Question

If the forward voltage in a semiconductor diode is changed from \[0.5V\] to \[0.7V\], then the forward current changes by \[1.0mA\]. The forward resistance of diode junction will be:
A) \[100\Omega \]
B) \[120\Omega \]
C) \[200\Omega \]
D) \[240\Omega \]

Answer 1

Hint: A diode is an electrical device allowing current to move through it in one direction with greater ease than in the opposite direction. When the polarity of the battery is such that current is allowed to flow through the diode, the diode is said to be forward-biased. The forward voltage is the amount of voltage needed to flow across a diode.
Formula Used: \[\Delta V=\Delta I\times R\]

Complete step by step solution:
As given in the question, the voltage changes from \[0.5V\] to \[0.7V\]
Hence change in the voltage \[\Delta V=0.7V-0.5V=0.2V\]
Change in the forward current \[\Delta I=1mA=1\times {{10}^{-3}}A\]
From ohm’s law, change in the forward voltage is equal to the product of the forward resistance and the change in the forward current, that is,
\[\Delta V=\Delta I\times R\]
Substituting the values in the above equation, we get
\[\begin{align}
  & 0.2V=1\times {{10}^{-3}}A\times R \\
 & \Rightarrow R=\dfrac{0.2V}{1\times {{10}^{-3}}A} \\
 & \Rightarrow R=\dfrac{0.2}{1}\times {{10}^{3}}\Omega \\
 & \Rightarrow R=200\Omega \\
\end{align}\]

Hence the forward resistance of the junction diode is \[200\Omega \] and (C) is the correct option.

Additional Information:
The value of forwarding resistance of a diode is very small, ranging between one and twenty-five. Ideally, the reverse resistance of a diode is considered to be infinite. This again helps fulfil the basic function of a diode, which is to allow current in only one direction.

Note:Although a junction diode is formed by fused semiconductors, we’ve applied ohm’s law to obtain our answer. This does not mean that ohm’s law is entirely valid for semiconductors as well. Ohm’s law is only applicable to devices that maintain a constant voltage to current ratio or for very low electric fields such as that given to us in the question. For higher fields, the current becomes almost independent of the applied field, and Ohm’s law is not obeyed in semiconductors.