Introduction to Bipolar Junction Transistors (BJT) PDF

Microelectronics Chapter 2 Bipolar Junction Transistor (BJT) Bipolar Junction Transistor Chapter 2 (BJT) Table of Contents 2.1 Introduction...............

Microelectronics Chapter 2 Bipolar Junction Transistor (BJT) Bipolar Junction Transistor Chapter 2 (BJT) Table of Contents 2.1 Introduction....................................................................2 2.2 The Action of a BJT.......................................................3 2.3 The Common Base Connection.....................................5 2.4 The Common Emitter Connection..................................7 2.5 The Common Collector Connection...............................8 1 Kolej Laila Taib  Microelectronics Chapter 2 Bipolar Junction Transistor (BJT) 2.1 Introduction The transistor is a semiconductor device that acts as an amplifier or an electronic switch. A transistor consists of a germanium or silicon crystal which contains three separate regions:  two p-type regions separated by an n-type region (p-n-p transistor)  two n-type regions separated by a p-type region (n-p-n transistor) The middle of the three regions is known as the base and the two outer regions are known as the emitter and the collector. The symbols for p-n-p and n-p-n transistors are shown below. The discussion throughout this chapter will be in terms of the n-p-n transistor. 2 Kolej Laila Taib  Microelectronics Chapter 2 Bipolar Junction Transistor (BJT) However, for the corresponding operation of a p-n-p transistor, just read electron for hole, hole for electron, negative for positive, and positive for negative. 3 Kolej Laila Taib  Microelectronics Chapter 2 Bipolar Junction Transistor (BJT) 2.2 The Action of a BJT The basic operation of the transistor will now be described using the p-n-p transistor of Fig. 3.3a. The operation of the npn transistor is exactly the same if the roles played by the electron and hole are interchanged. In Fig. 3.4a the p-n-p transistor has been redrawn without the base-to- collector bias. The depletion region has been reduced in width due to the applied 4 Kolej Laila Taib  Microelectronics Chapter 2 Bipolar Junction Transistor (BJT) bias, resulting in a heavy flow of majority carriers from the p - to the n -type material. Let us now remove the base-to-emitter bias of the p-n-p transistor of Fig. 3.3a as shown in Fig. 3.4b. Recall that the flow of majority carriers is zero, resulting in only a minority-carrier flow, as indicated in Fig. 3.4b. In summary, therefore: One p–n junction of a transistor is reverse-biased, whereas the other is forward-biased. In Fig. 3.5 both biasing potentials have been applied to a p-n-p transistor, with the resulting majority- and minority-carrier flows indicated. Note in Fig. 3.5 the widths of the depletion regions, indicating clearly which junction is forward-biased and which is reverse-biased. As indicated in Fig. 3.5, a large number of majority carriers will diffuse across the forward biased p–n junction into the n -type material. The question then is whether these carriers will contribute directly to the base current IB or pass directly into the p -type material. Since the sandwiched n - type material is very thin and has a low conductivity, a very small number of 5 Kolej Laila Taib  Microelectronics Chapter 2 Bipolar Junction Transistor (BJT) these carriers will take this path of high resistance to the base terminal. The magnitude of the base current is typically on the order of microamperes, as compared to milliamperes for the emitter and collector currents. The larger number of these majority carriers will diffuse across the reverse-biased junction into the p -type material connected to the collector terminal as indicated in Fig. 3.5. The reason for the relative ease with which the majority carriers can cross the reverse-biased junction is easily understood if we consider that for the reverse-biased diode the injected majority carriers will appear as minority carriers in the n -type material. In other words, there has been an injection of minority carriers into the n - type base region material. Combining this with the fact that all the minority carriers in the depletion region will cross the reverse-biased junction of a diode accounts for the flow indicated in Fig. 3.5. Applying Kirchhoff’s current law to the transistor of Fig. 3.5 as if it were a single node, we obtain and find that the emitter current is the sum of the collector and base currents. The collector current, however, comprises two components—the majority and the minority carriers as indicated in Fig. 3.5. The minority-current component is called the leakage current and is given the symbol I CO (IC current with emitter terminal Open). The collector current, therefore, is determined in total by 6 Kolej Laila Taib  Microelectronics Chapter 2 Bipolar Junction Transistor (BJT) For general-purpose transistors, IC is measured in milliamperes and ICO is measured in microamperes or nanoamperes. I CO, like I s for a reverse-biased diode, is temperature sensitive and must be examined carefully when applications of wide temperature ranges are considered. It can severely affect the stability of a system at high temperature if not considered properly. Improvements in construction techniques have resulted in significantly lower levels of ICO, to the point where its effect can often be ignored. 7 Kolej Laila Taib  Microelectronics Chapter 2 Bipolar Junction Transistor (BJT) 2.3 The Common Base Connection The notation and symbols used in conjunction with the transistor in the majority of texts and manuals published today are indicated in Fig. 3.6 for the common-base configuration with p-n-p and n-p-n transistors. The common-base terminology is derived from the fact that the base is common to both the input and output sides of the configuration. In addition, the base is usually the terminal closest to, or at, ground potential. Throughout this text all current directions will refer to conventional (hole) flow rather than electron flow. The result is that the arrows in all electronic symbols have a direction defined by this convention. 8 Kolej Laila Taib  Microelectronics Chapter 2 Bipolar Junction Transistor (BJT) Recall that the arrow in the diode symbol defined the direction of conduction for conventional current. For the transistor: The arrow in the graphic symbol defines the direction of emitter current (conventional flow) through the device. All the current directions appearing in Fig. 3.6 are the actual directions as defined by the choice of conventional flow. Note in each case that I E = IC + IB. Note also that the applied biasing (voltage sources) is such as to establish current in the direction indicated for each branch. That is, compare the direction of IE to the polarity of VEE for each configuration and the direction of IC to the polarity of VCC. 9 Kolej Laila Taib  Microelectronics Chapter 2 Bipolar Junction Transistor (BJT) 2.4 The Common Emitter Connection The most frequently encountered transistor configuration appears in Fig. 3.12 for the p-n-p and n-p-n transistors. It is called the common-emitter configuration because the emitter is common to both the input and output terminals (in this case common to both the base and collector terminals). Two sets of characteristics are again necessary to describe fully the behavior of the common emitter configuration: one for the input or base–emitter circuit and one for the output or collector–emitter circuit. 10 Kolej Laila Taib  Microelectronics Chapter 2 Bipolar Junction Transistor (BJT) 2.5 The Common Collector Connection The third and final transistor configuration is the common-collector configuration, shown in Fig. 3.20 with the proper current directions and voltage notation. The common-collector configuration is used primarily for impedance-matching purposes since it has a high input impedance and low output impedance, opposite to that of the common-base and common-emitter configurations. 11 Kolej Laila Taib  Microelectronics Chapter 2 Bipolar Junction Transistor (BJT) Summary Chart Parameter Common Base (CB) Common Emitter (CE) Common Collector (CC) Input Resistance Low Moderate High Output Resistance High Moderate Low Current Gain (β) Low (α) High (β) High (β) Voltage Gain Moderate High Low (≈ 1) Phase Shift 180° 180° 0° RF amplifiers, General-purpose amplifiers, Voltage buffers, Applications impedance matching switching circuits impedance matching 12 Kolej Laila Taib  Microelectronics Chapter 2 Bipolar Junction Transistor (BJT) 13 Kolej Laila Taib