Joining P- and N-Type Germanium

Like N-type germanium, P-type germanium is also electrically neutral (Fig. 1).

When N-type germanium and P-type germanium are joined some electrons and holes combine at the junction. In the region of the junction, N-type germanium loses some of its electrons. Thus, it is no longer neutral in this area; it now has a positive charge. The electrons it loses,combine with holes

from P-type germanium at the junction. The P-type germanium becomes negative. The majority carriers have combined at the junction, leaving charged atoms (ions) in the area near the junction. A potential difference (of the order of several tenths of a volt) exists between the N- and P-type germanium ions. If more electrons try to move from the N-type to the P-type, they are stopped by the negatively charged ions in the P-type germanium near the junction (Fig. 2). In a similar fashion holes from the P-type are prevented from crossing the junction by the building up of positively charged ions in the N-type germanium. The net effect of this action is to set up a barrier voltage that prevents further combination of electrons and holes. The area in which this voltage exists is called the barrier region.

Fig. 1 Doped germanium is electrically neutral

Fig. 2 Electrons are stopped near the junction

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