Page 50 - Alternative Energy Systems in Building Design

SOLAR CELL PHYSICS 27

Metals such as gold, copper, and iron are conductors; they have loosely bound elec-
trons in the outer shell or orbit of their atomic configuration. These electrons can be
detached when subjected to an electric voltage or current. On the contrary, atoms of
insulators, such as glass, have very strongly bonded electrons in the atomic configu-
ration and do not allow the flow of electrons even under the severest application of
voltage or current. Semiconductor materials, on the other hand, bind electrons midway
between that of metals and insulators.
Semiconductor elements used in electronics are constructed by fusing two adjacently
doped silicon wafer elements. Doping implies impregnation of silicon by positive and
negative agents, such as phosphor and boron. Phosphor creates a free electron that pro-
duces so-called N-type material. Boron creates a “hole,” or a shortage of an electron, that
produces so-called P-type material. Impregnation is accomplished by depositing the pre-
viously referenced dopants on the surface of silicon using a certain heating or chemical
process. The N-type material has a propensity to lose electrons and gain holes, so it
acquires a positive charge. The P-type material has a propensity to lose holes and gain
electrons, so it acquires a negative charge.
When N- and P-type doped silicon wafers are fused together, they form a PN junction.
The negative charge on the P-type material prevents electrons from crossing the junction,
and the positive charge on the N-type material prevents holes from crossing the junction.
A space created by the P and N, or PN, wafers creates a potential barrier across
the junction. Figure 3.2 illustrates flow of electrons and holes in a semiconductor PN
junction.
This PN junction, which forms the basic block of most electronic components, such
as diodes and transistors, has the following specific operational uses when applied in
electronics:

■ In diodes, a PN device allows for the flow of electrons and, therefore, current in one
direction. For example, a battery, with direct current, connected across a diode,
allows the flow of current from positive to negative leads. When an alternating sinu-
soidal current is connected across the device, only the positive portion of the wave-
form is allowed to pass through. The negative portion of the waveform is blocked.
■ In transistors, a wire secured in a sandwich of a PNP-junction device (formed by
three doped junctions), when polarized or biased properly, controls the amount of
direct current from the positive to the negative lead, thus forming the basis for current
control, switching, and amplification, as shown in Fig. 3.3.
■ In light-emitting diodes (LEDs), a controlled amount and type of doping material
in a PN-type device, connected across a dc voltage source, converts the electric
energy into visible light with differing frequencies and colors, such as white, red,
blue, amber, and green.
■ In solar cells, when a PN junction is exposed to sunshine, the device converts the
stream of photons (packets of quanta) that form the visible light into electrons (the
reverse of the LED function). This makes the device behave like a minute battery
with a unique characteristic voltage and current depending on the material dopants
and PN-junction physics.