Chapter 4 Physics of Bipolar Transistors. EE105 - Spring 2007 Microelectronic Devices and Circuits. Structure and Symbol of Bipolar Transistor

Transcription

1 EE105 - Spring 2007 Microelectronic Devices and ircuits Lecture 10 Bipolar ransistors 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 Structure and Symbol of Bipolar ransistor Forward Active Region 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. 3 Forward active region: BE > 0, B < 0. Figure b) presents a wrong way of modeling Figure a). 4

2 Accurate Bipolar Representation arrier ransport in Base ollector also carries current due to carrier injection from base. 5 6 ollector urrent Simple ransistor onfiguration S 2 AqDn E n i BE 1 NW E B BE S 2 E n i AqDn NW E B 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 voltagecontrolled element, thus a good candidate as an amplifier. 7 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. 8

3 onstant urrent Source Base urrent β B 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. 9 Base current consists of two components: Reverse injection of holes into the emitter and Recombination of holes with electrons coming from the emitter. 10 Emitter urrent Summary of urrents E E β B Applying Kirchoff s current law to the transistor, we can easily find the emitter current. + B β E B S 1 β β + 1 β S S β α β + 1 BE BE BE 11 12

4 Bipolar ransistor Large Signal Model Example: Maximum R L A diode is placed between base and emitter and a voltage controlled current source is placed between the collector and emitter. 13 As R L 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. 14 haracteristics of Bipolar ransistor Example: haracteristics 15 16

5 ransconductance isualization of ransconductance g g g m m m d BE S dbe 1 BE S ransconductance, g m 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. 17 g m can be visualized as the slope of versus BE. A large has a large slope and therefore a large g m. 18 Small-Signal Model: Derivation Small-Signal Model: BE hange 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

6 Small-Signal Model: E hange Small Signal Example 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. 21 g r π m β g m Ω 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 22 Small Signal Example A Ground Since the power supply voltage does not vary with time, it is regarded as a ground in small-signal analysis. n this example, a resistor is placed between the power supply and collector, therefore, providing an output voltage

7 Early Effect Early Effect llustration 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. With Early effect, collector current becomes larger than usual and a function of E Early Effect Representation Early Effect and Large-Signal Model 27 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. 28

8 Early Effect and Small-Signal Model Summary of deas Δ ro Δ E S A BE A Bipolar ransistor in Saturation Large-Signal Model for Saturation Region When collector voltage drops below base voltage and forward biases the collector-base junction, base current increases and the current gain factor, β, decreases

9 Overall / haracteristics Example: Acceptable Region R + ( 400 m) BE he speed of the BJ also drops in saturation. 33 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. 34 Deep Saturation PNP ransistor n deep saturation region, the transistor loses its voltage-controlled current capability and E becomes constant. 35 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. 36

10 A omparison between NPN and PNP ransistors PNP Equations with Early Effect he figure above summarizes the direction of current flow and operation regions for both the NPN and PNP BJ s. 37 B E EB S S EB β β + 1 EB S β EB E S 1+ A 38 Large Signal Model for PNP PNP Biasing Note that the emitter is at a higher potential than both the base and collector

11 Small Signal Analysis 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 Small Signal Model Example Small Signal Model Example Small-signal model is identical to the previous ones

12 Small Signal Model Example Small Signal Model Example Since during small-signal analysis, a constant voltage supply is considered to be A ground, the final smallsignal model is identical to the previous two