 1. The total wave vector and the total velocity of all electrons in the band is always zero

   
   
   

   2. The total wave vector and the total velocity of all electrons in the band is not necessarily zero

   
   
   

   3. Both (A) and (B) above

   
   
   

   4. None of the above

   
   
   

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   NA

   Correct Option: B

   NA

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230. Semiconductor A has a higher band gap than semiconductor B. If both A and B have the same dimensions, the same number of electrons at a given temperature and the same electron and hole mobilities, then:

1. 1. A has the same number of holes as B

   
   
   

   2. A has a larger number of holes than B

   
   
   

   3. A has less number of holes than B

   
   
   

   4. Any of the above statements (A), (B) or (C) could be true

   
   
   

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   NA

   Correct Option: B

   NA

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