1. BJT in Saturation Mode
Section 4.5

2. Schedule 9 2/11 Tuesday Physics of a BJT 4.1-4.3 L
Measure Beta of a transistor 10
2/13
Thursday
PNP
4.3, 11
2/18
BJT in saturation
4.5
BJT in saturation/BJT implementation of an NAND gate 12
2/20
Small signal model [homework: small eq. circuit]

3. Outline Modes of Operations Review of BJT in the active Region
BJT in Saturation Mode
Digital Integrated Circuits

4. Modes of Operation BE\BC Forward Biased Reverse Biased Saturation
Active
Reverse Active Mode
Cut-Off
Applications:
1. Saturation and cut-off mode are used in digital circuits.
2. Active mode is used in the amplifier design.

5. Voltage and Current Polarities of NPN and PNP transistors
A “fat” voltage between collector and emitter voltage places a transistor in the active region!
A “skinny” voltage between collector and emitter voltage places a transistor in the active region!

6. Review

7. Review
PN Junction
Reverse Bias
Forward Bias
BJT in the Active Mode

8. Review: Forward Bias Diode;
Depletion region shrinks due to charges from the battery.
The electric field is weaker.
Majority carrier can cross via diffusion;
Greater diffusion current.
Current flows from P side to N side

9. Review: PN Junction under Reverse Bias
Reverse: Connect
the + terminal to the
n side.
Depletion region widens.
Therefore, stronger E.
Minority carriers cross
the PN junction easily
through diffusion.
Current is composed
mostly of drift current contributed
by minority carriers.
np to the left and pn to the right.
Current from n side to p side,
the current is negative. E

10. Operation of an NPN Transistor in the Active Region
Electrons
are injected
into the BC junction
Electrons are injected into the B; holes to the E.
Electrons
are swept across
the reversed biased BC

11. Thin Base Region
The base region is made thin in order to reduce recombination
as electrons travel from BE junction to BC junction.

12. Highly Doped Emitter In order to emphasize the current contribution
due to the electrons (which can cross the BC junction),
the emitter is heavily doped by N type materials.

13. Base Current The proportional of hole current and electron current
is determined by dopants (ND and NA).
Even though the presence of holes are minimized, a small
number holes still must enter through the base.

14. Electrons in the Base Electrons are swept Into the collector;
low electron density at x2
Electrons injected into
the base; high electron
density at x1.
The electron gradient allows electrons to travel through diffusion.

15. Recombination Recombination
Base must supply holes that will enter the emitter and
for recombination with the electrons.

16. Extension of a PNP transistor
(NPN transistor)
(PNP transistor)
Base-emitter junction is forward
biased.
Holes are injected into the base.
Base-emitter junction is reverse
Biased.
Injected holes in the base is swept
across the base-collector junction by
the electric field.

17. BJT Current
Assumption:
BEJ: Forward Biased
BCJ: Reverse Biased

18. Large Signal Model of a BJT
Called “large” signal model
because this model is
applicable even if VBE
changes from 300 mV to 800 mV

19. Large-Signal Model of BJT Transistors
(NPN)
(PNP) C C E E

20. Experiments

21. Saturation Mode

22. BJT in Saturation Mode Key assumption so far: BE=Forward Biased
BC=Reverse Biased
What happens when these assumptions are not true?

23. Review: Forward Bias Diode;
Depletion region shrinks due to charges from the battery.
The electric field is weaker.
Majority carrier can cross the junction via diffusion;
Greater diffusion current.
Current flows from P side to N side

24. Hole Current into the Collector
A reverse biased BCJ keeps
holes in the base.
But as BCJ becomes forward
biased, the strong electric field
which opposes of the movement
of holes into the collector is weakened.
There is now a hole current into the collector.
Net Result: heavy saturation leads to a sharp rise in the base current and a rapid
fall in β.

25. A Large Signal Model of the BJT
The net collector current decreases as the collector enter into saturation

26. General Rules
As a rule of thumb, we permit soft saturation with VBC <400 mV because the current in the B-C junction is negligible, provided that various tolerances in the component values do not drive the device into deep saturation.
For a device in soft saturation or active region, we approximate IC as Isexp(VBE/VT)
In the deep saturation region, the collector-emitter voltage approaches a constant value called VCE, SAT (about 200 mV).

27. Voltage and Current Polarities of NPN and PNP transistors
A “fat” voltage between collector and emitter voltage places a transistor in the active region!
A “skinny” voltage between collector and emitter voltage places a transistor in the active region!

28. Use 2n3904 npn BJT in Simulation
(Error!, put 2n3904 here!)

29. Include 2n3904 (NPN) model

30. A NAND Gate Implemented With NPN Transistors

31. Optional Slides

32. BJT Inverter
(Define the input voltage as a variable)

33. Run Parametric Analysis

34. Parametric Analysis

35. Select a Wire to Plot

36. Use Calculator to Plot

37. Plot with Calculator (Under Tools)

38. RTL (Resistor-Transistor Logic)
Vout VA VB
First introduced in 1962! (50 years ago!)
What is the logic function?

39. RTL Based NOR A B Vout 3.6 V 34.05 mV 0 V 42.59 mV 0V
NOR is an universal gate! If you can build a NOR, you can build any logic.

40. Diode-Transistor Logic
What is the logic function?
This resistor allow charges
to be drained from the base

41. Sweep VB
VS: the input voltage at which the output is approximately 2V.
VS~2V
Condition: VA=4V, VC=4V. VB is swept from 0 to 4V

42. Diode-Transistor Logic
This resistor allow charges
to be drained from the base

43. Sweep VB Fixed VA=4V VCC=4V Sweep VB from 0 to 4 V
Increase the VS by about one diode drop.

44. Basic TTL Gate
Diode is replaced by TTL
A “relative “ of 7400LS Gate

45. Sweep VB
Fixed VA=4V
VCC=4V
Sweep VB from 0 to 4 V

46. 7400 NAND Gate
7400 Schematic
We will revisit this schematic in a couple of weeks!