2.2 The pn-junction

Joining two crystals

p-type and n-type materials are rarely used separately. It becomes interesting when p-type and n-type materials are joint in a so called pn-junction. The n-type material is represented as a structure of fixed positive charges, the nuclei and nearby surrounding electrons of the donor atoms, surrounded by negative free, or mobile, electrons. The p-type material is represented as a structure of fixed negative charges, the nuclei and nearby surrounding electrons of the acceptor atoms, surrounded by positive free holes. Obviously, the total material remains electrically neutral: no net charge is introduced by doping.

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Figure2.7 Schematic view on p-type and n-type crystals

When the materials are joint, the following happens. In the p-type material there is a high concentration of free holes, and they ‘see’ a very low concentration of free holes at the other side of the junction . This difference in concentration results in a so-called diffusion current: many holes close to the junction will diffuse to the other side, under the influence of a diffusion force. This process can be compared with a drop of ink in water: the ink will spread out in the fluid. The moved holes will recombine with the free electrons in the n-type material.

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Figure 2.8 A pn-junction is formed by joining p-type crystal with n-type crystal.

Similar effects are observed in the n-type material: many of the free electrons in the n-type material see a very low concentration of free electrons at the other side of the junction. They will diffuse to the p-type material and will recombine with the free holes. This results in a so called depletion layer or space charge layer in the vicinity of the junction. As a result, there is an excess of negative charges in the p-type material, and an excess of positive charges in the n-type material.

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Figure 2.9 An electric field will emerge when making a pn-junction

The built-in electric field

The region around the pn-junction is not anymore electrically neutral. An electric field has emerged that corresponds to an electric force that acts on the free holes and the free electrons. Note that this force is opposed to the diffusion force. An equilibrium of two opposite forces will emerge. The diffusion force ‘pushes’ the free electrons from the n-type material into the p-type material, but the resulting electric field ‘pushes’ them back. If not connected, a pn-junction will thus have a built-in electric field. This is a very crucial property, since this field is the basis for all diode functions, including solar cells.

Forward bias

When an electric voltage is applied across the pn-junction, with the p-type material electrically positive relative to the n-type material, the junction is said to be forward biased or forward polarized. The positive voltage at the end of the p-type material ‘pushes’ the free holes to the junction, while the negative voltage at the end of the n-type material pushes the free electrons to the junction. The space charge layer thus becomes smaller. If the applied voltage is sufficiently large, the space charge layer will disappear.  At this point, it becomes very easy for an electric current to go through the junction. In other words, a current can go through the junction when forward biased.

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Figure 2.10 The space charge layer and the electric field become smaller in forward bias.

Reverse bias

When the polarity of the electric voltage is altered, so the p-type material is electrically negative relative to the n-type material, one speaks of reverse bias. The negative voltage at the end of the p-type material ‘pulls’ the free holes away from the junction, while the positive voltage at the end of the n-type material pulls away free electrons from the junction. The depletion layer now becomes broader, and it is now very difficult for an electric current to go through the junction. At this point, there is no more electric conduction: for an electron it will be very difficult to move from one end in the n-type material to the other end of the p-type material. The reason for this is that the electric field would push it back, once it enters the space charge layer. For a hole, it will also be difficult to move in the opposite direction, for the same reason. So, when reverse biased, a current can not go through the pn-junction.

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Figure 2.11 The space charge layer and the electric field become larger in reverse bias.

Go to 2.3 The diode characteristic

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