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 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 I B 1 4 And equivalently IC The fraction 1 1 IE 1 5 is called For the transistors of interest 100 which corresponds to 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 No control VCE VBE 3 active region Base emitter junction forward biased Collector base junction is reverse biased Control I C 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 …