Fundamentals of Microelectronics

Transcription

1 Fundamentals of Microelectronics H1 Why Microelectronics? H2 Basic Physics of Semiconductors H3 Diode ircuits H4 Physics of Bipolar ransistors H5 Bipolar Amplifiers H6 Physics of MOS ransistors H7 MOS Amplifiers H8 Operational Amplifier As A Black Box 1 hapter 4 Physics of Bipolar ransistors 4.1 General onsiderations 4.2 Structure of Bipolar ransistor 4.3 Operation of Bipolar ransistor in Active Mode 4.4 Bipolar ransistor Models 4.5 Operation of Bipolar ransistor in Saturation Mode 4.6 he PNP ransistor 2 1

2 Bipolar ransistor n the chapter, we will study the physics of bipolar transistor and derive large and small signal models. H4 Physics of Bipolar ransistors 3 oltage-dependent urrent Source A out in KR L A voltage-dependent current source can act as an amplifier. f KRL is greater than 1, then the signal is amplified. H4 Physics of Bipolar ransistors 4 2

3 oltage-dependent urrent Source with nput Resistance Regardless of the input resistance, the magnitude of amplification remains unchanged. H4 Physics of Bipolar ransistors 5 Exponential oltage-dependent urrent Source A three-terminal exponential voltage-dependent current source is shown above. deally, bipolar transistor can be modeled as such. H4 Physics of Bipolar ransistors 6 3

4 Structure and Symbol of Bipolar ransistor Bipolar transistor can be thought of as a sandwich of three doped Si regions. he outer two regions are doped with the same polarity, while the middle region is doped with opposite polarity. H4 Physics of Bipolar ransistors 7 njection of arriers Reverse biased PN junction creates a large electric field that sweeps any injected minority carriers to their majority region. his ability proves essential in the proper operation of a bipolar transistor. H4 Physics of Bipolar ransistors 8 4

5 Forward Active Region Forward active region: BE > 0, B < 0. Figure b) presents a WRONG way of modeling figure a). H4 Physics of Bipolar ransistors 9 Accurate Bipolar Representation ollector also carries current due to carrier injection from base. H4 Physics of Bipolar ransistors 10 5

6 arrier ransport in Base H4 Physics of Bipolar ransistors 11 ollector urrent S A E qd n n N W S E E B A E qd n n N W B 2 i exp BE 2 i exp BE 1 Applying the law of diffusion, we can determine the charge flow across the base region into the collector. he equation above shows that the transistor is indeed a voltage-controlled element, thus a good candidate as an amplifier. H4 Physics of Bipolar ransistors 12 6

7 Parallel ombination of ransistors When two transistors are put in parallel and experience the same potential across all three terminals, they can be thought of as a single transistor with twice the emitter area. H4 Physics of Bipolar ransistors 13 Simple ransistor onfiguration Although a transistor is a voltage to current converter, output voltage can be obtained by inserting a load resistor at the output and allowing the controlled current to pass thru it. H4 Physics of Bipolar ransistors 14 7

8 onstant urrent Source deally, the collector current does not depend on the collector to emitter voltage. his property allows the transistor to behave as a constant current source when its base-emitter voltage is fixed. BJ Modes of Operation Mode EBJ BJ utoff Reverse Reverse Active Forward Reverse Saturation Forward Forward H4 Physics of Bipolar ransistors 15 Base urrent β B Base current consists of two components: 1) Reverse injection of holes into the emitter and 2) recombination of holes with electrons coming from the emitter. H4 Physics of Bipolar ransistors 16 8

9 9 H4 Physics of Bipolar ransistors 17 Emitter urrent Applying Kirchoff s current law to the transistor, we can easily find the emitter current. B E B E + + β β 1 1 H4 Physics of Bipolar ransistors 18 Summary of urrents α β β β β β exp 1 exp 1 exp BE S E BE S B BE S

10 Bipolar ransistor Large Signal Model A diode is placed between base and emitter and a voltage controlled current source is placed between the collector and emitter. H4 Physics of Bipolar ransistors 19 Example: Maximum R L As RL increases, x drops and eventually forward biases the collector-base junction. his will force the transistor out of forward active region. herefore, there exists a maximum tolerable collector resistance. H4 Physics of Bipolar ransistors 20 10

11 haracteristics of Bipolar ransistor H4 Physics of Bipolar ransistors 21 Example: haracteristics H4 Physics of Bipolar ransistors 22 11

12 ransconductance g g g m m m d S exp dbe 1 BE S exp BE ransconductance, gm shows a measure of how well the transistor converts voltage to current. t will later be shown that gm is one of the most important parameters in circuit design. H4 Physics of Bipolar ransistors 23 isualization of ransconductance gm can be visualized as the slope of versus BE. A large has a large slope and therefore a large gm. H4 Physics of Bipolar ransistors 24 12

13 ransconductance and Area When the area of a transistor is increased by n, S increases by n. For a constant BE, and hence gm increases by a factor of n. H4 Physics of Bipolar ransistors 25 ransconductance and c he figure above shows that for a given BE swing, the current excursion around 2 is larger than it would be around 1. his is because gm is larger 2. H4 Physics of Bipolar ransistors 26 13

14 Small-Signal Model: Derivation Small signal model is derived by perturbing voltage difference every two terminals while fixing the third terminal and analyzing the change in current of all three terminals. We then represent these changes with controlled sources or resistors. H4 Physics of Bipolar ransistors 27 Small-Signal Model: BE hange H4 Physics of Bipolar ransistors 28 14

15 Small-Signal Model: E hange deally, E has no effect on the collector current. hus, it will not contribute to the small signal model. t can be shown that B has no effect on the small signal model, either. H4 Physics of Bipolar ransistors 29 Small Signal Example g r π m m Here, small signal parameters are calculated from D operating point and are used to calculate the change in collector current due to a change in BE Ω β 375 Ω g H4 Physics of Bipolar ransistors 30 15

16 Small Signal Example n this example, a resistor is placed between the power supply and collector, therefore, providing an output voltage. H4 Physics of Bipolar ransistors 31 A Ground Since the power supply voltage does not vary with time, it is regarded as a ground in small-signal analysis. H4 Physics of Bipolar ransistors 32 16

17 Early Effect he claim that collector current does not depend on E is not accurate. As E increases, the depletion region between base and collector increases. herefore, the effective base width decreases, which leads to an increase in the collector current. H4 Physics of Bipolar ransistors 33 Early Effect llustration With Early effect, collector current becomes larger than usual and a function of E. H4 Physics of Bipolar ransistors 34 17

18 Early Effect Representation H4 Physics of Bipolar ransistors 35 Early Effect and Large-Signal Model Early effect can be accounted for in large-signal model by simply changing the collector current with a correction factor. n this mode, base current does not change. H4 Physics of Bipolar ransistors 36 18

19 Early Effect and Small-Signal Model r o E S A exp BE A H4 Physics of Bipolar ransistors 37 Summary of deas H4 Physics of Bipolar ransistors 38 19

20 Bipolar ransistor in Saturation When collector voltage drops below base voltage and forward biases the collector-base junction, base current increases and decreases the current gain factor, β. H4 Physics of Bipolar ransistors 39 Large-Signal Model for Saturation Region H4 Physics of Bipolar ransistors 40 20

21 Overall / haracteristics he speed of the BJ also drops in saturation. H4 Physics of Bipolar ransistors 41 Example: Acceptable Region R + ( 400 m ) BE n order to keep BJ at least in soft saturation region, the collector voltage must not fall below the base voltage by more than 400m. A linear relationship can be derived for and R and an acceptable region can be chosen. H4 Physics of Bipolar ransistors 42 21

22 Deep Saturation n deep saturation region, the transistor loses its voltagecontrolled current capability and E becomes constant. H4 Physics of Bipolar ransistors 43 PNP ransistor With the polarities of emitter, collector, and base reversed, a PNP transistor is formed. All the principles that applied to NPN's also apply to PNP s, with the exception that emitter is at a higher potential than base and base at a higher potential than collector. H4 Physics of Bipolar ransistors 44 22

23 A omparison between NPN and PNP ransistors he figure above summarizes the direction of current flow and operation regions for both the NPN and PNP BJ s. H4 Physics of Bipolar ransistors 45 PNP Equations Early Effect B E S S exp β β + 1 β exp S S EB EB exp exp EB EB 1 + E A H4 Physics of Bipolar ransistors 46 23

24 Large Signal Model for PNP H4 Physics of Bipolar ransistors 47 PNP Biasing Note that the emitter is at a higher potential than both the base and collector. H4 Physics of Bipolar ransistors 48 24

25 Small Signal Analysis H4 Physics of Bipolar ransistors 49 Small-Signal Model for PNP ransistor he small signal model for PNP transistor is exactly DENAL to that of NPN. his is not a mistake because the current direction is taken care of by the polarity of BE. H4 Physics of Bipolar ransistors 50 25

26 Small Signal Model Example H4 Physics of Bipolar ransistors 51 Small Signal Model Example Small-signal model is identical to the previous ones. H4 Physics of Bipolar ransistors 52 26

27 Small Signal Model Example Since during small-signal analysis, a constant voltage supply is considered to be A ground, the final small-signal model is identical to the previous two. H4 Physics of Bipolar ransistors 53 Small Signal Model Example H4 Physics of Bipolar ransistors 54 27