Tag: semiconductor electronics: materials, devices and simple circuits

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Questions Related to semiconductor electronics: materials, devices and simple circuits

Multiple choice zener diode special purpose diodes electronic devices semiconductor electronics: materials, devices and simple circuits physics

If, a Zener diode ($V _z = 5V$ and $I _z = 10 mA$) is connected in series with a resistance and 20 V is applied across the combination, then the maximum resistance one can use without spoiling zener action is

  1. 20 $k\Omega$
  2. 15 $k\Omega$
  3. 10 $k\Omega$
  4. 1.5 $k\Omega$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The total voltage is 20V and the Zener voltage is 5V, so the voltage across the series resistor is 20 - 5 = 15V. For maximum resistance, the circuit current must be at the minimum required Zener current, which is given as 10mA. Using Ohm's law, R = V / I = 15V / 10mA = 1.5 kOmega, making option D correct.

Multiple choice zener diode special purpose diodes electronic devices semiconductor electronics: materials, devices and simple circuits physics

The main cause of Zener breakdown is

  1. the base semiconductor being germanium.

  2. production of electron-hole pairs due to thermal excitation.

  3. low doping.

  4. high doping.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Zener breakdown occurs in heavily doped p-n junctions. The heavy doping makes the depletion layer extremely thin. So that, carriers cannot accelerate enough to cause ionization. Thus, current will increase in reverse bias only due to reverse breakdown voltage.

Multiple choice zener diode special purpose diodes electronic devices semiconductor electronics: materials, devices and simple circuits physics

The main cause of avalanche breakdown is

  1. collision by ionisation.

  2. high doping.

  3. recombination of electrons and holes.

  4. low doping.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Avalanche breakdown is caused by impact ionization of electron-hole pairs. A very little current flows under reverse bias conditions and depletion region increases. The electric field in the depletion region of a diode can be very high. Electron/holes that enter the depletion region undergo a tremendous acceleration. As these accelerated carriers collide with the atoms, they can knock electrons from their bonds, creating additional electron/hole pairs and thus additional current. As these secondary carriers are swept into the depletion region, they too are accelerated and the process repeats itself.

Multiple choice zener diode special purpose diodes electronic devices semiconductor electronics: materials, devices and simple circuits physics

The avalanche breakdown in p-n junction is due to

  1. shift of Fermi level.

  2. cumulative effect of conduction band electron.

  3. widening of forbidden gap.

  4. high impurity concentration.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The avalanche breakdown in p-n junction is due to cumulative effect of conduction band electron. In reverse bias, due to applied electric field, electrons acquire enough energy to free more electron bound to the atom. The abundant number of electron-hole pairs are created for conduction, this is cumulative effect.

Multiple choice zener diode special purpose diodes electronic devices semiconductor electronics: materials, devices and simple circuits physics

A Zener diode

  1. is a lightly doped junction diode.

  2. heavily doped junction diode.

  3. is either p-type or n-type.

  4. has no p-n junction.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The reverse breakdown voltage depends on doping of the diode. Hence, in the Zener diode the heavy doping of its p-n junction is done. The depletion region formed in the diode is very thin ($<1\ m$) and the reverse bias voltage of about $5\ V$ which is less than ordinary diode.

Multiple choice zener diode special purpose diodes electronic devices semiconductor electronics: materials, devices and simple circuits physics

The current flow in a Zener diode is mainly due to

  1. thermally generated charge carriers

  2. minority charge carriers

  3. collision generated charge carriers

  4. ions
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown. Avalanche breakdown occurs in reverse bias when the applied voltage is high enough, the free electron may move fast enough to knock other electrons free, creating more free-electron-hole pairs (i.e., more charge carriers), increasing the current. Thus, the current flow in a Zener diode is mainly due to collision generated charge carriers

Multiple choice zener diode special purpose diodes electronic devices semiconductor electronics: materials, devices and simple circuits physics

At breakdown voltage, the rate of creation of hole-electron pairs is _____  leading to the _______  in current. 

  1. increased, decrease

  2. increased, increase

  3. decreased, increase

  4. decreased, decrease
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

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.