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The P-N Junction

If a silicon crystal is doped with donor atoms at one end and with acceptor atoms at the other end, the crystal will have both P-type and N-type regions and there will be a junction (Fig. 1) between them. In Fig. 2 the plane AB is the P-N junction; only the free electrons and holes have been shown. Both regions include charge carriers of either sign but in the N-type region electrons are in the majority and in the P-type region holes predominate. In both regions the probability of a minority charge carrier meeting and recombining with a majority charge carrier is high and the minority charge carrier lifetime is short.

Fig. 1. The junction is strong

Fig.2. P-N junction

The free electrons and holes have completely random motions and wander freely in the lattice. However, since there are more electrons to the left of the P-N junction than to the right and more holes to the right of the junction than to the left, on average more electrons cross the junction from left to right than from right to left.

On average, therefore, the N-type region gains holes and loses electrons and the P-type region gains electrons and loses holes. This process is known as diffusion and may be defined as the tendency for charge carriers to move away from areas of high density.

Since the N-type region loses negative charge carriers and gains positive charge carriers and the P-type region loses positive charge carriers and gains negative charge carriers, the region to the left of the junction becomes positively charged and the region to the right of the junction becomes negatively charged. A hole passing into the N-type region, or an electron passing into the P-type region, becomes a minority charge carrier and will probably recombine with a carrier of opposite sign and disappear. However, one region has lost a positive (or negative) charge and the other region has gained a positive (or negative) charge. The movement of holes and electrons across the junction constitutes current and this is known as the diffusion current.

16. Fill in the gaps.

1. In the N-type region electrons are in … and in the P-type region holes … .

2. Since the N-type region loses … and gains … and the P-type region loses … and gains …, the region to the left of the junction becomes … and the region to the right of the junction becomes … .

3. If a silicon crystal is doped with … at one end and with … at the other end, the crystal will have both … and there will be … between them.

4. In both regions the probability of a minority charge carrier meeting and recombining with a majority charge carrier is … and the minority charge carrier lifetime is … .

5. The movement of holes and electrons across the junction … current and this is known as … .

17. Read the text “The P-N Junction“ again in your memory and answer the questions.

1. What is a sign of charge carriers in the N-type and P-type regions?

2. What regions include majority charge carriers and minority charge carriers?

3. What is their lifetime?

4. On average more electrons cross the junction from right to left than from left to right, don’t they?

5. How can you define the process of diffusion?

6. Do you know what the diffusion current is?

7. What will happen if a hole passes into the N-type region or an electron passes into the P-type region?

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