PRESENTACIÓN, ANÁLISIS E INTERPRETACIÓN DE DATOS

Any element can be categorized as either a conductor, semiconductor, or insula-tor. Conductors are elements, such as copper or silver, which will conduct elec-tricity readily. Insulators (non-conductors) do not conduct elecelec-tricity to any great degree and are therefore used to prevent a flow of electricity. Rubber and glass are good insulators. Material such as germanium and silicon are not good conductors, but cannot be used as insulators either, since their electrical characteristics fall between those of conductors and insulators. These are called semiconductors.

The electrical conductivity of matter is ultimately dependent upon the ener-gy levels of the atoms of which the material is constructed. In any solid material such as copper, the atoms which make up the molecular structure are bound together in a crystal lattice which is a rigid structure of copper atoms. Since the atoms of copper are firmly fixed in position within the lattice structure, they are not free to migrate through the material and therefore cannot carry the electricity through the conductor without application of some external force. However, by ionization, electrons could be removed from the influence of the parent atom and made to move through the copper lattice under the influence of external forces.

It is by virtue of the movement of these free electrons that electrical energy is transported within the copper material. Since copper is a good conductor, it must contain vast numbers of free electrons.

HOLE CURRENT AND ELECTRON CURRENT

The degree of difficulty in freeing valence electrons from the nucleus of an atom determines whether the element is a conductor, semiconductor, or an insulator. When an electron is freed in a block of pure semiconductor material, it creates a hole which acts as a positively charged current carrier. Thus, an elec-tron liberation creates two currents which are known as elecelec-tron current and hole current.

When an electric field is applied, holes and electrons are accelerated in opposite directions. The life spans (time until recombination) of the hole and the free electron in a given semiconductor sample are not necessarily the same. Hole conduction may be thought of as the unfilled tracks of a moving electron. Because the hole is a region of net positive charge, the apparent motion is like the flow of particles having a positive charge.

If suitable impurity is added to the semiconductor, the resulting mixture can be made to have either an excess of electrons, causing more electron current, or an excess of holes, causing more hole current.

Depending upon the kind of impurity added to a semiconductor, it will have more (or fewer) free electrons than holes. Both electron current and hole current will be present, but a majority carrier will dominate. The holes are called posi-tive carriers and the electrons, negaposi-tive carriers. The one present in the greater quantity is called the majority carrier; the other is called the minority carrier. The quality and quantity of the impurity are carefully controlled by the doping process.

N AND P TYPE MATERIALS

When an impurity like arsenic is added to germanium it will change the germani-um crystal lattice in such a way as to leave one electron relatively free in the crys-tal structure. Because this type of material conducts by electron movement, it is called a negative carrier (N-type) semiconductor. Pure germanium may be con-verted into an N-type semiconductor by doping it with a donor impurity consist-ing of any element containconsist-ing five electrons in its outer shell. The amount of the impurity added is very small.

An impurity element can also be added to pure germanium to dope the mate-rial so as to leave one electron lacking in the crystal lattice, thereby creating a hole in the lattice. Because this semiconductor material conducts by the movement of holes which are positive charges, it is called a positive carrier (P-type) semi-conductor. When an electron fills a hole, the hole appears to move to the spot previously occupied by the electron.

As stated previously, both holes and electrons are involved in conduction. In N-type material the electrons are the majority carriers and holes are the minority carriers. In P-type material the holes are the majority carriers and the electrons are the minority carriers.

Current flow through an N-type material is illustrated in Figure 5-2.

Conduction in this type of semiconductor is similar to conduction in a copper conductor. That is, an application of voltage across the material will cause the loosely bound electron to be released from the impurity atom and move toward the positive potential point.

Current flow through a P-type material is illustrated in Figure 5-3. Conduction in this material is by positive carrier (holes) from the positive to the negative terminal. Electrons from the negative terminal cancel holes in the vicinity of the terminal, while, at the positive terminal, electrons are being removed from the crystal lattice, thus creating new holes. The new holes then move toward the negative terminal (the electrons shifting to the positive terminal) and are canceled by more electrons emitted into the material from the negative terminal. This process continues as a steady stream of holes (hole current) move toward the negative terminal.

Figure 5-2. Electron Flow N-Type Material.

Figure 5-3. Electron Flow in P-Type Material.

P-N JUNCTION

Both N-type and P-type semiconductor materials are electrically neutral.

However, a block of semiconductor material may be doped with impurities so as to make half the crystals N-type material and the other half P-type material. A force will then exist across the thin junction of the N-type and P-type material. The force is an electro-chemical attraction by the P-type material for electrons in the N-type material. Due to this force, electrons will be caused to leave the N-type material and enter the P-type material. This will make the N-type material near to the junction positive with respect to the remainder of the N-type material. Also the P-type material near to the junction will be negative with respect to the remaining P-type material.

After the initial movement of charges, further migration of electrons ceases due to the equalization of electron concentration in the immediate vicinity of the junction. The charged areas on either side of the junction constitute a potential barrier, or junction barrier, which prevents further current flow. This area is also called a depletion region. The device thus formed is called a semiconductor diode.

SEMICONDUCTOR DIODE

The schematic symbol for the semiconductor diode is illustrated in Figure 5-4. The N-type material section, of the device is called the cathode and the P-type materi-al section the anode. The device permits electron current flow from cathode to anode and restricts electron current flow from anode to cathode.

Consider the case where a potential is placed externally across the diode, positive on the anode with respect to the cathode as depicted in Figure 5-5. This polarity of voltage (anode positive with respect to the cathode) is called forward bias since it decreases the junction barrier and causes the device to conduct appreciable current. Next, consider the case where the anode is made negative with respect to the cathode. Figure 5-6 illustrates this reverse bias condition.

Figure 5-4. Semiconductor Diode Symbol.

Theoretically, no current flow should be possible with reverse bias applied across the junction due to the increase in the junction barrier.

However, since the block of semiconductor material is not a perfect insulator, a very small reverse or leakage current will flow. At normal operating temperatures this current may be neglected. It is noteworthy, however, that leakage current increases with an increase in temperature. The characteristic curve of the typical diode is shown in Figure 5-7. Excessive forward bias results in a rapid increase of forward current and could destroy the diode. By the same token, excess reverse bias could cause a breakdown in the junction due to the stress of the electric field. The reverse bias point at which breakdown occurs is called the breakdown or avalanche voltage.

Some semiconductor diodes are made to operate in the breakdown or ava-lanche region, the most common being the zener diode which is discussed later.

Figure 5-5. Semiconductor Diode with Forward Bias.

Figure 5-6. Semiconductor Diode with Reverse Bias.

TRANSISTORS

By connecting two P-N junctions, either at their N sides or their P sides, and appropriately applying forward bias to one junction while reverse biasing the other junction, an interesting phenomenon occurs. The thin connecting section of material is the base, and the sections on either end of the junction are the emit-ter and collector respectively. This device is shown in Figure 5-8. Reverse bias applied to the base-collector junction causes a small reverse current as shown in Figure 5-7 for a typical P-N junction. By forward biasing the emitter-base junction, the base-collector junction is driven further into the breakdown, or avalanche region, resulting in a much larger collector current. What, then, is the difference between the simple junction diode and the transistor? If a small, varying signal is applied between the emitter and base, the bias across the base-emitter junction can be used to control the large current flow in the collector circuit, and if the bias is reversed, current flow ceases. This is the means for controlling a large current by varying a smaller one, which is the basis for amplification.

Figure 5-7. Semiconductor Diode Characteristic Curve.

Figure 5-8. Basic PNP Transistor Circuit.