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When n-type and p-type
materials are joined together, as shown in Fig. 5, an unusual but very important phenomenon occurs at the interface where the two materials meet (called the p-n junction). An interaction takes place between the two types of material at the junction as a result of the holes in one material and the excess electrons in the other.
When a p-n junction is formed, some of the free electrons from the n-type material
diffuse across the junction and recombine with holes in the lattice structure of the p-type material; similarly, some of the holes in the p-type material diffuse across the junction
and recombine with free electrons in the lattice structure of the n-type material. This interaction or diffusion is brought into equilibrium by a small space-charge region
(sometimes called the transition region or depletion layer). The p-type material thus acquires a slight negative charge and the n-type material acquires a slight positive charge.
Thermal energy causes charge carriers (electrons and holes) to diffuse from one side
of the p-n junction to the other side; this flow of charge carriers is called diffusion current. As a result of the diffusion process, however, a potential gradient builds up
across the space-charge region. This potential gradient can be represented, as shown in Fig. 6, by an imaginary battery connected across the p-n junction. (The battery
symbol is used merely to illustrate internal effects; the potential it represents is not directly measurable.) The potential gradient causes a flow of charge carriers, referred
to as drift current, in the opposite direction to the diffusion current. Under equilibrium conditions, the diffusion current is exactly balanced by the drift current so that the net
current across the p-n junction is zero. In other words, when no external current or voltage is applied to the p-n junction, the potential gradient forms an energy barrier that
prevents further diffusion of charge carriers across the junction. In effect, electrons from the n-type material that tend to diffuse across the junction are repelled by the slight
negative charge induced in the p-type material by the potential gradient, and holes from the p-type material are repelled by the slight positive charge induced in the n-type
material. The potential gradient (or energy barrier, as it is sometimes called), therefore, prevents total interaction between the two types of materials, and thus preserves the
differences in their characteristics.
Current Flow
When an external battery is connected across a p-n junction, the amount of current flow
is determined by the polarity of the applied voltage and its effect on the space-charge region. In Fig. 7(a), the positive terminal of the battery is connected to the n-type
material and the negative terminal to the p-type material. In this arrangement, the free electrons in the n-type material are attracted toward the posiitve terminal of the battery
and away from the junction. At the same time, holes from the p-type material are attracted toward the negative terminal of the battery and away from the junction. As a
result, the space-charge region at the junction becomes effectively wider, and the potential gradient increases until it approaches the potential of the external battery.
Current flow is then extremely small because no voltage difference (electric field) exists across either the p-type or the n-type region. Under these conditions, the p-n junction is
said to be reverse-biased.
In Fig. 7(b), the positive terminal of the external battery is connected to the p-type
material and the negative terminal to the n-type material. In this arrangement, electrons in the p-type material near the positive terminal of the battery break their electron-pair
bonds and enter the battery, creating new holes. At the same time, electrons from the negative terminal of the battery enter the n-type material and diffuse toward the junction.
As a result, the space-charge region becomes effectively narrower, and the energy barrier decreases to an insignificant value. Excess electrons from the n-type material
can then penetrate the space-charge region, flow across the junction, and move by way of the holes in the p-type material toward the positive terminal of the battery. This
electron flow continues as long as the external voltage is applied. Under these conditions, the junction is said to be forward-biased.
The generalized voltage-current characteristic for a p-n junction in Fig. 8 shows both
the reverse-bias and forward-bias regions. In the forward-bias region, current rises rapidly as the voltage is increased and is quite high. Current in the reverse-bias region
is usually much lower. Excessive voltage (bias) in either direction should be avoided in normal applications because excessive currents and the resulting high temperatures
may permanently damage the solid-state device.
On to Diodes
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