Physics of semiconductor devices. Andreas Dörfler. Students project. Seite 1 von
4. Bipolar transistors. Exam question: How does a bipolar transistor works?
Physics of semiconductor devices
Andreas Dörfler
Students project
Bipolar transistors Exam question: How does a bipolar transistor works? This exam question is separated in two parts: a) How does a bipolar transistor works? b) What is the Early effect in a bipolar transistor?
Solution: a) How does a bipolar transistor works? Basically bipolar transistors consist out of two pn- junctions and have three contacts (base, emitter and collector).
Figure 1: npn- Transistor
First pn- junction is located between E emitter and B the base. This pn- junction is forward biased and had a thin depletion width. The second pn- junction between base B and collector C is reverse biased and have a larger depletion width. The area outside the depletion regions are field free this means particularly for the region in the middle of the two pn- junctions, the base. The polarization of a bipolar transistor is called active mode. In the figure 2 the band structure of an npn- transistor is shown.
Seite 1 von 4
Physics of semiconductor devices
Andreas Dörfler
Students project
Figure 2: band- structure of a npn- bipolar transistor
Electrons flow from the emitter through the left pn- junction in the base. The most of these electrons can flow directly to the base-collector-junction. In this junction the electric field is strong because the pn-junction is reverse biased. The collector with his positive polarization absorbs the electrons from the emitter. For this reason the emitter must transmit electrons constantly. Also a charge current (npn: holes) must flow in the base. The main part of the electron current flows from the emitter to the collector but a small part is needed in the base for the recombination with the holes from the base connection. This part is necessary for the function of the transistor. The more electrons are available, the greater is the probability that a hole meets an electron and recombined. The (bigger) electron current in the emitter is directly proportional to the (small) hole current of the base.
b) What is the Early- effect in a bipolar transistor? Early effect or base width modulation: The early effect is the variation in the width of the base in a bipolar transistor due to a variation in the applied base-to-collector voltage. For example a greater reverse bias across the collector- base junction increases the collector-base depletion width. If VCE increases VCB increases too. E
