(introduction...)

Title: Semiconductor Review
Objectives:

- Know the difference between conductor and semiconductor
- Able to describe N- and P type semiconductor
- Understand the diode principle

Figure

Introduction

Conductor

A neutral copper atom has only one electron in its outer orbit. Since the single electron can be easily dislodged from its atom, it is called a free electron.

Semiconductor

Silicon is the most widely used semiconductor material. The number of electrons in the valence orbit is the key to conductivity. Conductors have one valence electron, semiconductors have four valence electrons, and insulators have eight valence electrons.

Silicon Crystals

Each silicon atom in a crystal has its four valence electrons plus four more electrons that are shared by the neighboring atoms. At room temperature, a pure silicon crystal has only a few thermally-produced free electrons and holes.

Intrinsic Semiconductor

An intrinsic semiconductor is a pure semiconductor. Intrinsic silicon acts as an insulator at room temperature.

Two Types of Flow

Flow of free electrons, flow of holes

Fig. 1-1: Types of Flow

Doping a semiconductor

Doping increases the conductivity of a semiconductor. A doped semiconductor is called an extrinsic semiconductor. When an intrinsic semiconductor is doped with pentvalent (donor) atoms (i.e. Arsenic atoms donates one free electron to the crystal), it has more free electrons than holes.

---> N-type semiconductor

When an intrinsic semiconductor is doped with trivalent (acceptor) atoms (i.e. Baron atoms in the crystal will create a hole which is capable of accepting an electron), it has more holes than free electrons.

---> P-type semiconductor

Diode

Unbiased Diode

An unbiased diode has a depletion layer at the PN-junction. The ions in this deplation layer produce a barrier potential. At room temperature, this barrier potential is approximately 0.7V for a silicon diode.

Fig. 1-2: Unbiased diode

Biased Diode

When an external voltage opposes the barrier potential, the diode is forward-biased. If the applied voltage is greater than the barrier potential, the current is large. In other words, current flows easily in a forward-biased diode.

When an external voltage aids the barrier potential, the diode is reverse biased. The width of the depletion layer increases when the reverse voltage increases. The current is approximately zero. The reverse biased diode acts like an open switch.

Breakdown

Too much reverse voltage will produce either avalanche or zener effect. Then, the large breakdown current destroys the diode.

Recap

Fig. 1-3: Forward biased diode

What happens to an electron in this circuit?

1. After leaving the source terminal, it enters the right end of the crystal.

2. It travels through the N-region as a free electron.

3. At the junction it recombines with a hole and becomes a valence electron.

4. It travels through the P-region as a valence electron.

5. After leaving the left end of the crystal, it flows into the positive source terminal.