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MIT 6 071J - Bipolar Junction Transistors

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Transistors: Bipolar Junction Transistors (BJT) General configuration and definitions The transistor is the main building block “element” of electronics. It is a semiconductor device and it comes in two general types: the Bipolar Junction Transistor (BJT) and the Field Effect Transistor (FET). Here we will describe the system characteristics of the BJT configuration and explore its use in fundamental signal shaping and amplifier circuits. The BJT is a three terminal device and it comes in two different types. The npn BJT and the pnp BJT. The BJT symbols and their corresponding block diagrams are shown on Figure 1. The BJT is fabricated with three separately doped regions. The npn device has one p region between two n regions and the pnp device has one n region between two p regions. The BJT has two junctions (boundaries between the n and the p regions). These junctions are similar to the junctions we saw in the diodes and thus they may be forward biased or reverse biased. By relating these junctions to a diode model the pnp BJT may be modeled as shown on Figure 2. The three terminals of the BJT are called the Base (B), the Collector (C) and the Emitter (E). CCBBEEnnp (a) npn transistor CCBBEEnpp (b) pnp transistor Figure 1. BJT schematics and structures. (a) npn transistor, (b) pnp transistor BECBase-Emitter junctionBase-Collector junctionBase-Collector junctionBase-Emitter junctionBEC Figure 2 22.071/6.071 Spring 2006, Chaniotakis and Cory 1Since each junction has two possible states of operation (forward or reverse bias) the BJT with its two junctions has four possible states of operation. For a detailed description of the BJT structure see: Jaeger and Blalock, Microelectronic Circuit Design, McGraw Hill. Here it is sufficient to say that the structure as shown on Figure 1 is not symmetric. The n and p regions are different both geometrically and in terms of the doping concentration of the regions. For example, the doping concentrations in the collector, base and emitter may be, , and respectively. Therefore the behavior of the device is not electrically symmetric and the two ends cannot be interchanged. 151017101910 Before proceeding let’s consider the BJT npn structure shown on Figure 3. Base-Emitter junctionBase-Collector junctionpnnBCERR- V + - V +IIIECBBE CBEC Figure 3. Biasing voltages of npn transistor With the voltage and as shown, the Base-Emitter (B-E) junction is forward biased and the Base-Collector (B-C) junction is reverse biased. BEVCBV The current through the B-E junction is related to the B-E voltage as ()/1BE TVVEsIIe=− (1.1) Due to the large differences in the doping concentrations of the emitter and the base regions the electrons injected into the base region (from the emitter region) results in the emitter current EI. Furthermore the number of electrons injected into the collector region is directly related to the electrons injected into the base region from the emitter region. Therefore, the collector current is related to the emitter current which is in turn a function of the B-E voltage. The voltage between two terminals controls the current through the third terminal. This is the basic principle of the BJT 22.071/6.071 Spring 2006, Chaniotakis and Cory 2The collector current and the base current are related by CBIIβ= (1.2) And by applying KCL we obtain ECBIII=+ (1.3) And thus from equations (1.2) and (1.3) the relationship between the emitter and the base currents is (1 )EBIIβ=+ (1.4) And equivalently 1CEIIββ=+ (1.5) The fraction 1ββ+ is called α. For the transistors of interest 100β= which corresponds to 0.99α= and CEII . The direction of the currents and the voltage polarities for the npn and the pnp BJTs are shown on Figure 4. CBEIEICIBVCEVCE++-- (a) npn transistor CBEIEICIBVCEVBE+--+ (b) pnp transistor Figure 4. Current directions and voltage polarities for npn (a) and pnp (b) BJTs 22.071/6.071 Spring 2006, Chaniotakis and Cory 3Transistor i-v characteristics A. Transistor Voltages Three different types of voltages are involved in the description of transistors and transistor circuits. They are: Transistor supply voltages: , . CCVBBV Transistor terminal voltages: , , CVCVEV Voltages across transistor junctions: , , BEVCEVCBV All of these voltages and their polarities are shown on Figure 5 for the npn BJT. BEVCCCVBBRBRCRE (a) BEVCCCVBBRBRCREVEVBVC+++---(b) B E VCCC VBBR B R C R E V BE VCE++-- V CB + - (c) Figure 5 22.071/6.071 Spring 2006, Chaniotakis and Cory 4Transistor Operation and Characteristic i-v curves The three terminals of the transistors and the two junctions, present us with multiple operating regimes. In order to distinguish these regimes we have to look at the i-v characteristics of the device. The most important characteristic of the BJT is the plot of the collector current, CI, versus the collector-emitter voltage, , for various values of the base current, CEVBI as shown on the circuit of Figure 6. BECVCE+-ICIEIB Figure 6. Common emitter BJT circuit for determining output characteristics Figure 7 shows the qualitative characteristic curves of a BJT. The plot indicates the four regions of operation: the saturation, the cutoff, the active and the breakdown. Each family of curves is drawn for a different base current and in this plot 432BBB 1BIIII>>> VCEICIB4IB3IB2IB1I=0BSaturationActiveBreakdownCutoff Figure 7. BJT characteristic curve 22.071/6.071 Spring 2006, Chaniotakis and Cory 5The characteristics of each region of operation are summarized below. 1. cutoff region: Base-emitter junction is reverse biased. No current flow 2. saturation region: Base-emitter junction forward biased Collector-base junction is forward biased Ic reaches a maximum which is independent of IB and β. No control. CE BEVV<3. active region: Base-emitter junction forward biased Collector-base junction is reverse biased Control, CBIIβ= (as can be seen from Figure 7 there is a small slope of CI with . CEVBE CE CCVVV<<4. breakdown region: CI and exceed specifications damage to the transistor CEV 22.071/6.071 Spring 2006, Chaniotakis and Cory 6Basic BJT Applications Switch Consider the circuit shown on Figure 8. If the voltage is less than the voltage required to forward bias the base-emitter junction then the current iv0BI= and thus the transistor is in the cutoff region and . Since 0CI = 0CI= the voltage drop across Rc is zero and so


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MIT 6 071J - Bipolar Junction Transistors

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