Electronics Engineering » Notes » Transistors
1. Transistors are three terminal devices that can be formed with the combination of two separate PN junction materials into one block.
2. An NPN transistor is formed with two PN junctions with the P−type material at the center, whereas a PNP transistor is formed with two PN junctions with the N−type material at the center.
3. The three terminals of a transistor, whether it is an NPN or PNP transistor, are identified as the emitter, the base, and the collector.
4. Transistors are used either as amplifiers or as electronic switches.
5. Like junction diodes, most transistors are made of silicon. Gallium Arsenide (GaAs) technology has been under development for several years and its advantage over silicon is its speed, about six times faster than silicon, and lower power consumption. The disadvantages of GaAs over silicon is that arsenic, being a deadly poison, requires very special manufacturing processes.
6. Since a transistor is a 3−terminal device, there are three currents, the base current, denoted as iB, the collector current, denoted as iC, and the emitter current, denoted as iE.
7. For any transistor, NPN or PNP, the three currentsare related as :
iB + iC = iE
8. A very useful parameter in transistors is the common−emitter gain β , a constant whose value ranges from 75 to 300. Its value is specified by the manufacturer. The base current iB is much smaller than the collector current iC and these two currents are related in terms of the constant β as :
iB = iC/β
9. Another important parameter in transistors is the common−base current gain denoted as α and relates the collector and emitter currents as :
iC = αiE
10. The parameters and are related as :
α = β/1 + β
β = α/1 – α
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- Transistors » Exercise - 1 20. For N-P-N transistor the value of β in common base is 100. The value of current gain α in common emitter is - (a) 0.99 (b) 0.92 (c) 1.01 (d) 0.95
- Transistors » Exercise - 3 117. For P-N-P transistor the value of α in common base is 0.98. the value of current gain β in common emitter is - (a) 4.9 (b) 49 (c) 0.49 (d) 0.98
- Transistors » Exercise - 3 112. The correct relationship between emitter current (IE) base current (IB)and collector current (IC) for a transistor ? (a) IE – IC =IB (b) IC + IE =IB (c) IC – IB =IE (d) IE + IB =IC