Intrinsic Semiconductor

When a semiconductor is in pure form, without mixing of any other impurity in it (like Si or Ge in pure form), such substance is called INTRINSIC SEMICONDUCTOR. Silicon and germanium are tetravalent elements. They have four valance electrons in their outermost orbit as shown below. The nucleus exerts a weak nuclear force on valence electrons. In silicon atomic structure, each valance electron is bonded with its neighboring silicon atom. This type of bonding is known as covalent bonding.

However, when some external energy (like voltage or strong light) is applied on a piece of pure silicon, the covalent bond breaks and valance electrons jump into conduction band to carry electric current. The energy required to break the bond is 1.12eV for silicon and 0.75eV for germanium. When an electron jumps into conduction band, it leaves a vacancy in its own atom. This vacancy is called as hole. Thus, the electron–hole pair is produced simultaneously in silicon atom and their number is ALWAYS EQUAL. A small section of silicon crystal is shown in the following figure –

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