hw3_soln - UNIVERSITY OF CALIFORNIA AT BERKELEY College of...

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UNIVERSITY OF CALIFORNIA AT BERKELEY College of Engineering Department of Electrical Engineering and Computer Sciences Homework #3 Solutions Due Tuesday, September 18, 2007 EE 105 Fall 2007 Prof. Liu Problem 1. BJT Operation and Design (Short-Answer Questions) a) As discussed in class (and mentioned in the hint), the minority carriers in the PN junction are unable to flow through the metal-to-semiconductor contact. Recall that a BJT requires a forward biased PN junction to inject carriers from the emitter into the base (in the case of an NPN, as shown here, those carriers would be electrons), where they diffuse to the base-collector depletion region to be collected. Electrons are the minority carriers of the p-type side of the base-emitter diode, meaning that they cannot flow from the base-emitter junction to the base-collector junction if there is a metal contact in-between, as there is in the diagram. That means they cannot be collected by the electric field formed by the reverse- biased base-collector junction, and the device cannot work like a properly constructed NPN transistor. b) i) If the base is doped too heavily, it can result in significant base current and hence low current gain. The base should be doped lightly as compared with the emitter, to ensure that the vast majority of the current that flows across the emitter-base junction is comprised of carriers injected from the emitter into the base. If the base is doped too
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