how bipolar transistor

2. N Type P Type
extra electrons holes (missing electrons)
depletion zone

3. N P N
emitter base collector

4. N P N
emitter base collector

5. Electron Flow
N P N
emitter base collector

6. Electron Flow
N P N
emitter base collector

7. • Emitter is grounded (zero voltage)
• Base has a small positive voltage
• Collector has a large positive voltage
• Electrons flow from emitter into P layer.
• Base (P layer) is lightly doped, so few
holes get filled by extra electrons.
• Most electrons build up in base (P layer).

8. Electron Flow
N P N
emitter base collector

9. • Positive voltage at base and collector
attract electrons in middle P layer.
• Some current flows from emitter to base.
• Most current flows from emitter to collector.
• Base voltage determines flow from emitter.
– If base voltage is too low, no current flows from
emitter to base or from emitter to collector.
– The higher the base voltage, the more base
current and collector current flows.
• Bipolar transistors are current amplifiers.

10. Hole Flow
N P N
emitter base collector

11. P N P
emitter base collector

12. P N P
emitter base collector

13. Current for Hole Flow
P N P
emitter base collector

14. Current for Electron Flow
P N P
emitter base collector

15. • Bipolar transistors act like 2 back-to-back
diodes
• base-emitter diode is forward biased
– current can flow from base to emitter
• base-collector diode is reverse biased
– current can not flow from collector to base
• some leakage current will flow
– if base current flows, causes collector to
emitter current to flow

16. No signal voltage on base,
So transistor turned off.
No base current
(emitter to base)

17. No signal voltage on base,
So transistor turned off.
Current can NOT flow from
Collector to base to input
No base to emitter current.

18. All supply voltage dropped on R2
because transistor is an open
No signal voltage on base,
So transistor turned off.
No collector to emitter current
No voltage from output to ground,
so no current flows
No base to emitter current.

19. Small signal voltage applied to base,
Transistor turned on.
LARGE current must flow to output
collector current flows
small base current flows

20. All supply voltage dropped on R2
All supply current to output
No collector to emitter current

21. No voltage on base
transistor is off

22. Voltage on the base
turns transistor turns on
small base current causes LARGE
collector current

23. Replace NPN with PNP and just
reverse the polarity

24. Replace NPN with PNP and just
reverse the polarity

25. Switches or Regulators
• Transistors are manufactured to be either
switches or regulators.
• Switches need to turn on and off FAST!
• Need enough base bias voltage to
overcome barrier
– 0.7 V for silicon transistor
– 0.3 V for germanium transistor

26. Switches or Regulators
• How does a transistor work as a regulator?

27. DC
AC voltage on base
causes AC current

28. DC
AC voltage on base
causes AC current

29. DC

30. DC
open base
allows collector
current
to flow

31. Small input signal current results in
larger output signal current

32. Output signal current is at same
frequency as input but larger
amplitude
Regulator transistors
can act as amplifiers