- After studying this section, you should be able to:
- • Describe the structure of a Silicon Atom.
- • Describe materials commonly used in diode and transistor manufacture.
- • Name materials commonly used for doping semiconductor materials.
- • Understand the purpose of doping semiconductor materials.
Fig.1.1.1 A Silcon Atom.
Some materials, such as copper, aluminium and brass pass electric currents very easily. As they conduct electricity readily, their ability to resist the flow of current is low. They therefore have very low resistivity and so are classed as conductors.
Other materials such as plastics or glass have extremely high resistivity so do not conduct electricity. These materials are called insulators.
Materials that have a resistivity mid-way between the conductors and insulators do conduct current, but very poorly at normal room temperatures, and so these are called semi-conductors.
How well or poorly any material conducts electricity depends on the atomic structure of the material. For an explanation of the atomic structure of materials as applied to electronics, see our page on The Atomic Structure of Matter.
Silicon and Germanium, as well as a number of other materials and mixtures of materials in the semiconductor group are widely used in the manufacture of transistors and diodes, as well as integrated circuits such as microprocessors.
To make such semiconductor materials suitable for use in transistors and diodes, the resistivity of the material is modified in a controlled way by first making very pure crystals of the semiconductor material. These crystals contain only atoms of one type (for example silicon) arranged in a regular lattice formation. This very pure material is then ‘doped’ by adding tiny amounts of impurity atoms (about 1 impurity atom in every 10 million).
The idea is that pure semiconductors conduct poorly, because the electrons in their lattice structure are mostly bound very tightly to their atoms, leaving only a few electrons free to move, from atom to atom, through the material, so forming a very weak electric current. By adding impurities with different atomic structures either more or in other cases less, free electrons are added. This controls the ability of the semiconductor to pass current, by effectively changing the resistivity of the material.
Some impurities such as Arsenic and Phosphorus add extra free electrons (negative charge carriers) to the material. This is called N type semiconductor.
Other impurities such as Aluminium and Boron can be added in order to remove free electrons, so that the resulting material has fewer free electrons than before. Each missing electron within the crystal structure is called a "hole". As free electrons are negative charge carriers, these holes in the structure are really positive charge carriers. Material doped in this way is called P type semiconductor. Putting P type and N type materials next to each other in a circuit creates a PN junction, and makes a useful device that is called a diode. When a voltage is applied across a diode a current will flow through the diode in one direction but not the other.
Making a three-part sandwich (PNP or NPN) and carefully controlling the levels of doping in the three layers creates a transistor capable of amplification, as well as many other useful functions.
Semiconductor Materials in Electronic Devices.
Transistors, diodes and integrated circuits can all be classified as semiconductor devices because they are made from semiconductor materials. Early types of transistors and diodes were made from Germanium (Ge), but Silicon (Si) is used today for the vast majority of devices. Germanium is very rarely used in modern transistors however, but does possess some properties that make it useful for devices such as photovoltaic cells to produce electricity in the presence of light. Gallium(Ga) is also used in electronic devices such as light emitting diodes (LEDs), usually as a compound material, i.e. in combination with other materials such as Gallium Arsenide(GaAs) or Gallium Arsenide Phosphide (GaAsP)
Top of Page.>