半导体物理学半导体 (64).pdf
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1、Base Width Modulation It is tempting to neglect the effects of base current in a transistor since the base current is usually much smaller than either the collector or the emitter current.Current CrowdingCross section of an npn bipolartransistor showing the basecurrent distribution and thelateral po
2、tential drop in the baseregionThe base region is typically less than a micrometer thick,so there can be a sizable base resistance.The nonzero base resistance results in a lateral potential difference under the emitter region.For the npn transistor,the potential decreases from the edge of the emitter
3、 toward the center.The emitter is highly doped,so as a first approximation the emitter can be considered an equipotential region The number of electrons from the emitter injected into the base is exponentially dependent on the BE voltage Current crowding effect:with the lateral voltage drop in the b
4、ase between the edge and center of the emitter,more electrons will be injected near the emitter edges than in the center,causing the emitter current to be crowded toward the edgesCross section of an npn bipolar transistor showing the emitter current crowding effect.Current CrowdingThe larger current
5、 density near the emitter edge may cause localized heating effects as well as localized high-injection effectsThe nonuniform emitter current also results in a nonuniform lateral base current under the emitter A two-dimensional analysis would be required to calculate the actual potential drop versus
6、distance because of the nonuniform base currentAnother approach is to slice the transistor into a number of smaller parallel transistors and to lump the resistance of each base section into an equivalent external resistancelarger current density near the edgen+Cross section of an npn bipolar transis
7、tor showing the emitter current crowding effect.Current CrowdingPower transistors:handle large currents,require large emitter areas to maintain reasonable current densitiesTo avoid the current crowding effect,these transistors are usually designed with narrow emitter widths and fabricated with an in
8、terdigitated designIn effect,many narrow emitters are connected in parallel to achieve the required emitter area(a)Top view and(b)cross section of an interdigitated npn bipolar transistor structureCurrent CrowdingThere are two breakdown mechanisms in a bipolar transistor.The first is called punch-th
9、rough.As the reverse-biased BC voltage increases,the BC space charge region widens and extends farther into the neutral base.BC depletion region to penetrate completely through the base and reach the BE space charge region,the effect called punch-through.Breakdown Voltage1.Punch-throughWhen a small
10、CB voltage,VR1,is applied,the BE potential barrier is not affected;thus,the transistor current is still essentially zero.When a large reverse-biased voltage,VR2,is applied,the depletion region extends through the base region and the BE potential barrier is lowered because of the CB voltage.The lower
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