14.4.1. n-type semiconductor

Suppose we dope Si or Ge with a pentavalent element as shown in Fig. 14.7. When an atom of +5 valency element occupies the position of an atom in the crystal lattice of Si, four of its electrons bond with the four silicon neighbours while the fifth remains very weakly bound to its parent atom.

Doping is the process of adding impurities to a semiconductor to enhance its electrical properties. In n-type semiconductors, we use pentavalent atoms (elements with five valence electrons), like arsenic or phosphorus. When these dopants are integrated into the silicon or germanium lattice, four out of their five electrons form strong covalent bonds with the surrounding silicon atoms. The fifth electron, however, is loosely attached and can easily move, which significantly contributes to the conductivity of the material.

Think of doping like adding a pinch of salt to a pot of water. Just as the salt dissolves and affects the flavor of the water, adding pentavalent atoms changes how easily electricity can flow through the semiconductor.

As a result the ionisation energy required to set this electron free is very small and even at room temperature it will be free to move in the lattice of the semiconductor.

The energy required to release the loosely bound fifth electron from a pentavalent atom is very low—only about 0.01 eV for germanium and 0.05 eV for silicon. This low ionisation energy means that even at room temperature, these electrons can be freed and participate in conduction. This is a crucial feature that differentiates n-type semiconductors from intrinsic semiconductors, where much higher energy is needed to free electrons.

Imagine trying to lift a feather as opposed to a heavy boulder. The feather is easily lifted (like the energy required to free an electron in n-type semiconductors), while the boulder requires significant effort (as in the case of intrinsic semiconductors).

Thus, with proper level of doping the number of conduction electrons can be made much larger than the number of holes. Hence in an extrinsic semiconductor doped with pentavalent impurity, electrons become the majority carriers and holes the minority carriers.

In n-type semiconductors, the number of free electrons—contributed by the dopant—is much greater than the number of holes generated intrinsically. This creates a scenario where electrons are the majority charge carriers, while holes are the minority. A correctly doped n-type semiconductor can have an abundance of free electrons available for conduction, making it an effective conductor of electricity.

Think of a crowded club. If there are 80 people (electrons) dancing on the floor and only 20 people (holes) sitting at tables, the dancers greatly outnumber those sitting. Similarly, in an n-type semiconductor, the free electrons are like the dancers who are active participants in conducting electricity.

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