Question

Beyond the threshold voltage, resistance offered by the diode is:
A. Zero
B. Very low
C. Very low
D. Infinite

Answer 1

Hint: You can start by explaining p-type and n-type semiconductors briefly. Then explain diodes and also include the formation of depletion regions as it will help later on. Then explain what forward bias condition and threshold voltage is. Use the concept of threshold voltage to reach the solution.

Complete step by step answer:
P-type semiconductor – When a trivalent (having a valency of three) impurity like Boron (B) or Gallium (Ga) is added to a pure semiconductor (made from silicon or germanium). This addition leads to a decrease in the number of electrons in the crystal lattice of semiconductors, thus creating the positive holes.
N-type semiconductor - When a pentavalent (having a valency of five) impurity like Phosphorous (P) and Arsenic (As) is added to a pure semiconductor (made from silicon or germanium). This addition leads to an increase in the number of free electrons in the crystal lattice of semiconductors.
Diode is essentially a semiconductor that has the ability to act as a one-way switch for current. Unlike normal switches which allow the flow of current in both directions easily, diodes allow current to flow easily in one direction but heavily resist the flow of current in the opposite direction. Diodes also have an Anode (positive terminal) and Cathode (negative terminal).
Diode is p-type and n-type semiconductors joined together. The p-type acts as the anode and n-type acts as the cathode. In diode electrons from the n-type diffuse towards the holes from p-type creating a region at the junction called depletion region. So in the depletion region we have more electrons on the side of p-type semiconductor and more holes on the side of n-type semiconductor, thus giving them a negative charge and positive charge respectively. This potential difference creates an electric field from the n-type to the p-type which opposes further diffusion of electrons and holes.
When the diode is connected in a forward bias condition (the p-type connected to higher potential and n-type connected to lower potential), initially when we apply a voltage across the diode there is little flow of current as the electric field of the depletion region is opposing the flow of current. As we keep on increasing the voltage, after one point we overcome the electric field and the flow of current increases very rapidly. This point is called the threshold voltage. After threshold voltage the resistance offered by the diode becomes very low.

So, the correct answer is “Option B”.

Note:
Diodes are very special electrical equipment, they are used in rectifiers to change the alternating current (AC) into direct current (DC). Remember this direct current does not have a constant amplitude like simple direct current, but it has a constantly varying amplitude which changes according to the amplitude of the alternating current used.