P-N junctions form the basis for most semiconductor devices. The unique electrical properties attributed to P-N junctions allow production of semiconductor devices of great variety and versatility, including diodes, thyristors, transistors, solar cells, LEDs, and more.
Properties of P-N Junctions
In order to understand how P-N junctions are used to construct semiconductor devices, one must have a general understanding of the concept of doping.
Semiconductor doping can be defined as the process of intentionally introducing impurities of other metals into a pure semiconductor in order to modify its conducting ablities. There are two types of doping:
- P-Doping - introduction of impurity atoms with one less valence electron than silicon (acceptor impurities), resulting in available positive charge carriers (holes)
- N-Doping - introduction of impurity atoms with one more valence electron than silicon (donor impurities), resulting in available negative charge carriers (electrons)
Most semiconductor devices are formed from junctions of dissimilar material. The junctions can be formed between p- and n-type doped materials, or between a metal and a semiconductor.
A P-N Junction is the boundary between n-doped and p-doped materials (e.g. silicon). The surrounding region of this boundary may be conductive or nonconductive depending on the relative voltages of the p- and n-type materials.
Figure 1. Current-Voltage characteristic of P-N junctions
In the absence of an electric field, electrons in the n-type material near this boundary tend to migrate, or diffuse, into the p-type material, resulting in the creation of positively charged ions in the n-type material and negatively charged ions in the p-type material. This can be equivalently thought of as the diffusion of holes from the p-type material into the n-type material. The electric field generated by the displaced charges, refered to as the jonction's built-in potential, opposes the diffusion, and an equilibrium is reached where there exists charged regions in the p- and n-type materials near the boundary. This area is called the depletion zone, or depletion layer, since the diffusion of electrons and holes leaves this region depleted of charge carriers available for conduction.
If a voltage is applied across the P-N junction, with the positive terminal connected to the p-type material and the negative terminal connected to the n-type material (forward bias), electrons in the n-type material and holes in the p-type material are repeled towards the boundary, reducing the depletion zone, and thus increasing the conductivity of the material so that current may flow.
If, however, a voltage is applied across the P-N junction, with the positive terminal connected to the n-type material and the negative terminal connected to the p-type material (reverse bias), the diffused electrons and holes are pulled further from the boundary, increasing the depletion zone and the resistance of the material. In this case, there will not be current flow in the forward direction, but there will be a nominal flow of current in the reverse direction. This reverse current will be negligible and nearly constant up to a certain voltage refered to as the breakdown voltage.
|Voltage polarity||Current Flow||Depletion Zone|
|Forward Direction||negative pole on n-type||Yes||None|
|No Applied Voltage||---||No||Narrow|
|Reverse Direction||positive pole on n-type||only reverse current||Wide|
Table 1. P-N Junctions when voltage is applied
Reverse Bias in P-N Junctions
As mentioned above, reverse current occurs when a reverse voltage (with positive pole to N-type material) is applied accross the PN junction. For small values of reverse voltage, the amount of reverse current remains negligible and nearly constant, until a particular value of reverse voltage, known as the breakdown voltage, is reached. Beyond this point, reverse current increases sharply.
Figure 2. Generation of Reverse Current
The reverse current is produced exclusively in the space charge region, while the rest of the silicon is effectively free of electric fields.
Reverse Bias - Breakdown in P-N Junctions
The two phenomenon that cause current to increase through reverse biased P-N junctions at breakdown are:
- The zener effect
- Avalanche effect
If there are extremely strong fields within the depletion region, the field effectively breaks the covalent bonds of the atoms in the material, resulting in the creation of electron-hole pairs. The reverse current rises as a result of the increase in available charge carriers, a phenomenon known as the Zener effect. The Zener effect occurs only in cases of high silicon doping.
If an electric sufficiently high, free electrons are accelerated such that their energy is great enough to free other electrons from their bonds (impact ionization). The new electrons and the holes that occur simultaneously are also accelerated and produce free charge carriers through impact ionisation. In this way, the number of free charge carriers increases rapidly and there is a steep increase in reverse current (avalanche breakdown).
Figure 3. Breakdown in a P-N junction
For more information, please read: