Transistor

1. UNIT 3
TRANSISTORS

2. WHAT IS ATRANSISTOR?
• Semiconductors: ability to change from
conductor to insulator
• Can either allow current or prohibit current to
flow
• Useful as a switch, but also as an amplifier
• Essential part of many technological
advances.

3. A BRIEF HISTORY
• Guglielmo Marconi invents radio in 1895
• Problem: For long distance travel, signal must be amplified
• Lee De Forest improves on Fleming’s original vacuum tube to amplify
signals
• Made use of third electrode
• Too bulky for most applications

4. THE TRANSISTOR IS BORN
• Bell Labs (1947): Bardeen,
Brattain, and Shockley
• Originally made of
germanium
• Current transistors made of
doped silicon

7. • Construction
• two polarities:
npn and pnp

11. RECALL P-N JUNCTION
N
> 0
-
N
W +
P
W
+
Vappl
Forward bias, + on P, - on N
(Shrink W, Vbi)
Allow holes to jump over barrier
into N region as minority carriers
I
V
P
-
Vappl < 0
Reverse bias, + on N, - on P
(Expand W, Vbi)
Remove holes and electrons away
from depletion region
I
V

13. • Transistor action

16. • In PNP transistor majority charge carriers are holes
• In NPN transistor majority charge carriers are electrons
• Emitter junction always should be forward biased and collector base junction is always
reverse

17. TRANSISTOR CONFIGURATION

20. TRANSISTOR CHARACTERISTICS
• Common Base : Input & Output Characteristics

22. COMMON EMITTER

24. COMMON COLLECTOR

26. CONCLUSION
PARAMETERS CB CE CC
Input Resistance Very low Medium High
Output Resistance Very high Medium Low
Current Gain Less than 1 More than 1 More than 1
Power gain Lower than CE High Higher than CB

27. QUESTIONS
• Explain the working of NPN transistor
• Draw the input and output characteristics of common emitter (CE)
configuration.
• Compare three configurations of transistor.

30. DC CURRENT GAIN
𝑑𝑐 𝐼𝐸
•𝛼 = 𝐼𝑐
(Ratio of Collector Current to Emitter
Current)
𝑑𝑐 𝐼𝐵
• = 𝐼𝑐
(Ratio of Collector Current to Base Current)