1. 1 Bipolar Junction Transistors (BJTs)

2. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith2 Introduction - The invention of the BJT in 1947 at the Bell Laboratories ushered in the era of solid-state circuits. Bardeen, Shockley, and Brattain at Bell Labs - Brattain and Bardeen invented the bipolar transistor in 1947. The first germanium bipolar transistor. Roughly 50 years later, electronics account for 10% (4 trillion dollars) of the world GDP.

3. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith3 BJT >> MOSFET until 1980s. Now, BJT << MOSFET, CMOS Nevertheless, the BJT remains a significant device that excels in certain applications. - For instance, the reliability of BJT circuits under severe environmental conditions makes them the dominant devices in automobile electronics, an important and still-growing area. - The BJT remains popular in discrete-circuit design, in which a very wide selection of BJT types are available to the design. The characteristics of the BJT are so well understood that we can design transistor circuits whose performance is remarkably predicted and quite insensitive to variations in device parameters. The BJT is still preferred device in RF circuits, very-high-speed digital logic circuit family, such as emitter-coupled logic. BJT+CMOS = BiCMOS : high-input impedance, low-power operation (MOSFET) very-high-frequency operation, high-current-driving capability (BJT) Chap. 6, 7, 9, 11

4. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith4 Figure 5.1 A simplified structure of the npn transistor. 5.1 Device Structure and Physical Operation Figure 5.2 A simplified structure of the pnp transistor. Amplifier Switch Limited application Electrons and holes participate in the current-conduction process. – Bipolar!

5. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith5 Figure 5.3 Current flow in an npn transistor biased to operate in the active mode. (Reverse current components due to drift of thermally generated minority carriers are not shown.) 5.1.2 Operation of the npn Transistor in the Active Mode Heavily doped emitter Lightly doped base V V Figure 5.4 Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor operating in the active mode: v BE  0 and v CB  0. Very thin! Thin enough to neglect recombination. Should be zero since the positive collector voltage causes the electron at the end to be swept across CBJ depletion region. 1. Due to forward-biased EB junction, current flows. - Base current by recombined holes and electrons. (small) - This means E, B act as a resistance that is function of the base voltage V BE. 2. Then, electrons from V CB arrives at BC junction. 3. Electrons flow to C due to V CB. 4. Collector current from C to E! 5. This Ic is a function of base voltage! 6. Amplifier if V CE > V BE !!!

6. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith6 The Collector Current Since base region is very thin and lightly doped, * I C is; - inversely proportional to the base width W. - Proportional to the area of the EBJ (scale current). - Typically in the range of 10 -12 A to 10 -18 A. - Proportional to n i 2. (doubling for every 5 o C rise) The Base Current - Base current = current due to holes injected from base to emitter.(i B1 ) + current due to holes to supply recombined holes (i B2 ).

7. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith7 * β is; - Highly depend on the base width W. - Highly depend on relative dopings (N A /N D ) The Emitter Current Recapitulation and Equivalent Circuit model

8. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith8 5.1.3 Structure of Actual Transistors Figure 5.6 Cross-section of an npn BJT. Figure 5.7 Model for the npn transistor when operated in the reverse active mode (i.e., with the CBJ forward biased and the EBJ reverse biased).

9. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith9 5.2 Current-Voltage Characteristics 5.2.1 Circuit Symbols and Conventions Figure 5.13 Circuit symbols for BJTs. Figure 5.14 Voltage polarities and current flow in transistors biased in the active mode. An npn transistor whose EBJ is forward biased will operate in the active mode as long as the collector voltage does not fall below that of the base by more than 0.4 V. Otherwise, saturation mode. An pnp transistor whose EBJ is forward biased will operate in the active mode as long as the collector voltage does not allowed to rise above that of the base by more than 0.4 V. Otherwise, saturation mode. BJT voltage-current relationship in the active mode

10. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith10 Figure 5.15 Circuit for Example 5.1. EXAMPLE 5.1 Design the circuit such that i C =2 mA, υ C =5 V. CBJ is reverse biased, active mode!

11. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith11 Figure 5.19 (a) Conceptual circuit for measuring the i C – v CE characteristics of the BJT. (b) The i C – v CE characteristics of a practical BJT. 5.2.3 Dependence of i C on the Collector Voltage -The Early Effect Common Emitter Configuration Due to the change of effective base width cf) channel length modulation in MOSFET Figure 5.20 Large-signal equivalent-circuit models of an npn BJT operating in the active mode in the common-emitter configuration. It is rarely necessary to include the dependence of i C on υ CE in dc bias design and analysis. However, The finite output resistance can have a significant effect on the gain.

12. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith12 Figure 5.21 Common-emitter characteristics. Note that the horizontal scale is expanded around the origin to show the saturation region in some detail. 5.2.4 The Common-Emitter Characteristics. The Common-Emitter Current gain β V CEsat and R CEsat Figure 5.23 An expanded view of the common-emitter characteristics in the saturation region. ΔiCΔiC ΔiCΔiC Therefore, β in saturation mode is smaller than β in active mode! To make a transistor operate in saturation mode (switch mode, given I C =I Csat ), designer should establish V CE ≈0.1~0.2 V, and

13. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith13 Figure 5.24 (a) An npn transistor operated in saturation mode with a constant base current I B. (b) The i C – v CE characteristic curve corresponding to i B = I B. The curve can be approximated by a straight line of slope 1/R CEsat. (c) Equivalent-circuit representation of the saturated transistor. (d) A simplified equivalent-circuit model of the saturated transistor.

14. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith14 5.3 The BJT as an Amplifier and as a Switch

15. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith15 Figure 5.26 (a) Basic common-emitter amplifier circuit. 5.3.1 Large-Signal Operation-The Transfer Characteristic (b) Transfer characteristic of the circuit in (a). The amplifier is biased at a point Q, and a small voltage signal v i is superimposed on the dc bias voltage V BE. The resulting output signal v o appears superimposed on the dc collector voltage V CE. The amplitude of v o is larger than that of v i by the voltage gain A v. R C : to establish a desired dc bias voltage. to convert the collector signal current i c to an output voltage.

16. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith16 5.3.2 Amplifier Gain Q (quiescent) point determined by a bias voltage, R C and V BE. - The voltage gain of CE amp is the ratio of the dc voltage drop across R C to the thermal voltage V T (~25 mV at room temp). - For a given V CC, to increase V RC, we have to operate at a lower V CE. - But, too low V CE results in negative peaks of output will be flattened.

17. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith17 EXAMPLE 5.10 Find the voltage at all nodes and currents through all the branches. (β = 100) 1. Find Thevenin’s equivalent circuit of base bias circuit. 2. Find I B and I E using loop equation around L.

18. Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith18 Figure 5.60 (a) A common-emitter amplifier using the structure of Fig. 5.59. (b) Equivalent circuit obtained by replacing the transistor with its hybrid-  model. Figure 4.43 (a) Common-source amplifier based on the circuit of Fig. 4.42. (b) Equivalent circuit of the amplifier for small-signal analysis.