BJT1.V4 - L 302-1.V4 Drexel University Electrical and...

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L302-1.V4 1-1 Drexel University Electrical and Computer Engr. Dept. Electrical Engineering Laboratory II, ECEL 302 E. L. Gerber BIPOLAR TRANSISTORS (X2) Object The object of this experiment is to learn some of the properties and electrical characteristics of the bipolar junction transistor (BJT), and to learn the fundamentals of a common emitter amplifier. Introduction The P-N junction is the basic unit of all electronic devices. The bipolar transistor has two P-N junctions. And all electronic systems are made from P-N junctions: op-amps, logic chips, microprocessors, memory chips, PALs - the computer. There are three types of transistors: bipolar (BJT), junction field effect (JFET), and metal oxide field effect (MOSFET). The first transistor (BJT) was developed in 1948 at Bell Labs in New Jersey. These first devices were made of germanium semiconductors. Now almost all transistors are made of silicon. In the 1960s the U.S. Department of Defense (DOD) poured $2 billion into the development of silicon integrated circuits (ICs) in order to replace large, heavy, power draining vacuum tubes. That is why the U.S. was the leader in ICs for decades. Theory 1) BJT Structure: The BJT is formed by chemically “doping” a pure semiconductor with both negative and positive ion elements. The process results in a structure of n-type semiconductor sandwiched between two p-types. The PNP device is shown below. A BJT can also be fabricated with a NPN structure as well. Each interface of PN material defines a basic PN junction. A PN junction is forward biased when the P-side is connected to a positive voltage. This causes large amounts of current to flow. The volt-amp equation for a PN junction is, I = I o e qV / kT 1 ( ) = I o e 40 V 1 ( )
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L302-1.V4 1-2 In this equation V is positive in the forward biased PN junction. At room temperature q/kT 40. The current increases exponentially with the positive voltage, V, applied. When the junction is reversed biased (P-side is connected to a negative voltage), very little current flows; I = - I o . I o is the reverse saturation current of the junction and is very small (<10 -8 amp). The diagram above shows a PNP device with the first junction forward biased and the second reversed. This is normal or active mode biasing and is required for amplifier and switching applications of the BJT. The diagram also defines the three terminals: emitter, base, and collector. The three terminal currents and voltages are also defined in this diagram. 2) BJT Biasing: In the normal-biased circuit above almost all of the emitter current (I E ) is due to positive holes and flows across the two junctions into the collector. The collector current, I C , consists of two terms: its own reverse saturation current, I CO , which is very small, plus a large fraction of the emitter current, α I E . The parameter α ( 1) is a property of the structure. The total collector current is written below. Since I CO is extremely small the relationship can be simplified. I
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BJT1.V4 - L 302-1.V4 Drexel University Electrical and...

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