Test: Intrinsic Semiconductors - Electronics and Communication Engineering (ECE) MCQ

• We know that in a semiconductor the total current is the sum of the electron current and the hole current.
• Therefore if the I_e and I_h are the electrons and the hole current respectively for the intrinsic semiconductor, then the total current is given as,
  • ⇒ I = I_e + I_h

Hence, option 3 is correct.

Semiconductor: 

• Semiconductors are materials that have a conductivity between conductors and insulators.
  • Semiconductors are made of compounds such as gallium arsenide or pure elements, such as germanium or silicon.
• Holes and electrons are the types of charge carriers accountable for the flow of current in semiconductors. 
  • ​Holes (valence electrons) are the positively charged electric charge carrier whereas electrons are the negatively charged particles.
• Intrinsic semiconductors are made of pure elements.
• The mobility of electrons and holes depends on their effective masses. The effective mass of electrons is less than that of holes hence electrons have higher mobility than holes.

Hence, option 3 is correct.

• In an intrinsic semiconductor, conduction occurs only when the electrons in the valence band can move to the conduction band.
• To cross over the energy gap between the valence band and conduction band, the electrons will require some amount of energy.
• Hence, at higher temperatures, the electrons will gain energy and jump into the conduction band.
• 0 K is also known as absolute zero. It is the temperature at which the particles in a substance are motionless.
• At 0 K, the electrons will not have the energy to move into the conduction band.
• Thus, there is no conductivity at 0 K in intrinsic semiconductors and it behaves as an insulator.

• When the external voltage becomes greater than the value of the potential barrier for the diode the potential barriers opposition will be overcome and the current will start to flow. This is the conventional current.
• The potential barrier is approx 0.3 volts for germanium and 0.7 volts for silicon while for the carbon it is 7eV.
• This is far much higher for Carbon to make it a semiconductor that has lower forbidden energy gaps (Si and Ge).
• This is why Carbon is not a semiconductor.

So the correct answer is option 3.

• In n-type semiconductors, electrons are the majority carriers and holes are the minority carriers. 
  • A common dopant for n-type silicon is phosphorus or arsenic.

The correct option is 1.

• An ideal, perfectly pure semiconductor (with no impurities) is called an intrinsic semiconductor.
• At absolute zero temperature valence band is full of electrons and the conduction band is empty, hence there are no free electrons in the conduction band and holes in the valence band.
• The charge carrier concentration is zero. Hence intrinsic semiconductor behaves like an insulator

Hence, at zero temperature the intrinsic semiconductor behaves like an insulator. The correct option is 3.

• An intrinsic semiconductor is made up of only a single type of element.
• Germanium (Ge) and Silicon (Si) are the most common type of intrinsic semiconductor.
• When the temperature rises, due to collisions, some covalent bonds break and few electrons are unbounded and become free to move through the lattice, thus creating an absence in its original position (hole).​ Hence, option 3 is correct.

• Option 1: The conductivity of an intrinsic semiconductor is very low at room temperature. It means very low or no current flows at room temperature.
  • So this is an incorrect statement.
• Option 2: Since intrinsic semiconductors are pure semiconductors they have the same number of holes and electrons. 
• Option 3 and 4: The intrinsic semiconductor is a pure semiconductor and an extrinsic semiconductor is a semiconductor with impurity.
  • So option 4 gives the correct statement.

So the correct answer is option 4.

• In an intrinsic semiconductor, the number of electrons is equal to the number of holes. Hence at room temperature, no free electron is available for conduction
• If some energy is applied to the atoms of intrinsic semiconductor the covalent bonds will break and electrons will be free 
• Each electron will leave behind a hole and the process will continue, and charge flows through the intrinsic semiconductor

Therefore intrinsic semiconductor conductivity is due to breaking off the covalent bond. Hence option (4) is correct

• Intrinsic semiconductors are made of pure elements.
• Since no impurity is mixed in the intrinsic semiconductors, so the numbers of electrons and holes remain equal in an intrinsic semiconductor.

Hence, option 1 is correct.