1. Bipolar Junction Transistor
Chapter 6
Lecture 7 & 8

2. N P N The bipolar junction transistor has 3 doped regions.
COLLECTOR (medium doping) P
BASE (light doping) N
EMITTER (heavy doping)

3. In a properly biased NPN transistor, the emitter electrons
diffuse into the base and then go on to the collector. RC N
VCE RB P
VCC N
VBE
VBB

4. Bipolar Junction Transistors (BJTs)
The bipolar junction transistor is a semiconductor device constructed with three doped regions.
These regions essentially form two ‘back-to- back’ p-n junctions in the same block of semiconductor material (silicon).
The most common use of the BJT is in linear amplifier circuits (linear means that the output is proportional to input). It can also be used as a switch (in, for example, logic circuits). 
The description ‘bipolar’ arises because the device depends on the flow of both majority and minority carriers through the device. (bi means two). We shall look at field effect transistors later. They rely on just one carrier type (e.g. electrons) and can be regarded as ‘unipolar’ (uni means one).
Smith and Dorf pages , , (bias)

5. npn-BJT Structure n-type p-type
The ‘npn’ version of the BJT consists of two n regions separated by a p region (as the name suggests). A schematic of an npn transistor is shown.
n-type
p-type

6. BJT Structure
The three regions are known as the emitter, base and collector regions.
Electrical connections are made to each of these regions.

7. npn-BJT Structure E Emitter (n-type) Base (p-type) Collector (n-type)

8. npn BJT Symbol

9. npn BJT Symbol E B C

10. pnp BJT Symbol
In the symbol for a pnp BJT transistor the direction of the arrow on the emitter is reversed E B C

11. Still remember about BJT?
The emitter current (iE) is the sum of the collector current (iC) and the base current (iB)
iB << iE and iC OTHER PRAMETERS &
EQUATIONS?

12. BJT Basic structure and schematic symbol pnp type npn type approximate
equivalents
transistor
symbols
pnp type
npn type

13. Refresh.. Common-emitter current gain, β Common-base current gain, α
Range: 50 < β < 300
Common-base current gain, α
Range: always slightly less than 1
The current relationship between these 2 parameters are as follows:

14. Refresh.. BJT as amplifying device B-E junction is forward-biased
B-C junction is reverse-biased

15. BIASING OF BJT Remember…! for normal operation AND
emitter-base junction is always forward-biased
AND
collector-base junction is always reverse-biased

16. FORWARD BIASING E/B JUNCTION

17. REVERSE BIASING C/B JUNCTION

18. BIASING NPN TRANSISTOR

19. Common-Emitter Circuit
with an npn transistor
with a pnp transistor
with a pnp transistor biased with a positive voltage source

20. IC IB IE IC IB IE
Conventional flow
Electron flow
IE = IC + IB IE
IB << IC
adc = IC IE
bdc = IC IB

21. RC RB VCE VCC VBE VBB The common emitter connection has two loops:
the base loop and the collector loop. RC RB
VCE
VCC
VBE
VBB

22. Subscript notation
When the subscripts are the same, the voltage represents a source (VCC).
When the subscripts are different, the voltage is between two points (VCE).
Single subscripts are used for node voltages with ground serving as the reference (VC).

23. The base circuit is usually analyzed with the
same approximation used for diodes.
VBB - VBE RC
IB = RB
VCE
VCC RB
VBE
VBB

24. A graph of IC versus VCE 100 mA 14 12 80 mA 10 60 mA IC in mA 8 40 mA
(Note that each new value of IB presents a new curve.)
100 mA 14 12
80 mA 10
60 mA
IC in mA 8
40 mA 6 4
20 mA 2
0 mA 2 4 6 8 10 12 14 16 18
VCE in Volts
This set of curves is also called a family of curves.

25. Regions of operation Cutoff - - - used in switching applications
Active used for linear amplification
Saturation used in switching applications
Breakdown can destroy the transistor

26. Transistor circuit approximations
First: treat the base-emitter diode as ideal and use bIB to determine IC.
Second: correct for VBE and use bIB to determine IC.
Third (and higher): correct for bulk resistance and other effects. Usually accomplished by computer simulation.

27. The second approximation:
VBE = 0.7 V
bdcIB
VCE

28. IB =
VBB - VBE RB
5 V V
IB =
= 43 mA RC
100 kW
100 kW
VCC RB
VBE = 0.7 V
VBB
5 V

29. IC = bdc IB
IC = 100 x 43 mA = 4.3 mA RC
100 kW
bdc = 100
VCC RB
IB = 43 mA
VBB
5 V

30. VRC = IC x RC
VRC = 4.3 mA x 1 kW = 4.3 V
1 kW RC
IC = 4.3 mA
100 kW
12 V
VCC RB
IB = 43 mA
VBB
5 V

31. IC = 4.3 mA
VCE = VCC - VRC
VCE = 12 V V = 7.7 V
1 kW RC
VCE
100 kW
12 V
VCC RB
IB = 43 mA
VBB
5 V

32. Typical Breakdown Ratings
VCB = 60 V
VCEO = 40 V
VEB = V
Note: these are reverse breakdown ratings

33. A graphic view of collector breakdown14 12 10
IC in mA 8 6 4 2 50
VCE in Volts

34. Typical Maximum Ratings
IC = mA dc
PD = 250 mW (for TA = 60 oC)
PD = 350 mW (for TA = 25 oC)
PD = 1 W (for TC = 60 oC)

35. Typical “On Characteristics”
IC in mA hFE(min) hFE(max) ___ 1 70 ___ ___ ___

36. Troubleshooting Look for gross voltage errors.
First approximation and mental estimates will usually suffice.
Resistors don’t short but circuit boards can.
Circuit boards can and do open.
Junctions can and do short.
Junctions can and do open.