Understanding Bipolar Junction Transistors: Construction and Operating Principles

2. chapter 2 transistors (BJT) • 2.1 Transistor classification • 2.2 Bipolar junction transistors (BJT) construction • 2.3 Transistor action and operating • 2.4 Quiescent Operating Point • 2.5 Bipolar transistor characteristics • 2.6 Transistor parameters • 2.7 Current gain • 2.8 Typical BJT characteristics and maximum ratings • 2.9 Transistor operating configurations

3. 2.1 Transistor classification

4. 2.2 Bipolar junction transistors (BJT) construction • Bipolar transistors generally comprise n-p-n or p-n-p junctions of either silicon (Si) or germanium (Ge) material. • N: phosphorus or arsenic P: boron or gallium • The junctions are, in fact, produced in a single slice of silicon by diffusing impurities through a photographically reduced mask. • Silicon transistors are superior when compared with germanium transistors in the vast majority of applications

5. ◆The symbols and simplified junction models for n-p-n and p-n-p transistors are shown in Figure 2.3. It is important to note that the base region is extremely narrow. • Figure 2.3 The symbols and simplified junction models for n-p-n and p-n-p transistors

6. E – Emitter B – Base C - Collector Electronics-BTEC Slide - 6

7. 2.3 Transistor action • ◆In the n-p-n transistor, transistor action is accounted for as follows: • ◆ the base-emitter junction is forward biased and the base-collector junction is reverse biased • ◆ Around 99.5% of the electrons leaving the emitter will cross the Base collector junction and only 0.5% of the electrons will Recombine with holes in the narrow base region. • Figure 2.4 Transistor action of n-p-n transistor

9. ◆the base-emitter junction is forward biased and the base-collector junction is reverse biased • Figure 2.5 Transistor action of p-n-p transistor

10. 2.2.2 leakage current • ◆For an n-p-n transistor, the base-collector junction is reversed biased for majority carriers, but a small leakage current, ICBO , flows from the collector to the base due to thermally generated minority carriers (holes in the • collector and electrons in the base), being present. The base-collector junction is forward biased to these minority carriers. • ◆ With modern transistors, leakage current is usually very small (typically less than 100nA) and in most applications it can be ignored. • ◆ The control of current from emitter to collector is largely independent of the collector-base voltage and almost wholly governed by the emitter-base voltage.

11. 2.2.3 bias and current flow • ◆In normal operation (i.e. for operation as a linear amplifier) the base-emitter junction of a transistor is forward biased and the collector-base junction is reverse biased. • ◆The current flowing in the emitter circuitis typically 100 times greater than that flowing in the base. • Figure 2.7 bias and current flow

12. Leakage current ICBO 2.2.3 bias and current flow

13. 2.2.3 bias and current flow

14. 2.2.4 Transistor operating configurations • ◆Three basic circuit configurations are used for transistor amplifiers. • ◆ These three circuit configurations depend upon which one of the three transistor connections is made common to both the input and the output. • ◆ In the case of bipolar junction transistors, the configurations are known as common emitter, common collector (or emitter follower), and common base. • Figure 2.8 Transistor operating configurations

15. 2.2.5 bipolar transistor characteristics • ◆The characteristics of a bipolar junction transistor are usually presented in the form of a set of graphs relating voltage and current present at the transistors terminals. • Figure 2.9 measurement circuit of bipolar transistor characteristics

16. ◆In this mode, the input current is applied to the base and the output current appears in the collector. • Figure 2.10 Typical input characteristic

17. ◆ Each curve corresponds to a different value of base current. Note the ‘knee’ in the characteristic below VCE =2V. • ◆ Also note that the curves are quite flat. • ◆ For this reason (i.e. since the collector current does not change very much as the collector-emitter voltagechanges) we often refer to this as a constant current characteristic. • Figure 2.11 Output characteristics

18. ◆ Here IC is plotted against IB for a small-signal general-purpose transistor. • ◆ The slope of this curve (i.e. the ratio of IC to IB) is the common-emitter current gain of the transistor. • Figure 2.12 Transfer characteristic

19. 2.2.6 Bipolar transistor parameters • ◆In particular, the three characteristic graphs can be used to determine the following parameters for operation in common-emitter mode:

20. 2.2.6 Bipolar transistor parameters • ◆In particular, the three characteristic graphs can be used to determine the following parameters for operation in common-emitter mode:

21. 2.2.6 Bipolar transistor parameters • ◆In particular, the three characteristic graphs can be used to determine the following parameters for operation in common-emitter mode:

22. 2.2.6 Bipolar transistor parameters

24. 2.2.7 Current gain • ◆We use the symbol hFE to represent the static value of common-emitter current gain. • ◆ Similarly, we use hfe to represent the dynamic value of common-emitter current gain. • ◆ Note that hFE is found from corresponding static values while hfe is found by measuring the slope of the graph. • ◆ Furthermore, most transistor parameters (particularly common-emitter current gain, hfe) are liable to wide variation from one device to the next.

25. 2.2.8 Typical BJT characteristics and maximum ratings • Table 2.2 Transistor characteristics and maximum ratings PTOTmax is the maximum device power dissipation.

26. 2.4 The junction field-effect transistor • Figure 2.13 Conformation of N channel J.F.E.T

27. 2.4 The junction field-effect transistor • N channel JFET • P channel JFET • Figure 2.14 Symbol of JFET

28. 2.4 The junction field-effect transistor • Figure 2.15 Operation of N channel JFET

29. 2.5 Metal-oxide-semiconductor field-effect transistor 2.5.1 depletion-type MOS FET • N channel • P channel Construction of N channel depletion-type MOS FET • Figure 2.16 depletion-type MOS FET

30. 2.5 Metal-oxide-semiconductor field-effect transistor 2.5.2 Enhancement-type MOS FET • N channel • P channel Construction of N channel enhancement-type MOS FET • Figure 2.16 depletion-type MOS FET

31. 2.4 Quiescent Operating Point