two functions that are fundamental to the design of electronic circuits:
amplification and switching.
• amplification consists of magnifying a signal by transferring energy to it
from an external source;
• whereas a transistor switch is a device for controlling a relatively large
current between or voltage across two terminals by means of a small
control current or voltage applied at a third terminal
• There are two types of transistor:
(a) bipolar junction transistor (BJT)
(b) field-effect transistors (FET)
Transistors History
• Before transistors were invented, circuits used vacuum tubes:
Fragile, large in size, heavy, generate large quantities of heat, require a
large amount of power.
• The first transistors were created at Bell Telephone Laboratories in 1947
William Shockley, John Bardeen, and Walter Brattain created the
transistors in and effort to develop a technology that would overcome the
problems of tubes.
• The first patents for the principle of a field effect transistor were
registered in 1928 by Julius Lillenfield.
• Shockley, Bardeen, and Brattain had referenced this material in their work.
• Today an advanced microprocessor can have as many as 1.7 billion transistors.
• The device in which conduction takes place due to two types of carriers,
electrons and holes is called a Bipolar Device.
• As p-n junctions exist in the construction of the device, it is a junction device.
• When there is transfer of resistance from input side which is Forward Biased
( low resistance) to output side which is Reverse Biased ( high resistance ), it is a
Trans Resistor or Transistor Device.
• A BJT is formed by joining three sections of semiconductor material, each with a
different doping concentration.
• The three sections can be either a thin n region sandwiched between p+ and
players, or a p region between n and n+ layers, where the superscript “plus”
indicates more heavily doped material. The resulting BJTs are called pnp and npn
transistors, respectively.
• Thus, there are two types of BJT namely the the pnp and the npn.
 |
| npn transistor |
 | ||||||||||||||||||
| pnp transistors | • Emitter: It is more heavily doped to supply majority charge carriers (either electron or | hole) | • Base: It is lightly doped as compared to emitter and collector. It is always constructed | to be very thin, sandwiched between the emitter and the collector. | • Collector: Its function is to collect majority carrier passing through the emitter, | through the base to the collector. | • If the arrow mark is towards the base, it is PNP transistor. If it is away then it is NPN | transistor. The arrow mark on the emitter specifies the direction of current when the | emitter base junction is forward biased. |
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