Speaking

20. Draw a picture of a p-n junction in your own interpretation and describe it.

Start your description like this:

This plane AB is the P-N junction.

20. Act as an interpreter. Translate the description of the p-n junction given by your group mate from English into Russian.

21. Divide into two groups. Group a translates text a “The p-n Junction”, group b translates text b “Joining p- and n-Type Germanium” with a dictionary in writing. Text a

The P-N Junction

If the crystal was neutral before diffusion took place, it must be neutral afterwards. Further, because both regions were also originally neutral they must contain equal and opposite charges after diffusion. These charges have an attractive electric force between them and are not able to diffuse away from the vi­cinity of the junction . The two charges are concentrated immediately adjacent to the junction, and produce a potential barrier across the junction.

The polarity of the potential barrier is such as to oppose the further diffusion of majority charge carriers across the junction, but to aid the movement of minority charge carriers; this gives rise to a minority charge carrier current in the opposite direction to the diffusion current.

The difference in potential from one side of the junction to the other is called the height of the potential barrier and is measured in volts. The height of the potential barrier attains such a value that the majority charge carrier (diffusion) currents and minority charge carrier current are equal and so the net current across the junction is zero. Any charge carriers entering the region on either side of the junc­tion over which the barrier potential is effective are rapidly swept out of it, and hence this region is depleted of charge carriers.

Text b 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