Consider the following statements about the V-I characteristic of a p-n junction diode:
(A) The cut-in voltage is about 0.2 V for a germanium diode and about 0.7 V for a silicon diode.
(B) Under reverse bias, the current is of the order of microampere and stays almost constant as the bias changes.
(C) The dynamic resistance of a diode is the ratio of a small change in voltage to the small change in current it causes.
(D) Under forward bias, the current increases in direct proportion to the voltage, starting from zero voltage.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A), (B) and (C) only
The V-I curve of a diode is not a straight line. Its forward part and its reverse part behave very differently.
(A) The cut-in voltage is about 0.2 V for a germanium diode and about 0.7 V for a silicon diode. — True. These are the threshold (cut-in) voltages of the two diodes. Beyond this voltage the forward current rises sharply.
(B) Under reverse bias, the current is of the order of microampere and stays almost constant as the bias changes. — True. This small reverse current is called the reverse saturation current. It hardly changes with the reverse voltage.
(C) The dynamic resistance of a diode is the ratio of a small change in voltage to the small change in current it causes. — True. The dynamic resistance is \( r_d=\Delta V/\Delta I \), taken over a small part of the curve.
(D) Under forward bias, the current increases in direct proportion to the voltage, starting from zero voltage. — False. Below the cut-in voltage the current rises very slowly. After it, the current rises exponentially. So the current does not grow in proportion to the voltage.
NCERT (2026-27):
p-n junction diode under reverse bias – “… This voltage is called the threshold voltage or cut-in voltage (~0.2V for germanium diode and ~0.7 V for silicon diode).”
p-n junction diode under reverse bias – “For the diode in reverse bias, the current is very small (~μA) and almost remains constant with change in bias. It is called reverse saturation current.”
p-n junction diode under reverse bias – “… For diodes, we define a quantity called dynamic resistance as the ratio of small change in voltage ΔV to a small change in current ΔI:”
\( \displaystyle r_d=\frac{\Delta V}{\Delta I} \qquad (14.6) \)
p-n junction diode under reverse bias – “… in forward bias, the current first increases very slowly, almost negligibly, till the voltage across the diode crosses a certain value.”