The change of the collector current when changing the collector-emitter voltage is primarily due to the variation of the base-collector voltage, since the base-emitter junction is forward biased and a constant base current is applied. The Early voltage, V A, is also indicated on the figure. It is indicated on the figure by the horizontal arrow.Ĭollector current increase with an increase of the collector-emitter voltage due to the Early effect. The Early voltage equals the horizontal distance between the point chosen on the I- V characteristics and the intersection between the tangential line and the horizontal axis. The Early voltage, V A, is obtained by drawing a line tangential to the transistor I- V characteristic at the point of interest. The Early effect is observed as an increase in the collector current with increasing collector-emitter voltage as illustrated with Figure 5.4.2. This effect is referred to as the Early effect. The gradient of the minority-carrier density in the base therefore changes, yielding an increased collector current as the collector-base current is increased. Variation of the minority-carrier distribution in the base quasi-neutral region due to a variation of the base-collector voltage.Ī variation of the base-collector voltage results in a variation of the quasi-neutral width in the base. This causes the collector current to vary with the collector-emitter voltage as illustrated in Figure 5.4.1. We conclude this section with the temperature dependence and breakdown mechanisms in BJTs.Īs the voltages applied to the base-emitter and base-collector junctions are changed, the depletion layer widths and the quasi-neutral regions vary as well. High injection effects, base spreading resistance and emitter current crowding are described next. These include the base-width modulation effects and the current due to recombination in the depletion layers. Non-ideal effects Chapter 5: Bipolar Junction Transistorsĥ.4. Non-ideal effects 5.4.1. Base-width modulation 5.4.2. Recombination in the depletion region 5.4.3. High injection effects 5.4.4. Base spreading resistance and emitter current crowding 5.4.5. Temperature dependent effects in bipolar transistors 5.4.6. Breakdown mechanisms in BJTsĪ variety of effects occur in bipolar transistors, which are not included in the ideal transistor model. The old germaium alloy junctions could take about 30V reverse base emitter, same as the Vcb.5.4. Thats why modern silicon planars only hav a reverse base emitter breakdown capability of of about 5 to 7V. Reduce capacitance and hence improve high frequency performance. Increase transistor efficiency by easing the flow hole/electrons into the collector region. The base emitter has very thin region to, Unlike modern silicon planars were the junction widths are delibrately asymetric to get good voltage breakdown between base /collector. We just had to do a thorough visual exmination. Hence no electrical test could identify a line wiring mistake. The voltage and current capability was equal either way around. The problem was they were simple alloy junction transistors where the collector and emitter regions were the same size. One of the problems was identifying germanium transistors in the control circuitry that had been wired in the wrong way around. I remember production line testing linear adjustable bench power supplies around 1970. RE: Reversing collector and emitter on a PNP What other information can anyone tell be about using a PNP transistor with the collector and emitter reversed? What other characteristics do transistors have when used this way? Since the transistor is driven by a current source it is protected. I can find little data on using a PNP transistor with collector and emitter reversed, just a Wikipedia reference about how the gain is much, much lower. Reversing this transistor keeps other opamps off the rail and solves temperature drift problems of the circuit as a whole. This transistor is part of a multivibrator circuit (NPN and PNP in a SCR-like arrangement) being fed from a 11 ma constant current source and capacitor through a double emitter transistor (to create a ramp). Through shear accident, it was discovered that the problems with the design could be fixed by reversing the collector and emitter on a 2N3906 PNP. But I need to fix it with minimum changes. In other words, the engineers who designed it are long gone, and the circuit approach is not-at-all what would be done today. This industrial product was designed around 1970, and consist of a mixture of discrete transistors, and LM741/LM308 opamps.
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