Intrinsic Semiconductor

An INTRINSIC semiconductor is pure — no dopants. Examples: ultra-pure Si and Ge.

At T = 0 K: VB fully filled, CB empty ⇒ behaves as insulator.

At room T: a few electrons thermally excited to CB, leaving an equal number of HOLES in VB ⇒ n = p = n_i.

Carrier density depends exponentially on temperature: n_i ∝ T^(3/2) · exp(−E_g/2k_BT).

For intrinsic Si at 300 K: n_i ≈ 1.5 × 10¹⁰/cm³. For Ge: ~2.4 × 10¹³/cm³.

Holes drift in the OPPOSITE direction to electrons in an applied field ⇒ both contribute to current in the SAME direction.

Intrinsic conductivity at room T is too small for practical devices ⇒ doping is essential.