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Semiconductor Electronics: Materials, Devices and Simple Circuits Test 13

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Semiconductor Electronics: Materials, Devices and Simple Circuits Test 13
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  • Question 1
    1 / -0
    Optoelectronic devices work on :
    Solution
    Optoelectronic devices and components are those electronic devices that operate on both light and electrical currents. This can include electrically driven light sources such as laser diodes and light-emitting diodes, components for converting light to an electrical current such as solar and photovoltaic cells and devices that can electronically control the propagation of light.
  • Question 2
    1 / -0
    A thin wire of resistance $$\displaystyle 4\Omega $$ is bent to form a circle. The resistance across any diameter is:
    Solution
    Given that the resistance of the total wire is $$\displaystyle 4\Omega $$
    Here, $$\displaystyle ACB\left( 2\Omega  \right) $$ and $$\displaystyle ADB\left( 2\Omega  \right) $$ are in parallel.
    So, the resistance across any diameter is
    $$\displaystyle \Rightarrow \quad \frac { 1 }{ R } =\frac { 1 }{ 2 } +\frac { 1 }{ 2 } =\frac { 2 }{ 2 } =1\Rightarrow R=1\Omega $$

  • Question 3
    1 / -0
    When semiconductor is doped half with trivalent and half with pentavalent impurities, junction formed is known as
    Solution
    $$Answer:-$$ A
    As pentavalent impurities contribute or donate electrons to the semiconductor, these are called donor impurities and similarly as these impurities contribute negative charge carriers in the semiconductor this we refer as n - type impurities. The semiconductor doped with n - type impurities is called n-type semiconductor.
    Since trivalent impurities contribute excess holes to semiconductor crystal, and these holes can accept electrons, these impurities are referred as acceptor impurities. As the holes virtually carry positive charge, the said impurities are referred as positive - type or p - type impurities and the semiconductor with p type impurities is called p-type semiconductor.
  • Question 4
    1 / -0
    Which of the following is/are optoelectronic devices?
    Solution
    P-n junctions are an integral part of several optoelectronic devices. These include photodiodes, solar cells light emitting diodes (LEDs) and semiconductor lasers.
  • Question 5
    1 / -0
    A hole is
    Solution
    A doping occurs when atoms from group 3 or group 5 of periodic table replace some of the atom of the group 4 elements being doped. Doing creates hole 'unfilled covalent  bond' if group 3 atom is used or provides  excess electron if group 5 atom is used.
  • Question 6
    1 / -0
    The output of NOT gate when its input is $$0$$:
    Solution
    Truth table of a NOT gate is:

      Input Output
     X 0 1
     Y 1 0
    We can clearly see that when the input of a NOT gate is $$0$$, the output is $$1$$.
  • Question 7
    1 / -0
    In the depletion region of a pn junction, there is a shortage of
    Solution
    Depletion region of a p-n junction is formed due to the shortage of holes and electrons.
  • Question 8
    1 / -0
    At breakdown voltage, the rate of creation of hole-electron pairs is _____  leading to the _______  in current. 
    Solution
    When the reverse voltage across a diode is very large, the valance electrons become free due to applied high electric field and get enough acceleration to make other electrons free, thus create a lot of electron-hole pairs in a short time. As the number of charge carriers increases, current also increases.
    Therefore, at breakdown voltage, the rate of creation of hole-electron pairs is increased leading to the increase in current.
  • Question 9
    1 / -0
    The pn junction can be formed by which of the following methods?
    Solution
    Grown Junction Diode:
    Diodes of this type are formed during the crystal pulling process. P and N-type impurities can be alternately added to the molten semiconductor material in the crucible, which results in a P-N junction when crystal is pulled. After slicing, the larger area device can then be cut into a large number of smaller-area semiconductor diodes. Though such diodes, because of larger area, are capable of handling large currents but larger area also introduces more capacitive effects, which are undesirable. Such diodes are used for low frequencies.

    Alloy Type or Fused Junction Diode:
    Such a diode is formed by first placing a P- type impurity (a tiny pellet of aluminium or some other P- type impurity) into the surface of an N-type crystal and heating the two until liquefaction occurs where the two materials meet. An alloy will result that on cooling will give a P-N junction at the boundary of the alloy substrate. Similarly, an N-type impurity may be placed into the surface of a P- type crystal and the two are heated until liquefaction occurs. Alloy type diodes have a high current rating and large PIV (peak inverse voltage) rating. The junction capacitance is also large, due to the large junction area.

    Diffusion Diode: 
    Diffusion is a process by which a heavy concentration of particles diffuse into a surrounding region of lower concentration. The main difference between the diffusion and alloy process is the fact that liquefaction is not reached in the diffusion process. In the diffusion process heat is applied only to increase the activity of elements involved. For formation of such diodes, either solid or gaseous diffusion process can be employed. The process of solid diffusion starts with formation of layer of an acceptor impurity on an N- type substrate and heating the two until the impurity diffuses into the substrate to form the P-type layer. A large P-N junction is divided into parts by cutting process. Metallic contacts are made for connecting anode and cathode leads.
  • Question 10
    1 / -0
    A pn junction is a/an
    Solution
    $$answer:-$$ B
    In p-n junction current flow is unidirectional from anode to cathode. In other direction current flow is very small or negligible.

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