Chapter 2. Bipolar Junction Transistor

Transcription

1 Chapter 2 ipolar Junction Transistor 2.0 History The name bipolar is used because both types of carriers namely hole and electron are used in the transistor, as opposed to field effect transistor, which is considered a unipolar device. Transistor was invented by J. ardeen ( ), W. Shockley ( ), and W. rattain ( ) in Fig. 2.1 shows the picture of three transistor inventors. n 1950 the junction transistor was made using molten germanium. The bipolar transistors produced in 1950s were typically made with alloyed junction. The planar technology developed around 1960 started to use silicon as the semiconductor material. Today bipolar junction transistor enjoys a large market but they have been challenged by MOSFETs because of cost, yield, power, miniaturization, and etc. Figure 2.1: nventors of transistor John ardeen, William Shockley, and Walter rattain (left to right) Figure 2.2 shows the picture of first transistor made by three mentioned inventors

2 Figure 2.2: First transistor built in 1948 n 1999, a vertical replacement gate transistor of 50 nm thick was invented by scientist in Lucent laboratory. Figure 2.3 shows the picture of the Lucent Transistor. There was other smaller transistor reported during that time by research laboratory of a university in Ohio State. Figure 2.3: 50nm transistor built by Lucent Laboratory Transistors both bipolar and field effect type, are a three terminal semiconductor device used primarily for signal amplification and switching. t is also formed the fundamental element for integrated circuit design such as the VLS microprocessor. ipolar junction transistor can be divided into two types namely npn and pnp

3 Field effect transistor FET may be broadly divided into JFET, MESFET, MODFET, and MOSFET, where MOSFET can further be divided into depletion-enhancement type and enhancement type. n today's VLS design, complimentary MOSFETs are used to reduce power consumption and fast switching application. 2.1 ipolar Junction Transistor ipolar junction transistor JT can be viewed as two pn junctions connected back to back to form np-pn or pn-np structures namely as npn or pnp transistors. Figure 2.4 illustrates the structure showing two pn junctions. Figure 2.4: The structure of a transistor showing two pn junctions ipolar junction transistor is manufactured in miniature formed by the fabrication process, which involved oxidation, photolithography, etching, deposition, ion implantation, diffusion, chemical/mechanical polishing processes, metallization, and etc. Like pn junction, the current components of JT come from two carrier types, which are the hole and electron. This is also the reason why the transistor is called bipolar junction transistor. The current components are diffusion hole, diffusion electron, drift hole, and drift electron, which are illustrated in Fig

4 Figure 2.5: t illustrates the current components of a p + np transistor Design Concept of ipolar Junction Transistor ipolar junction transistor has three terminals. One terminal is used to inject carrier name as emitter E, one is used to control the passage of the carrier named as base, and one is used to collect the carrier named as collector C. Traditional way of designing a bipolar junction transistor is designed in such that the doping concentration of its emitter is higher than the doping concentration of the base and collector. The order of doping concentration is highest for emitter cm -3, followed by collector cm -3 and then base cm -3. To ensure almost 100% of the injected carrier from emitter is collected by collector, the diffusion carriers have to out number the diffusion carriers of base and to out number recombination of carrier in the base. The base is also designed to be much shorter than the diffusion length L n or L p of the minority carriers. This is used to minimize the chance of recombination of minority carrier with majority carrier in the base. ipolar junction transistor that meets the design concept would have high emitter efficiency and high current gain beta β value Type of ipolar Junction Transistor ipolar junction transistor can be divided into two types namely npn and pnp types which have their structures shown in Fig

5 Figure 2.6: Structure of pnp and npn transistor Symbol of ipolar Junction Transistor The symbols of npn and pnp bipolar junction transistors are shown in Fig The terminal with arrow sign signifies the emitter side. The tail of the arrow shows the p-type, whilst the head of arrow shows the n-type. (a) npn transistor (b) pnp transistor Figure 2.7: Symbol of npn and pnp bipolar junction transistor Power the ipolar Junction Transistor n normal operation of JT, the emitter-to-base junction of the bipolar junction transistor is always forward biased. The collector-to-base is always reversebiased. Figure 2.8: Voltage bias configuration of pnp transistor

6 The corresponding energy band diagram of a pnp bipolar junction transistor is shown in Fig Figure 2.9: Energy band diagram of pnp transistor under voltage bias dc Operation Mode of ipolar Junction Transistor Figure 2.10 shows the current components of a bipolar junction transistor under normal bias. Figure 2.10: iasing configuration of pnp transistor There are three current types flowing in bipolar junction transistor. There are collector current C, emitter current E, and and base current. y Kirchhoff's Current Law KCL, E + C (2.1) There is a small portion of the injected carrier recombines with majority carrier in the base to form part of the base current. Thus, the emitter current E is equal to ( C + )

7 The ratio of collector current to emitter current is called α, which is also named as h-parameter h F. This parameter is commonly known as common base gain. α C / E (2.2) The typical value of α ranges from from 0.95 to For a good transistor, its α value is closed to one. The ratio of collector current to base current is β, which also denoted as h- parameter-h FE. This parameter is commonly known as common emitter gain. β C / (2.3) The typical range value of β is between 20 to 500. Substitute equation (2.1) into equation (2.3), it yields E (β + 1) (2.4) The relationship of α and β parameters shall be or β α/(1 - α) α β/(β + 1) (2.5) Example 2.1 A transistor has 0.08mA and E 9.60mA. Determine its collector current C, α, and β. Solution C E mA mA 9.52mA α 9.52mA/9.60mA β 9.52mA/0.08mA

8 or Characteristics of eta β β α/(1-α) / The beta β of transistor increases as the junction temperature of JT increases. Thus, it will affect the quiescent Q-point of the transistor. However, if the JT is properly biased, the effect is insignificant. The basic reason for the increase is due to increase of electron-hole pair EHP at higher temperature as compared to lower temperature eversed ias Mode of ipolar Junction Transistor When the transistor is in reverse-biased mode, there are leakage currents, which can be measured. They are minority currents, which are drift currents in the collector-to-emitter and collector-to-base junctions. n engineering sense, they are referred as leakage currents. Two types of leakage current namely CO at open emitter mode and CEO at open base mode are of interest because it affects the operating Q-point of the transistor as the operating temperature increases. n the industrial analogue circuit design, CEO and CO values are normally considered as very small in nanoampere range, which can be ignored. Figure 2.11 shows the set up to measure CO with emitter left open i.e. C CO. Figure 2.11: CO measurement

9 Figure 2.12 shows the set up to measure CEO current with base open i.e. C CEO. Figure 2.12: CEO measurement Taking into account CEO and CO, the real α (real) and β (real) parameters shall be re-calculated. Since Figure 2.13: t illustrates the CO and CEO leakage current components β α, β +1 C C(real) + CO (2.6) α (real) ( C - CO )/ E (2.7) C α (real) E + CO (2.8) E -(α (real) E + CO ) (2.9)

10 E - CO (2.10) β +1 β (real) C + CO CO (2.11) C β (real) + CO (β+1) (2.12) n open base condition, the collector current C is C E + CEO (2.13) Thus, the open base collector-to-emitter current CEO is CEO CO (β+1) (2.14) Note that CEO value in equation (2.14) is indeed the cutoff base current of a bipolar junction transistor. Example 2.2 A silicon bipolar junction transistor has β 100 and CO of 0.01µA. Calculate the value of α, C and assuming E 1mA. Solution α β β C α E + CO 0.99(1mA) µA ma E - CO β +1 1mA/ µA 9.89µA Collector Characteristic Curves y fixing the base current and varying the V CC voltage and knowing β value, the characteristic curve of collector current C versus collector-to-emitter voltage V CE can be plotted as shown in Fig y changing the value of base current

11 , a new collector characteristic curve can be obtained by varying the collectorto-emitter voltage V CE and measuring the collector current C. When base current is zero i.e. 0, the transistor is said to be at cutoff. When base current is increased, the collector current C is also increased, and collector-to-emitter voltage V CE is decreased. V CE will decrese until it is equal to V CE(sat), which is approxiamtely equal to 0.1 ~ 0.2V. At this condition, the transistor is saturated because C will not increase any further and base-tocollector junction becomes forward bias (equation (2.19); V C V CE - V E ). n this condition, the current gain β formula will not follow. Note also that at cutoff, V CE is almost equal to V CC and likewise at saturation, collector-to-emitter voltage is almost zero. i.e.v CE 0V. Figure 2.14: Collector characteristic curves 2.2 dc Configuration of ipolar Junction Transistor There are three biasing configurations for the bipolar junction transistor. They are common-base, common-emitter, and common-collector configurations. We shall study each of the configurations in details

12 2.2.1 Common-ase C Configuration The transistor is connected with base as common ground terminal as shown in Fig is called common-base configuration. The current gain is α, which is the ratio of collector current and emitter current C / E. The input is at emitter terminal, whilst the output is at collector terminal. Figure 2.15: Common-base C configuration Common-Emitter CE Configuration The transistor is connected with emitter as the common or ground is called common-emitter configuration as shown in Fig Figure 2.16: Common-emitter configuration The current gain of this configuration is β, which is the ratio of collector current C and base current. β is also called as static forward transfer current ratio h fe. The input is at base terminal, whilst the output is at collector terminal

13 dc Analysis eference to common-emitter configuration shown Fig. 2.17, there are three currents and three voltages, which are base current, emitter current E, collector current C, base-to-emitter voltage V E, collector-to-base voltage V C, and collector-to-emitter voltage V CE. For any other dc biasing configuration, there always have these currents and voltages. Figure 2.17: Common-emitter current and voltage At room temperature 25 0 C, the base-to-emitter voltage V E is approximately equal to 0.7V, which is the forward voltage drop of a diode. The voltage across base resistor V is V V -V E (2.15) Therefore, the base current is V V E (2.16) Knowing the beta value, using equation (2.2) and (2.3), collector current C and emitter current E can be determined. The voltage drop across collector resistor C is V C, which is

14 V C (2.17) C C The voltage drop across collector and emitter V CE shall then equal to V CE V CC - C C (2.18) The voltage drops across collector and base shall follow equation (2.19), which is V C V CE - V E (2.19) Example 2.3 Determine if the transistor shown in circuit is in saturation. Assume that V CE(Sat) is small enough to be neglected. Solution The collector saturation current is VCC VCE(sat) 10V 10mA. f is large 1kΩ C (sat) C enough to produce C(sat) then the base current is V 0.7V 2.3V 0.23mA. 10kΩ The collector current is C β (50)(0.23mA) 11.5mA. This shows that with the specified β, this base current is capable of producing the collector current C greater than C(sat). Since the transistor is saturated, the collector current value of 11.5mA is never attained

15 2.2.3 Common-Collector CC Configuration The transistor is connected with collector as the common is called commoncollector configuration as shown in Fig Figure 2.18: Common-collector configuration The current gain is β+1 since E C +. This shall mean that emitter current is E (β + 1). The input is at base terminal, whilst the output is at emitter terminal dc Analysis The current and voltage shown in Fig depend on the transistor characteristics and external circuit values. Figure 2.19: Currents and voltage of common collector configuration The voltage at base is V. The voltage at emitter V E is (V -V E ), where V E 0.7V for silicon at room temperature. Thus, the emitter current E is

16 V E E E (2.20) E V V E E (2.21) Since emitter current is + E and beta is β C / C E β + 1 (2.22) The collector-to-emitter voltage V CE is V CE V CC - V E V CE V CC - E E (2.23) The collector-to-base voltage V C is V C V CC - V E - V E (2.24) Example 2.4 Determine current, C, E and voltage at each transistor terminal with respect to ground and V CE voltage in the figure. β 200. Solution Emitter current is V V 10V 0.7V 10kΩ E E E 0.93mA

17 β C E β mA E 0.93mA 4.43µ A β V C V CC 20V and V V 10V V E E E (0.93mA)(10kΩ) 9.3V V 9.2V + 0.7V 10V. V CE 20V - 9.3V 10.7V 2.3 dc Operating Point The dc operating point is referred to Q-point (quiescent point). t is a point on the transistor characteristic curve. f one chooses collector current C versus collector-to-emitter voltagev CE characteristics curve then Q-point is the point on the curve determined by collector current C and collector-to-emitter voltagev CE for a fixed value of base current derived from the biasing of circuit. Using the transistor biasing circuit shown in Fig. 2.20, the Q-point on the characteristics curve can be determined by finding the values of C and V CE for a given base current determined by the circuit. The line joining the Q-point is known as dc load line. (a) ias circuit (b) Characteristic curve Figure 2.20: (a) iasing circuit for determining Q-point and (b) showing Q-point and dc load line f there is a sine wave of amplitude 1.0V superimposed on base voltage V as shown in Fig The base current varies 100.0µA above and 100.0µA

18 below the Q-point. The collector current C will vary between 20.0mA to 40.0mA, which is ±10.0mA above and below Q-point of 30.0mA. The collector-to-emitter voltage V CE will vary from 2.0V to 6.0V as shown in Fig. 2.22, which is 2.0V above and below Q-point of 4.0V. Figure 2.21: t shows the transitor with ac signal superimposed on dc Figure 2.22: t shows the transistor dc load line

19 2.3.1 Distortion of Output The location of Q-point can cause distortion of the output and it determines the maximum input voltage. The output signal is clipped if the input is driven into either saturation or cutoff area. Fig illustrates the conditions of output distortion. (a) Driven in saturation (b) Driven into cutoff (c) Driven both into saturation and cutoff Figure 2.23: Conditions of output distortion Example 2.5 Determine the Q-point for circuit shown in the figure and the peak value of base current for linear operation. Given that the beta value β of the npn transistor is

20 Solution Q-point is defined by collector current C and collector-to-emitter voltage V CE on the output characteristic curve. The base current is V VE 10V 0.7V 186 A and the collector current C is 50kΩ µ C β 200x186µA 37.2mA. The collector-to-emitter voltage V CE is V CE V CC - C C 20V ma(300ω) 20V V 8.84V Thus, first Q-point is at C 37.2mA and V CE 8.8V for 186.0µA The next Q-point shall be determined at saturation. C at this point is C(sat) V CC / C 20V/300Ω 66.7 ma Now a dc load line can be drawn as shown in the figure. From the graph, V CE at cutoff is found to be 20.0V

21 From the graph, the operating C range is 37.2mA ± 29.5mA. Thus, the peak base current value is b(peak) 29.5mA/ µA. 2.4 dc iasing a Transistor Amplifier The purpose of dc bias is to make transistor to work as amplifier or alternative one can say to keep the transistor alive. All three terminals of the bipolar junction transistor must be biased. Showing here is the most common type of dc biasing for transistor, which are base bias, emitter bias, voltage-divider bias, and collector feedback bias. The constant curent bias shall also be discussed briefly. dc analysis and the effect of temperature for each biasing type shall be discussed detail. The advantage and disadvantage of each bised type shall also be discussed ase ias Circuit in Fig illustrates the base biasing of a bipolar junction transistor. The base of the bipolar junction transistor is biased using V CC voltage instead of a separated voltage

22 dc Analysis Figure 2.24: ase bias circuit of bipolar junction transistor The voltage drop across base resistor is (V CC -V E ). Therefore, base current is (V CC - V E )/ (2.25) Also collector-to-emitter voltage is V CE V CC - C β (2.26) From the above dc analysis, it shows that collector-to-emitter voltage V CE is dependent on β parameter. Since beta β increases with temperature, it shall mean that collector current C will also increase. ncrease of collector current reduces collector-to-emitter voltage V CE. Thus, it affects the Q-point. ased on the analysis, the bipolar junction transistor baised with base bias technique is not a good biasing technique unless the operating temperature can be kept constant Emitter ias Emitter bias of bipolar junction transistor is shown in Fig The emitter is normally biased

23 Figure 2.25: Emitter biasing circuit dcanalysis At base-emitter loop, + V E + E E V EE (2.27) ( C /β) + V E + β + β 1 C E V EE Thus, the collector current C is VEE VE ( β + 1) / β + C (2.28) E / β Since /β is small as compared to E and (β + 1)/β 1, the effect of collector current C with temperature is minimum. Thus, emitter bias is a good biasing technique for linear circuit design. The collector voltage V C is V C V CC - C C (2.29) and collector-to-emitter voltage V CE is

24 V CE V C -V E (2.30) Example 2.6 Determine how much the Q-point of the circuit shown in the figure will change over temperature where β increases from 50 to 100 and V E decreases from 0.7V to 0.6V. Solution For β 50 and V E 0.7V C VEE VE E ( β + 1) / β + / β 20V 0.7V 10kΩ(51/ 50) + 10kΩ / mA V C V CC - C C 20V - (1.86mA)(5kΩ) 10.72V Therefore, the emitter voltage V E is V E -1.86mA/50x10kΩ - 0.7V V and the collector-to-emitter voltage is

25 V CE V C -V E 10.72V - (-1.072V) 11.79V For β 100 and V E 0.6V C VEE VE E ( β + 1) / β + / β 20V 0.6V 10kΩ(101/100) + 10kΩ / mA V C V CC - C C 20V - (1.90mA)(5kΩ) 10.49V Therefore, the emitter voltage and collector-to-emitter voltage are V E -1.90mA/100x10kΩ- 0.6V -0.79V V CE V C -V E 10.49V - (-0.79V) 11.28V The % change in collector current C as β changes from 50 to 100 is 1.90mA 1.86mA 1.86mA C x100% 2.15% The % change in collector-to-emitter voltage V CE is 11.79V 11.28V 11.79V V CE x100% 4.32% From the results, one can conclude that the emitter bias circuit is a good way to stabilize Q-point due to change of β caused by temperature Voltage-Divider ias Voltage-divider bias is the most widely used technique for linear circuit design. The base voltage V is biased based on device circuit shown in Fig

26 Figure 2.26: Voltage-Divider bias circuit f the base current is very small as compared to current 2 flows in 2, then the divider circuit can be simplied and depends on 1 and 2. Otherwise, the input resistance N(base) at the base needs to take into consideration. Figure 2.27: (a) Divider circuit without input resistance and (b) with input resistance

27 The input resistance of base N(base) is defined as N(base) (β+1) E as shown in in Fig n most cases, N(base) is very large as compared to 2. Thus, it can be ignored in the calculation. Since (β+1) β, then (β+1) E β E From Thévenin's theorem, an equivalent base-to-emitter circuit is shown in Fig and its dc model circuit is shown in Fig The dc model can be used for the case where N(base) is considered as part of input and also the base current is not assumed to be zero. Figure 2.28: Thévenin's equivalent circuit of base-to-emitter circuit Figure 2.29: dc model circuit of voltage divider amplifier

28 dc Analysis Consider circuit shown in Fig. 2.30, the base voltage V at point A is equal to V ( β + 1) 2 E VCC (2.31) 1 + { 2 ( β + 1) E } f term (β+1) E >> 2 then the base voltage is approximately equal to V 2 VCC (2.32) Knowing base voltage is V and emitter voltage is V E V - V E, the emitter current is equal to E V V E E (2.33) Since collector current is C α E and emitter voltage is V E E E, the collector-to-emitter voltage V CE is equal to V CE V CC - C C - E E V CC - E (α C + E ) (2.34) Usually the value of input base resistance N(base) (β + 1) E is much larger as compared to 2. Therefore, Q-point is only slightly effected by β, which is temperature dependent. Example 2.7 Using the circuit shown in the figure, determine the values of base voltage V and emitter current E. f the transistor is replaced with one that has β 250, what is the change of base voltage V?

29 Solution n this example, N(base) cannot be ignored since it involves β parameter. N(base) 51(1kΩ) 51kΩ V N(base) N(base) VCC 50kΩ 51kΩ 10V 100kΩ + 50kΩ 51kΩ 2.01V E (V -V E )/ E (2.01V- 0.7V)/1kΩ 1.31mA f beta β increases to 250, input base reistance N(base) is 251kΩ and base votlage V is V 50kΩ 251kΩ 10V 100kΩ + 50kΩ 251kΩ 2.94V ase voltage V increases from 2.01V to 2.94V

30 2.4.4 Collector Feedback ias The circuit of collector feedback bias or voltage feedback bias is shown in Fig t provides base-to-emitter bias. This circuit is good to stablize the effect β on Q-point caused by temperature dc Analysis The base current is Figure 2.30: Collector feedback bias VC V E (2.35) The collector-to-emitter voltage V CE is V CE V C V CC ( C + ) C V CC - C C (2.36) Also the base current is equal to V CC ut C /β, thus, C C V E

31 C β V V CC C C V VE + / β E 2 ipolar Junction Transistor CC C (2.37) C Normally the value of /β is small as compared to C. Thus, collector current C is fairly independent of β iasing Using Current Source Current source biasing has advantage because emitter current is independent of resistance and β value of the transistor as shown in Fig. 2.31(a). Thus, can be made as large as possible to increase the input impedance without disturbing the stability of the bias. Current source also leads to significant design simplification. t keeps the collector voltage at point V greater than voltage at base (-V EE + V E ). The circuit in Fig. 2.31(b) has the current ratio / EF which is depending on the design aspect ratio (A/ω b ) of the transistor Q 1 and Q 2. Thus, the relationship / (A / ω b Q2 EF (A / ωb ) Q1 ) β β 2 1 is established. f transistor Q 1 and Q 2 have same design geometry then is a replica or mirror of current reference EF. Thus it is also called the current mirror, a name that is used irrespective of the ratio of device dimenison. EF V CC ( V EE ) V E (2.38) EF is also equal to the sum of collector current C flows in transistor Q 1 and the base current flows in both transistors. Thus, also EF C + 2 EF C +2 C /β and C, therefore, current same β value. β β + 2 x EF for transistor Q 1 and Q 2 that have

32 (a) asic current source (b) Current mirror Figure 2.31: ipolar transistor biased using current source 2.5 Output mpedance r o With reference to Fig. 2.14, we assume that the output impedance r o of the bipolar junction transistor is infinite at amplification region. Thus, we ignore r o at the gain calculation. n reality the output impedance of the transistor for a specified current is depending on the Early voltage V A and the collector current C. This is illustrated in Fig Thus, the output impedance of the transistor is r o V + V A C CE V A. C At saturation and upon further increase V CE voltage, the depletion thickness at collector-to-base region increases in such that the effective width W of the base is reduced. This causes an increase of minority carrier, which is the source of reversed saturation current S. Knowing that S is inversely proportional to W and C V E / V T S e, thus there is an increase of C current

33 Figure 2.32: The figure shows that the output resistance r O has finite value 2.5 Transistor As a Switch n digital electronic, transistor is configured as a switch that operates between cutoff and saturation regions. Consider an npn bipolar junction transistor circuit shown in Fig f the input voltage V in is equal to V CC and the ratio of the base resistance to collector resistance / C or the ratio of collector current to base current C / is less than β value, then the transistor is be driven into saturation. Likewise, if the input voltage V in is less than the base-to-emitter voltage V E of 0.7V, the transistor will be at cutoff. Under these conditions, the transistor acts like an inverter switch. ( Vin V From circuit shown in Fig. 2.33, base current is equal to E ). However, at saturation collector current C is equal to C V CC / C. Knowing that C β, thus, the result of / C at saturation is equal to Vin V VCC β E CC E. f V in V CC, then is less than one. This shall C VCC mean that ratio of / C is less than β for a transistor to operate as a switch. This result infers that the ratio of C / current is less than β for a bipolar junction transistor to work as switch. V V

34 Tutorials Figure 2.33: An npn bipolar junction transistor used as an inverter switch 2.1. The majority carrier in base region of an npn transistor is Explain the purpose of a thin, lightly doped base and a heavily doped emitter Why collector current C is less than emitter current E? 2.4. Discuss how the base-emitter terminal and collector-emitter terminal of a bipolar junction transistor should be biased for normal functioning A base current of 50µA is applied to a transistor in figure below and a voltage of 5V is dropped across resistor C. Determine α and β for the transistor

35 2.6. Find V CE, V E, and V C of the transistor shown in figure below. Deduce whether or not the transistor is saturated Calculate the V CE(max) and C(sat) for the amplifier shown in figure below and draw its dc load line. What is the ac range can be applied at the base without distortion given that β 100?

36 2.8. efers to circuit of Q2.7, if you need to be 10.0µA, what will be the values of V and the Q-point of this amplifier? You may take and β Among the dc biasing circuits for transistor that you have learnt, name the one that its Q-point will be greatly affected by temp.variation. State the reason. eferences 1. Theodore F. ogart Jr., Jeffrey S. easley, and Guillermo ico, Electronic Devices and Circuit, sixth edition, Prentice Hall, Thomas L. Floyd, "Electronic Devices", Prentice Hall nternational, nc., Adel S. Sedra and Kenneth C. Smith, "Microelectronic Circuits", fourth edition, Oxford University Press, obert L. oylestad, and Louis Nashelsky, Electronic Devices and Circuit Theory, eighth edition, Prentice Hall,