Electronics Bipolar transistor

Transcription

1 Electronics Bipolar transistor Prof. Márta Rencz, Gábor Takács BME DED 01/10/ / 30

2 Transistors I. Transistors are the most important semiconductor devices. They are used in analog circuit as amplifiers: the input power of an amplifier is smaller than its output power, the energy need for amplification is provided by the supply voltage, a transformer is not an amplifier as the power at its terminals is equal (if the voltage is larger at the output then the current is smaller), both analog and digital circuits as switches: large power can be switched by a small input power, logic gates can be realized by controlled switches. 2 / 30

3 Transistors II. The types of transistors: bipolar transistor: controlled by current, its outputs are not interchangeable bipolar device 1 field-effect transistor: controlled by voltage, unipolar device. 1 it means that charge carriers of both polarities (electrons and holes) participate in the current-conduction process 3 / 30

4 The bipolar transistor I. The bipolar transistor (BJT) consists of two p-n junctions placed very close to each other. There are two types depending on structure: npn, pnp. Both types are widely used. The npn transistors operate faster, so they are more widespread. The difference in speed is due to the fact that in npn transistors the current is made up of electrons, while in pnp it s made up of holes and electrons move faster than holes in semiconductors. 4 / 30

5 The bipolar transistor II. The bipolar transistors have three terminals: 1 Emitter (E) 2 Base (B) 3 Collector (C) 5 / 30

6 The symbol of the bipolar transistor I. The currents and voltages of the two types are exactly the opposite. We ll discuss the npn transistors everything is the same in pnp transistors, only the directions are the opposite. The arrow in the symbol: is between the base and the emitter, shows the forward direction of the p-n junction. 6 / 30

7 The symbol of the bipolar transistor II. The currents of the bipolar transistor comply to the KCL: I E = I C + I B The direction of the voltages is determined by the p-n junctions: the emitter-base junction is assumed to be open, the collector-base junction is assumed to be closed. This is the most widely used operating mode of the transistor. 7 / 30

8 The structure of the BJT I. The BJT consists of two p-n junctions in a proximity of a few microns (or less). The figure shows a discrete transistor there is only one transistor in a package. The structure is planar: its width is much bigger than its depth (just as diodes). 8 / 30

9 The structure of the BJT II. The collector is lightly doped and is n-type in npn transistors. The base is inside the collector, has an average doping and is p-type in npn transistors. The emitter is inside the base, it is highly doped and is n-type in npn transistors. 9 / 30

10 The structure of the BJT III. In the leftmost figure the size of the chip is mm. The collector terminal is the metal base that the chip is mounted onto. Golden wires connect the emitter and base to the leads of the package. The wires are connected to the the chip by thermocompression bonding. Small power transistors are packaged in plastic, power transistors are packaged in metal packages. 10 / 30

11 The structure of the BJT IV. The device is asymmetrical due to the inhomogeneous doping densisties. The densities are determined by the technology. The doping of the two p-n junctions is different. 11 / 30

12 The operating modes of the BJT There are four operating modes determined by the direction of the two junctions currents. The most important is the normal active mode. The operating modes B-E junction B-C junction normal active open closed inverse active closed open saturation open open cut-off closed closed 12 / 30

13 The normal active operation mode I. ++ n p n + i E E i E i En i Ep electrons holes i C recombination C i C i B1 i B2 v BE B i B v CB The B-E junction is open, thus the majority charge carriers of the two sides are crossing the junction. The B-C junction is closed, there is a large field in the space charge region, that forces minority charge carriers across the junction. The doping density of the emitter is much higher than that of the base thus electrons make up most of the B-E current. 13 / 30

14 The normal active operation mode II. ++ n p n + i E E i E i En i Ep electrons holes i C recombination C i C i B1 i B2 v BE B i B v CB The electrons arriving in the base are forced away from the B-E junction by diffusion. When they reach the proximity of the collector, they are drifted across the junction by the field as they are minority carriers in the base. Although the B-C junction is closed, its current is large due to the large number of electrons that enter the base from the emitter, diffuse towards the B-C junction and then drift over the reverse biased junction. 14 / 30

15 The normal active operation mode III. ++ n p n + i E E i E i En i Ep electrons holes i C recombination C i C i B1 i B2 v BE B i B v CB The emitter emits charge carriers to the base, hence its name. The charge carriers in the base are collected by the collector. The narrower the base, the bigger the chances that electrons get through to the collector without recombining with holes. The collector current is almost equal to the emitter current: the difference is the amount of electrons lost to recombination during their way across the base. 15 / 30

16 The normal active operation mode IV. ++ n p n + i E E i E i En i Ep electrons holes i C recombination C i C i B1 i B2 v BE B i B v CB The relationship between the emitter current and the collector current: I C = A N I E where A N is common base, normal active, DC current gain of the transistor (A N = ). This operating mode is used for amplification. 16 / 30

17 The common-emitter configuration I. The collector current is proportional to the emitter current but the current gain is smaller than 1. The difference between I E and I C is the small I B. By controlling I B, a large current gain can be obtained. In the common-emitter configuration the base current is the input and the collector current is the output. 17 / 30

18 The common-emitter configuration II. According to the KCL: I C = A N I E = A N (I C + I B ) I C = A N 1 A N I B = B N I B B N is the common emitter, normal active, DC current gain, and B N = B N is larger than 1, thus this configuration amplifies current. The N in the index is usually omitted: I C = BI B. In some textbooks A is denoted with α and B with β. 18 / 30

19 The other operating modes Inverse active mode: the role of the emitter and collector are swapped. Due to the inhomogeneous structure, the transistor effect is, although present, much smaller. This mode is very scarcely used (it was used in traditional TTL gates). Saturation: both p-n junctions are open. Large current flows through the device while the collector-emitter voltage is small. The transistor is in this mode when it s operated as a switch that is turned on. Cut-off region: both junctions are closed. Only the saturation currents flow in the device. These can be neglected as they are in the range of na. This is the operating mode of a switch that is turned off. When the transistor is operated as a switch, it switches between saturation and the cut-off region. 19 / 30

20 Characteristic curves of the BJT The currents of the transistor are depicted as a function of the voltages. As the device has three terminals, at least two current-voltage pairs are needed to describe the operating point. The most widely used characteristic curves are the common-emitter curves: I B = f(v BE ) I C = g(v CE, I B ) 20 / 30

21 Common-emitter characteristic curves I. Input characteristic curve Output characteristic curve The input characteristic curve: depicts the relationship between the input quantities. It resembles the diode s characteristic curve I B is an exponential function of V BE. This is due to the fact the there is a diode operating in the forward direction between the B and E terminals. Output characteristic curves: depict the collector current as a function of the collector-emitter voltage and the base current (I B4 > I B3 > I B2 > I B1 ). 21 / 30

22 Common-emitter characteristic curves II. The output characteristic curves: They are a set of V CE I C curves for increasing I B values. The area where the curves are close to being horizontal is the normal active region. The steep part of the curves constitute the saturation region. In saturation the collector and emitter forward currents flow in opposite directions, thus their difference appears as a macroscopic current. 22 / 30

23 Voltage amplification using a transistor The transistor is in a common-emitter configuration. The base voltage is sinusoidal with an offset. The collector is connected to the supply voltage through a resistor. The output is the collector. 23 / 30

24 The operating point of the circuit I. The DC input voltage (V IN ) determines a base current (I B ) it can be found using the input characteristic curve. With the help of I B, the curve that holds the operating point can be chosen from the set of output characteristic curves. The exact OP is defined by the supply voltage and the resistor as V CE also affects I C, though only slightly. 24 / 30

25 The operating point of the circuit II. The linear elements surrounding the transistor determine the load line. The intersection of the load line and the characteristic curve is the operating point. According to the KCL the load line is: V CC = R C I C + V CE I C = V CC V CE R C 25 / 30

26 The operating point of the circuit III. The load line can be found in an easier way: When I C = 0 the collector s potential has to be equal to the supply voltage (Ohm s law for R C ). If V CE = 0, the entire supply voltage is dropped on R C, thus I C = V CC /R C By connecting these two points, we get the load line. This is due to the fact that the two points were determined according to the characteristic equation of the linear elements (R C ), thus they have to be on the load line. 26 / 30

27 Small-signal calculation Small-signal analysis is performed at the operating point. The characteristic equation is substituted with its tangent a linear equation. A small-signal model of the circuit is created which consists of linear elements only. Such a circuit describes the AC behaviour only. The value of the elements in the circuit is determined by the operating point currents and voltages. The small-signal model is easy to calculate. It neglects the non-linearity of the characteristic equation, thus the results are not exactly accurate. 27 / 30

28 Common-emitter, small-signal equivalent circuits The collector current is proportional to the base current: this can be modelled with a current controlled current source: β = I C I B B as I C = B I B The B-E diode can be substituted with its differential resistance: r e = V BE/ I E = V T /I E, but in this case the input resistance is r e(β + 1). 28 / 30

29 Calculation of the gain using the small-signal equivalent I. The AC equivalent circuit: the transistor is substituted with its small-signal equivalent, the DC supply voltage sources are substituted with short circuits (as the changes pass through them). 29 / 30

30 Calculation of the gain using the small-signal equivalent II. The calculation: The base current as a function of the input current: v in i b = (β + 1)r e The collector current: i c = βi b The output voltage: v out = i cr c = βi b R C = β β + 1 R C r e v in R C r e v in The negative sign shows that changes at the output occur in the opposite direction as at the input. 30 / 30