Question

Which of the following semiconductors is electrically negative?

• A. Intrinsic semiconductor
• B. Silicon doped with pentavalent impurities
• C. Silicon doped with trivalent impurities
• D. None of the alternatives (A), (B) and (C) is correct

Hint:

Remember: \[ \text{Pentavalent doping} \Rightarrow n\text{-type} \] \[ \text{Trivalent doping} \Rightarrow p\text{-type} \] \(n\)-type semiconductors are electrically negative because electrons are the majority carriers.

Answer 1

The Correct Option is B

Step 1: What electrically negative means.
A semiconductor is called electrically negative when its majority charge carriers are electrons, which carry negative charge. This is the $n$-type material.

Step 2: Look at an intrinsic semiconductor.
A pure (intrinsic) semiconductor has equal numbers of electrons and holes. With no excess of either, it is neutral, not electron-rich.

Step 3: Doping with a pentavalent impurity.
A pentavalent atom has five valence electrons. After forming four bonds with silicon, one extra electron is left free. This gives plenty of free electrons.

Step 4: Doping with a trivalent impurity.
A trivalent atom has only three valence electrons, so it leaves a hole. Holes are the majority carriers here, making it $p$-type, the opposite of what we want.

Step 5: Pick the electron-rich case.
Only silicon doped with pentavalent impurities has electrons as majority carriers, so it is the electrically negative ($n$-type) one.

Step 6: Final choice.
\[ \boxed{\text{Silicon doped with pentavalent impurities}} \]