Question: Write two points of difference between intrinsic and extrinsic semiconductors.
Step 1 (Definition & Governing Principle): [1 Mark]
An intrinsic semiconductor is a pure semiconductor material (e.g., pure $Si$ or $Ge$) where the number of free electrons ($n_e$) is equal to the number of holes ($n_h$), such that $n_e = n_h = n_i$. In contrast, an extrinsic semiconductor is a semiconductor doped with a specific impurity (trivalent or pentavalent) to modify its electrical properties, resulting in $n_e \neq n_h$.
Step 2 (Comparative Analysis): [1 Mark]
The differences are tabulated based on the charge carrier concentration and the effect of temperature:
- Carrier Concentration: In intrinsic semiconductors, the carrier concentration is determined solely by thermal excitation. In extrinsic semiconductors, the carrier concentration is controlled by the doping level ($N_D$ or $N_A$).
- Electrical Conductivity: The conductivity of intrinsic semiconductors is low at room temperature. Extrinsic semiconductors exhibit significantly higher conductivity due to the presence of additional charge carriers introduced by doping.
Step 3 (Final Concluded Answer): [2 Marks Total]
The two primary points of difference are:
- Purity: Intrinsic semiconductors are pure elements (e.g., $Si$, $Ge$), whereas extrinsic semiconductors are doped with impurities (e.g., $P$, $As$, $B$, $Ga$).
- Carrier Density: In intrinsic semiconductors, $n_e = n_h$. In extrinsic semiconductors, the concentration is dominated by either electrons ($n_e \gg n_h$ for n-type) or holes ($n_h \gg n_e$ for p-type).
💡 EL10 Concept Insight: Think of an intrinsic semiconductor like a quiet, empty room where only a few people (electrons) are moving around. An extrinsic semiconductor is like adding a "booster" group of people to that room, which makes the whole room much more active and better at conducting electricity!