5 Bipolar Junction Transistors Bipolar Junction Transistors (BJT’s) are three terminal devices in which voltage applied across the two terminals control the current flowing in the third terminal. The BJT can be used as both a voltage-controlled current source (analog device) and a voltage-controlled switch (digital device). The Bipolar Junction Transistor three terminals are labeled Emitter (E), Base (B), and Collector (C), and are connected to the three regions within a monolithic (single-piece) structure. The name Bipolar implies, namely both mobile carriers, electron and holes, are contributing to the charge transport, hence current. The Bipolar Junction Transistor is called an npn transistor If the transistor’s emitter region n-type, base region p-type, and collector region are n-type semiconductors. Likewise, the tr ansistor is pnp transistor if the transistor’s emitter region p-type, base region, n-type, and collector region, p-type semiconductor materials. Silicon is the dominant material in transistor fabrication because of temperature and breakdown voltage advantages over other semiconductor materials. BJTs are available in different packaging scheme. These packaging schemes are based on three broad categories of BJTs, namely general-purpose transistors, power transistors and RF (radio frequency and microwave) transistors. General-purpose or small-signal transistors are used for low or medium-power amplifiers or switching circuits. They are packaged either in plastic cases as shown in figure 5-1 for both npn and pnp transistor, or metal cases as shown in figure 5-2 for npn transistor and for pnp transistor as shown in figure 5-3.2 Bipolar Junction Transistors Chapter 5 Figure 5-1: npn TO-92 or TO-226AA and pnpTO 92- or TO 226AE Figure 5-2: npnTO-18 or TO-206AA, TO-52 or TO-206AC, TO39-or TO-205AD and Pin configuration bottom view Emitter is closest to tab Figure 5-3: pnpTO-46 or TO-206AB and Pin configuration bottom view Emitter is closest to tab Certain types of packages contain multiple transistors as shown in figure 5-4 for dual metal can and dual ceramic flat-peak as shown in figure 5-5. npnChapter 5 Microelectronic Circuits Analysis and Design 3 Figure 5-4: Dual metal can npn Figure 5.5: Dual ceramic flat-peak The SOT packaging scheme is shown in figure 5-6. Figure 5.6: SOT-23 or TO-236AB Some of the multiple transistor packages such as the dual-in-line (Dip) as shown in figure 5-7 and small-outline (SO) are the same as those used for many integrated circuit as shown in figure 5-8. Figure 5-7: Quad dual in-line (DIP) and quad flat-pack. Dot indicates pin 14 Bipolar Junction Transistors Chapter 5 Figure 5-8: Quad small outline (SO) package for surface-mount technology Power transistors are used to handle current more than 1 Ampere and or large voltages. Some package configuration for power amplifier and their pin assignments are shown in figure 5-9. Figure 5-9: TO-3 or TO-204AE, TO-225AA, Surface-mount technology, TO-218AC, TO-220AB and TO-218 RF transistors are used in communications systems, designed to operate at extremely high frequencies. Gallium arsenide GaAs) transistor is used for microwave (very high frequency). Their packaging are designed to optimized certain high-frequency parameters. Some of their packaging schemes are shown in figure 5-10. Figure 5-10: RF transistors After the removal of packaging material, pending the transistor is fabricated as a discrete or integrated circuit structure as shown in figure 5-11 and 5-12 will revealChapter 5 Microelectronic Circuits Analysis and Design 5 n+p-substratepn+nn+Collector EmitterBaseMetal02Si Figure 5-11: Planar-diffused IC bipolar junction transistor structure of a few squares micrometer -substratepn+nCollectorEmitterBasen+Metal2SioMetal Figure 5-12: Planar-diffused discrete transistor structure of several squares millimeter The one dimensional idealized pnp+ and npn+ are shown in figure 5-13 and 5-14. Emitter CollectorBasepnCEBp++−++−−ECVEBVCBVCIEI−++−ECVEBCIII=+pnp+ECEBCBVVV=−BEC+−EBVBCVBI Figure 5-13: Idealized pnp+Bipolar Junction Transistor6 Bipolar Junction Transistors Chapter 5 Emitter CollectorBasenpCEBn++−−−++CEVBEVBCVCIEI+−−+CEVEBCIII=+npn+CE BE BCVVV=−BEC−+BEVCBVBI Figure 5-14: Idealized npn+Bipolar Junction Transistor When the arrow points away from the base, it signifies npn+ transistor and when the arrow points into the base, it signifies pnp+transistors.Chapter 5 Microelectronic Circuits Analysis and Design 7 xx()0Bn()0nEP0nEPn+EmitternCollecorpBaseEffective basenCnCxEx−0x=BWxxxρCEFEiEVEρEEBECV0nCpnEn()pBnWDepletionregionDENDepletionregionABNEmitterBaseCollectorDCN Figure 5-15: A simplified structure of the npn+ bipolar junction transistor, its energy band diagram, voltage, electric field, change density and carrier distribution in various regions in active mode of operation.8 Bipolar Junction Transistors Chapter 5 Example 5-1 To come Solution: Bipolar Junction Transistor modes of operation The Bipolar Junction transistors can be viewed as two pn junction diodes connected, back-to-back, in series, namely the emitter–base junction diode and the collector-base junction diode. The different modes of operation of bipolar junction transistor are possible by forward or reverse biasing the two junction as shown in table 1-1. Mode of operation E-B junction C-B junction Saturation Forward bias Forward bias Active Forward bias Reverse bias Reverse active Reverse bias Forward bias Cutoff Reverse bias Reverse bias Table 5-1: Bipolar junction modes of operation and their junctions biasing The combination of the bipolar junction transistor emitter-base junction and collector-base junction voltages for the four modes of operations are given in Figure 5-16. The junction voltage polarities are a given for both pnp and npn transistors. ()EBV pnp()BEVnpnActiveSaturationCutoffInverted()CBVpnp()BCV npn Figure 5-16: pnp and npn Bipolar Junction Transistor mode of operations and their junction biases The output IV− characteristics for bipolar junction transistor in common-emitter configuration for all mode of operation is given in figure 5-17. Where CI is collectorChapter 5 Microelectronic Circuits Analysis and Design 9 current flowing out of the transistor for pnp and flowing into the transitor for npn . The output voltage is ECV for pnp and CEV for npn . CI0CBV=0BI >Activecutoffcutoff0BI=()ECV pnp()CEV