1. Semi Conductors
Between conductors and insulators are materials, which allow an electric current to flow only under certain circumstances. These are called semiconductors.

2. Semiconductors
To understand semiconductors, we need to understand the structure of a semiconductor atom.
Four electrons in the valence band.

3. Semiconductors
There are several types of semiconductor material available, but the most commonly used materials are:-
Silicon
& Germanium

4. Semiconductors Both these materials have the structure shown below.
core. representing nucleus and inner bands
4 electrons in valence band

5. Semiconductors
If we imagine that there is another band just outside the Valance band, called the conductor band, that electrons could move into and this conductor band gives the atom similar properties to a conductor. Then the conduction abilities of a Semiconductor material can be altered.

6. Semiconductors germanium conduction band
In SC materials the gap (forbidden gap) between the Valance band and conduction band is very small. So a little extra energy (heat), closes the gap, enabling the electrons to pass into the conduction band.
Forbidden gap
germanium valence band

7. Intrinsic Semiconductors
It is this property that we can use for NTC (Negative Temperature Coefficient) sensors.
Hence Germanium is adopted for use in NTC temperature sensors.
What does NTC mean?

8. Intrinsic Semiconductors
As the temperature of the Germanium material is increased more electrons are given enough energy to jump into the conducting band and hence lower its resistance.
NTC

9. Intrinsic Semiconductors
The semiconductor materials that naturally act in this way are normally referred to as Intrinsic semiconductors.
For other purposes other than temperature sensing, changing the properties of the natural semiconductor material are required.

10. Extrinsic Semiconductor
An Intrinsic semiconductor material can be altered by adding minute quantities of another substance, the electrical properties of the material can be altered.
Impurities such as phosphorus, arsenic, antimony and bismuth are added to the natural material to give an excess of electrons.
Impurities such as indium, aluminum, gallium and boron are added to give an excess of holes.

11. Extrinsic Semiconductor
These doped semiconductor materials are man made and are referred to as Extrinsic semiconductors.
Semiconductor materials with an excess of electrons are called N – Type.
Semiconductor materials with an excess of holes are called P – Type.

12. N-Type Semiconductor Materials
Doping elements used to make N – Type materials all have 5 electrons in there outer shell and are referred to as pentavalent elements.
When added to silicon or germanium, four of these electrons form covalent bonds with the semiconductor atoms. However, the fifth electron, although temporarily bound to its additive atom, can easily be detached.

13. N-Type Semiconductor Materials
A semiconductor containing these additives thus has many free electrons to form an electric current.
The conductivity of the semiconductor is now a thousand times greater.

14. N-Type Semiconductor Materials
Extra electron

15. P-Type Semiconductor Materials
We have seen that introducing atoms, which have extra electrons, can increase the conductivity of a semiconductor. Introducing atoms, which produce extra holes, can also increase it. Doping elements used to make P – Type materials all have 3 electrons in there outer shell and are referred to as trivalent elements.

16. P-Type Semiconductor Materials
Extra Hole

17. The PN Junction Diode
Up to now we have examined p-type and n-type material separately. However, the properties of these extrinsic semiconductor materials become useful only when the two types are combined in some way.
The area where the two types meet is called a pn junction, and the resulting properties allow it to be used as a diode.

18. The PN Junction Diode
A diode is the electrical equivalent of a one-way valve, which normally allows current to flow through it in one direction only. They can be made to handle currents varying from a few micro amps to many hundreds of amps.

19. The PN Junction Diode FORWARD BIAS
With the N layer connected to the negative terminal and P layer connected to the positive terminal, electrons flow through the diode.
FORWARD BIAS

20. The PN Junction Diode REVERSE BIAS
If the battery terminals are reversed then there will be no flow, as the electrons and holes are attracted away from the junction to there opposites in the battery.
REVERSE BIAS

21. Transistors
An even more useful device is made when three layers of semiconductor material are formed together in a single continuous crystal of silicon or germanium as an 'NPN' sandwich, or a 'PNP' sandwich.

22. Transistors
Transistors act like a minute relay. When the base is connected to the opposite source (e.g. N to positive/ earth) it connects the emitter to the collector and current flows through.

23. Transistors
FLOW E C B
NO FLOW

24. Small Current to the Base Switches the Collector and Emitter on.
Transistor Operation
Small Current to the Base Switches the Collector and Emitter on.

25. Transistors Switching device
Uses for Transistors
Switching device
As the base requires very small current to actually activate the transistor, it can be used instead of a relay. Benefits include no mechanical parts at all (solid state) and with such small currents being used to switch them on, they can be controlled by control units.

26. Transistors Amplifier
Uses for Transistors
Amplifier
If two or more transistors are used in series with each other then, this tiny current applied to the base can be used to allow very large currents to flow.