Transistors: Bipolar Junction Transistors (BJT)

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). C C C C n p B p B B n n B p E E E (a) npn transistor E (b) pnp transistor Figure 1. BJT schematics and structures. (a) npn transistor, (b) pnp transistor C C Base-Collector junction B Base-Collector junction B Base-Emitter junction Base-Emitter junction E E Figure 2 22.071/6.071 Spring 2006, Chaniotakis and Cory 1 Since 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, 1015 , 1017 and 1019 respectively. Therefore the behavior of the device is not electrically symmetric and the two ends cannot be interchanged. Before proceeding let’s consider the BJT npn structure shown on Figure 3. Base-Collector junction Base-Emitter junction E n IE n C IC B RE - p V BE + IB - RC VCB + Figure 3. Biasing voltages of npn transistor With the voltage VBE and VCB as shown, the Base-Emitter (B-E) junction is forward biased and the Base-Collector (B-C) junction is reverse biased. The current through the B-E junction is related to the B-E voltage as I E = I s ( eVBE / VT − 1) (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 I E . 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 2 The collector current and the base current are related by IC = β I B (1.2) I E = IC + I B (1.3) And by applying KCL we obtain And thus from equations (1.2) and (1.3) the relationship between the emitter and the base currents is I E = (1 + β ) I B (1.4) And equivalently IC = The fraction β 1+ β β 1+ β IE (1.5) is called α . For the transistors of interest β = 100 which corresponds to α = 0.99 and I C  I E . The direction of the currents and the voltage polarities for the npn and the pnp BJTs are shown on Figure 4. C C IC B IB + V CE - + V CE IE E (a) npn transistor IC B IB V BE + V CE + IE E (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 3 Transistor 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: VCC , VBB . Transistor terminal voltages: VC , VC , VE Voltages across transistor junctions: VBE , VCE , VCB All of these voltages and their polarities are shown on Figure 5 for the npn BJT. RC RB VBB C RC VCC B E C RB VBB RE (a) B + VB - + VC - RC VCC RB E RE (b) + VE - VBB C + VCB - B + VBE E RE VCC + VCE - (c) Figure 5 22.071/6.071 Spring 2006, Chaniotakis and Cory 4 Transistor 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, I C , versus the collector-emitter voltage, VCE , for various values of the base current, I B as shown on the circuit of Figure 6. IC C + VCE - B IB E IE 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 I B 4 > I B 3 > I B 2 > I B1 IC Saturation Breakdown IB4 Active IB3 IB2 IB1 IB=0 Cutoff VCE Figure 7. BJT characteristic curve 22.071/6.071 Spring 2006, Chaniotakis and Cory 5 The 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. VCE < VBE 3. active region: Base-emitter junction forward biased Collector-base junction is reverse biased Control, I C = β I B (as can be seen from Figure 7 there is a small slope of I C with VCE . VBE < VCE < VCC 4. breakdown region: I C and VCE exceed specifications damage to the transistor 22.071/6.071 Spring 2006, Chaniotakis and Cory 6 Basic BJT Applications Switch Consider the circuit shown on Figure 8. If the voltage vi is less than the voltage required to forward bias the base-emitter junction then the current I B = 0 and thus the transistor is in the cutoff region and I C = 0 . Since I C = 0 the voltage drop across Rc is zero and so Vo=Vcc. If the voltage vi increases so that VBE forward biases the base-emitter junction the transistor will turn on and IB = vi − VBE RB (1.6) Once the transistor is on we still do not know if it is operating in the active region or in the saturation region. However, KVL around the C-E loop gives VCC = I C RC + VCE (1.7) VCE = VCC − I C RC = V0 as shown on Figure 8. (1.8) And so Note that VCE VCC RC load vi RB IB + VBE - IC + VCE - Vo Figure 8. npn BJT switch circuit 22.071/6.071 Spring 2006, Chaniotakis and Cory 7 Equation (1.8) is the load line equation for this circuit. In graphical form it is shown on Figure 9. IC Largest current (saturation) IB3 VCC/RC Load line IB2 IB1 VCC VCE Figure 9 As the base current increases the transistor may operate at points along the load line (thick dashed line on Figure 9). In the limit, the base current I B 3 results in the largest current I C . This is the saturation current and when the transistor operates at this point it is said to be biased in the saturation mode. In saturation, the base-collector junction is forward biased and the relationship between the base and the collector current is not linear. Therefore the collector current at saturation is I C ( sat ) = VCC − VCE ( sat ) RC (1.9) In saturation the collector-emitter voltage, VCE , is less than the VBE . Typically, the VCE at saturation is about 0.2 Volts. 22.071/6.071 Spring 2006, Chaniotakis and Cory 8 Digital Logic. The circuit on Figure 10 shows the fundamental inverter circuit. VCC RC vi IC Vo RB IB Figure 10. Basic BJT inverter circuit If the voltage vi is zero (low) the transistor is in the cutoff region, the current I C = 0 and the voltage V0 = VCC (high). By contrast if the voltage vi is high, say equal to VCC , the transistor is driven into saturation and the output is equal to VCE ( sat ) which is low. With this fundamental circuit as the basis we are able to construct any other logic operation. Problem: For the circuit shown below, complete the logic table: V1 High Low Low high V2 Low High Low high V0 VCC RC V1 IC Vo RB IB1 V2 RB IB2 22.071/6.071 Spring 2006, Chaniotakis and Cory 9 Amplifier Circuit. The basic inverter circuit also forms the basic amplifier circuit. The voltage transfer curve (output voltage as a function of the input voltage) is the fundamental characterization of an amplifier. For the circuit shown on Figure 11 the voltage transfer curve is shown on Figure 12. Note the large slope of the curve in the active mode. A small change in the input voltage VI induces a large change on the output V0 – an amplification. (we will explore this in the laboratory) VCC RC vi IC Vo RB IB Figure 11. BJT inverter amplifier V0 Cutoff Forward active mode (large slope) Saturation VCE(sat) VBE(on) VCC VI Figure 12. Inverter amplifier voltage transfer characteristic curve 22.071/6.071 Spring 2006, Chaniotakis and Cory 10