The advantages and disadvantages of semiconductor devices are: Advantages
Components made from semiconductor materials are often referred to as solid-state components, because they are made from solid materials. Semiconductors have largely replaced vacuum tubes, which were made of glass and therefore very fragile, and which consumed large amounts of power, since they required heaters to operate them. Semiconductors are additionally much smaller, lighter, and are much cheaper than vacuum tubes.
Disadvantages
Semiconductors are highly susceptible to temperature changes, and are easily damaged by excessive heat. For optimal operation they require highly sophisticated temperature control. Solid-state devices are also damaged if supply voltage polarity is not correct.
CONSTRUCTION OF A SEMICONDUCTOR
A semiconductor is a material that, under certain conditions, can act as either a conductor or an insulator. Silicon (Si) and germanium (Ge) are both semiconductive elements, of which silicon is the most popular. Each atom of silicon has four electrons in the outer (valence) shell, as shown in the diagram. Semiconductors are electronically stable, however, doping creates a surplus or deficit of electrons which gives the specific characteristics of semiconductor devices.
Chapter 15 Semiconductor Devices
Single atoms of silicon are of little use, so they are grown into large crystals, which are then cut into wafers for the manufacture of electronic components. The silicon atoms link up with neighbouring atoms to share electrons. A cluster of silicon atoms sharing outer electrons forms a matrix called a Crystal, as shown below.
The four electrons in the outer shell of each atom are shared with the electrons from the adjoining atoms via Covalent Bonding, and result in the valence shell of each atom in the crystal effectively holding eight electrons. These bonds are so strong that at absolute zero temperature (-273°C), there are no free electrons, and the silicon crystal assumes the properties of an electrical insulator. If the crystal of silicon is subsequently heated or a voltage applied across it, the covalent bonds break down and its characteristics change. The electrons break away from the atom and leave behind a hole in the atom’s outer shell. The free electrons then travel through the silicon as negative electrical charges. As the electrons move from one atom to another, the holes appear as if they are moving from one atom to another in the opposite direction. The movement of holes and electrons forms the basis of a semiconductor.
DOPING
Silicon in its pure state is not particularly useful in electronics, so doping is carried out, where the silicon atoms are contaminated with other materials such as phosphorous (P) or boron (B), to give them useful electronic properties. This contamination leaves the silicon atoms with incomplete outer valence shells and a hole is formed in the shell. The holes, which replace the missing electrons, act as positive charges and attract any free electrons within the crystal.
P-TYPE MATERIAL
If silicon is doped with indium, it produces a P-type material. Indium atoms only have 3 electrons in their outer shells (trivalent) and are acceptor atoms. This results in vacant electron openings or holes, which are positively charged, being left in the silicon crystal, as shown below.
COVALENT BONDS
Semiconductor Devices Chapter 15
Electrics 15-3
An electron from an adjacent silicon atom then falls into the hole, and the hole appears to move to another location. The electrons move through the material from left to right, whilst the holes move in the opposite direction.
N-TYPE MATERIAL
If silicon is doped with phosphorous, it produces an N-type material. Phosphorous atoms have 5 electrons in their outer shell (pentavalent) and are known as donor atoms. Extra electrons, which are negatively charged, are left floating around in the crystal, as shown below.
An N-type semiconductor contains many donor atoms that contribute free electrons, and these are free to drift through the material. The loss of an electron leaves the donor atoms with an overall positive charge and forms positive ions. Electrical current flows in the normal manner due to the movement of the free electrons. Like P-type silicon, it can also flow due to the migration of holes.
Chapter 15 Semiconductor Devices
P-N JUNCTION DIODE
Both P and N-type silicon conduct electricity at different rates, depending on the amount of doping. Both types function as resistors and conduct in both directions. The N-type material contains mobile electrons and an equal number of positive ions, which provide an overall neutral charge. The P-type material similarly contains mobile holes and an equal number of negative ions. Each part is initially neutral. If a junction is made by joining a piece of P and N-type material together, electrons will only flow in one direction through the junction, from N to P.
When the two materials are placed together, some of the free electrons in the N-type material cross the junction and fill the holes in the P-type material close to the junction. As the free electrons cross the junction, the N-type material becomes depleted of electrons near the junction and the holes in the P-type material become filled, depleting the holes near the junction. The region where the holes and electrons become depleted is known as the depletion layer.
This leaves the N-type material with an excess of positive ions and the P-type material with an excess of negative ions near the junction. The material close to the junction is in a charged state. The N-side is positively charged and the P-side negatively charged, which is known as a diode. This is an electronic one-way valve and is represented by the symbol shown below.
Semiconductor Devices Chapter 15
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If the negative terminal is connected to the N-type material, the diode is forward biased and current flows (i.e. it is in a conducting state), as shown above. If the diode is reverse biased, the positive terminal attracts electrons in the N-type material away from the junction. The negative terminal similarly attracts the holes in the P-type material, increasing the thickness of the depletion layer, as shown below.
If the diode is forward biased, electrons are attracted from the N-type material across the depletion layer to the positive terminal and the holes are attracted to the negative terminal, as shown below.
Chapter 15 Semiconductor Devices