Chapter 24.6, Problem 4PPC

Interpretation Introduction

Interpretation:

The reason for the given statement that combination of pure elements from group $4A$ considered as semiconductors should be explained.

Concept Introduction:

Semiconductors are substances that conduct electricity either by addition of an impurity or by the effects of temperature on it.  Semiconductors electrically conductivity lies between conductor and insulator.  Semiconductors have small energy gap between valence band and conduction band.

Addition of impurity to a semiconductor is termed as doping. Doping alters the conductivity of a semiconductor.  The addition of an element having either more or less number of valence electrons than the natural semiconductor decides the combination as the following two types of semiconductor.

• n- type semiconductor:  (conduction is due to movement of extra electrons)

The element added will have more valence electron than the natural semiconductor. Therefore, the extra electron from the added element resides in conduction band and increase the conductivity.

Example: Silicon (natural semiconductor) and Phosphorus

• p-type semiconductor: (conduction is due to movement of holes)

The element added will have less valence electron than the natural semiconductor.  Here, instead of extra electron, there will be “holes” at the places, where a semiconductor is replaced by added element.  A p-type semiconductor increases conductivity because the holes (effective positive charge; lies at valence band) move through the natural semiconductor rather than electrons.

Example: Silicon (natural semiconductor) and Gallium