Conductors are elements, such as copper and silver, that will conduct a flow of electricity very readily. Due to their good conducting abilities, they are formed into wire and used whenever it is desired to transfer electrical energy from one point to another.
Insulators (non-conductors), on the other hand, do not conduct electricity to any great degree and are therefore used when it is desirable to prevent a flow of electricity. Substances like sulfur, rubber, and glass are good insulators.
Materials like germanium and silicon are not good conductors but also cannot be used as insulators, since their electrical characteristics fall between those of conductors and insulators. These materials that do not make good conductors or good insulators are classified as ''semiconductors.''
The degree of difficulty in dislodging 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 two current carriers: a "hole" (which acts as a positively charged current carrier) and a free electron (which acts as a negatively charged current carrier).
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.
An analogy of hole motion is the movement of the hole as balls are moved through a tube, as shown in '''Figure 1'''. When ball number 1 is removed from the tube, a space is left. Ball number 2 then fills this space. Ball number 3 then moves into the space left by ball number 2. This action continues until all the balls have moved one space to the left, at which time there is a space left by ball number 8 at the right-hand end of the tube.
'''Figure 1: Analogy of Hole Movement'''
A pure semiconductor material will have an equal number of free electrons and holes, the number depending on the temperature of the material and the type and size of the material. Such a material is called an ''intrinsic semiconductor,'' and the current, which is borne equally by hole conduction and electron conduction, is called ''intrinsic conduction.''
If a suitable "impurity" is added to the semiconductor, the resulting mixture can be made to have either an excess of electrons, thus causing more electron current, or an excess of holes, thus causing more hole current. An impure semiconductor material is known as an ''extrinsic semiconductor.''
In the pure form, semiconductor materials are of little use in electronics. When a certain amount of impurity is added, however, the material will have more (or less) free electrons than holes, depending on the kind of impurity added. Both forms of conduction will be present, but the "majority carrier" will be dominant. The one present in the greatest quantity is called the ''majority carrier''; the other is called the ''minority carrier.'' The process of adding impurities to semiconductor material is known as ''doping.''
The first type of impurity loses its extra electron easily, which increases the conductivity of the material by contributing a free electron. This type of impurity has five valence electrons and is called a ''pentavalent impurity.'' Arsenic, antimony, bismuth, and phosphorous are pentavalent impurities. Because these materials give up, or donate, one electron to the material, they are called ''donor impurities.''
The second type of impurity tends to compensate for its deficiency of one valence electron by acquiring an electron from its neighbor. Impurities of this type in the lattice structure have only three electrons and are called ''trivalent impurities.'' Aluminum, indium, gallium, and boron are trivalent impurities. Because these materials accept one electron from the material, they are called ''acceptor impurities.''
When a pentavalent (donor) impurity like arsenic is added to germanium, it will form covalent bonds with the germanium atoms. '''Figure 2''' shows an arsenic atom (As) in a germanium lattice structure.
'''Figure 2: Germanium Lattice Structure (Donor Impurity Added)'''
The arsenic atom has five valence electrons in its outer shell but uses only four of them to form covalent bonds with the germanium atoms, leaving one electron relatively free in the crystal structure. Because this type of material conducts by electron movement, it is called a ''negative carrier type,'' or ''N-type,'' semiconductor. The amount of the impurity added is very small; it is of the order of one atom of impurity in 10 million atoms of germanium, or 100 parts per billion.
A trivalent (acceptor) impurity element can also be added to pure germanium to "dope" the material. In this case, the impurity has one less electron than it needs to establish covalent bonds with four neighboring atoms. Thus, in one covalent bond, there will be only one electron instead of two. This arrangement leaves a hole in that covalent bond. '''Figure 3''' shows the germanium lattice structure with the addition of an indium (In) atom.
'''Figure 3: Germanium Lattice Structure (Acceptor Impurity Added)'''
The indium atom has one electron less than it needs to form covalent bonds with the four neighboring atoms and thus creates a hole in the structure. Because this semiconductor material conducts by the movement of holes that are positive charges, it is called a ''positive'' carrier type, or ''P-type,'' semiconductor material. When an electron fills a hole, as shown in '''Figure 4''', the hole appears to move to the spot previously occupied by the electron.
'''Figure 4: Germanium Lattice Structure (Hole Movement)'''
Current flow through an N-type material is illustrated in '''Figure 5'''.
'''Figure 5: Current Flow in N-Type Material'''
Conduction in this type of semiconductor is similar to conduction in a copper conductor. That is, the 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 6'''.
'''Figure 6: Current Flow in P-Type Material'''
Conduction in the material is by positive carriers (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 covalent bonds, 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 from the negative terminal. This process continues as a steady stream of holes (hole current) moving toward the negative terminal. In both N-type and P-type materials, current flow in the external circuit is out of the negative terminal of the battery and into the positive terminal.
Elements with atoms that contain four valence electrons are classified as ''semiconductors.'' Examples of such elements are germanium and silicon. These semiconductor materials are of little use in electronics in their pure, or intrinsic, form. However, when they are doped with a small amount of impurity material, they form the basis for a myriad of solid-state electronic devices. The impurity elements, added to semiconductor material, fall into one of two categories: '''pentavalent,''' those with five valence electrons, and '''trivalent,''' those with three valence electrons. When a pentavalent impurity is added to a semiconductor material, the result is called N-type material. When a trivalent impurity is added, P-type material is formed.
A single block of intrinsic semiconductor material may be doped with both pentavalent and trivalent impurities to produce both N-type and P-type semiconductor materials on the same block with a junction in between.
Some of the electrons in the N-type material will migrate across the junction to the P-type material due to their attraction for the holes in the P-type material. This will make the N-type material in proximity to the junction positive with respect to the rest of the N-type material. The P-type material in proximity to the junction becomes negative with respect to the remainder of the P-type material. This is shown in '''Figure 7.''' Migration of electrons stops after the concentration of electrons has equalized in the immediate vicinity of the junction. This region is called a ''depletion region.''
'''Figure 7: P-N Junction'''
Several facts must be emphasized. The junction barrier exists only for a minute distance on either side of the junction. The formation of a barrier occurs only in a homogeneous crystal that has been properly doped. That is, doping two separate sections of crystal and then placing them in contact will not produce the desired phenomenon. Finally, the barrier is formed at the instant the crystal is manufactured, and the magnitude of the barrier is a function of the particular crystal.
The device described above is a semiconductor junction diode. '''Figure 8''' shows the schematic symbol for a semiconductor diode.
'''Figure 8: Semiconductor Diode Symbol'''
This device allows current flow in one direction while restricting current flow in the other direction. The N-material section of the device is called the cathode, and the P-material section is called the anode. In Figure 9, a potential is placed externally across the diode, with positive voltage on the anode with respect to the cathode.
'''Figure 9: Forward Bias'''
The applied voltage, called ''bias'', is in opposition to the junction barrier potential. If this voltage is increased from zero, the junction barrier will be progressively reduced, and current flow through the device will increase. This is depicted in '''Figure 10'''.
'''Figure 10: Effect of Forward Bias on Barrier Width'''
Eventually, the barrier will be eliminated, and current flow will increase rapidly with an increase in voltage. This polarity of voltage (anode positive with respect to the cathode) is called ''forward bias'', since it caused the device to conduct an appreciable current flow.
Next, consider where the anode is made negative with respect to the cathode. '''Figure 11''' illustrates this ''reverse bias'' condition.
'''Figure 11: Reverse Bias'''
Note that the reverse bias voltage aids the junction barrier potential. '''Figure 12''' depicts how the barrier is increased.
'''Figure 12: Effect of Reverse Bias on Barrier Width'''
It would seem that no current flow should be possible under this reverse bias condition. However, since the block of semiconductor material is not a perfect insulator, a very small reverse, or leakage, current will flow. '''Figure 13''' is a graph of the current flow through a semiconductor diode, plotted by values of anode-to-cathode voltage.
'''Figure 13: Semiconductor Diode Characteristic Curve'''
With reverse bias applied, a very small reverse current exists. This reverse current increases minutely with an increase in reverse current. However, if an excessive reverse voltage is applied, the structure of the semiconductor material may be broken down. This may result in damage to the diode.
The value of reverse voltage at which this breakdown occurs is called the ''breakdown voltage.'' At this voltage, current increases rapidly with small increases in reverse bias. Certain semiconductor devices are designed and doped to operate in the breakdown region without harm.
Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve, and early diodes were actually called ''valves.''
'''Figure 14''' shows a typical operating curve for a P-N junction diode. It illustrates current flow in both the forward and reverse bias conditions.
'''Figure 14: Diode Characteristic Curve'''
Small amounts of forward bias cause very little current flow, until the internal barrier potential is overcome. The potential difference varies from diode to diode but is usually no more than a few tenths of a volt, usually 0.3V for germanium and 0.4V for silicon.
Reverse bias produces a very small amount of reverse current before the breakdown point is reached. This leakage current is very small with respect to the forward current (microamperes vs. milliamperes). Beyond the breakdown point, a very rapid increase in reverse current occurs for a small change in voltage.
Rectifier diodes are used primarily in power supplies to convert alternating current (AC) to direct current (DC), a process called ''rectification''. They are also used elsewhere in circuits where a large current must pass through the diode. These diodes are primarily of the silicon type because of this material’s inherent reliability and higher overall performance compared to germanium. Silicon allows higher forward conductance, lower reverse leakage current, and operation at higher temperatures compared to germanium. The major electrical characteristics of rectifier diodes are listed below:
Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA. Signal diodes offer much faster response times than rectifier diodes. They are usually used where high-speed switching is required. A signal diode’s major electrical characteristics are:
'''Figure 15''' shows the schematic symbol for the rectifier or signal diode. The forward current flows into the point of the arrow, and reverse current is with the arrow.
'''Figure 15: Diode Schematic Symbol'''
Rectifier and signal diodes also have specified power ratings. These rating are expressed in watts or milliwatts at a specific temperature, usually 25C. If the diode is operated at a temperature above the specified temperature, all ratings must be reduced to ensure the diode will not be damaged.
The Zener diode is unique compared to other diodes in that it is designed to operate with reverse bias in the breakdown region, without being damaged. Zener diodes differ from rectifier diodes in that they are more heavily doped. Zeners are used primarily as voltage regulators, clippers, and coupling devices.
In the breakdown region, a very small change in voltage produces a very large change in current. This is because the resistance of the Zener diode drops considerably as the voltage across it is increased beyond the breakdown point. As a result, when a Zener diode is used with a dropping resistor and the voltage across the diode tends to increase, the current through the diode rises out of proportion and causes a sufficient increase in voltage drop across the dropping resistor to lower the output voltage back to normal.
Similarly, when the voltage across the diode tends to decrease, the current through the diode goes down out of proportion so that the dropping resistor drops much less voltage to raise the output voltage to normal. The zener diode is always used in the reverse direction. '''Figure 16''' shows zener diode operation. Zener current flows in the direction of the arrow.
'''Figure 16: Zener Diode Operation'''
Light-emitting diodes are major indicating devices used in electrical/electronic circuits. An LED is a regular P-N junction device doped with gallium phosphide or gallium arsenide phosphide to produce its light-emitting characteristics. Like regular junction diodes, these devices have a low forward voltage threshold. When the threshold is overcome, the junction has a low opposition and current flows easily. The symbol for an LED is shown in '''Figure 17'''.
'''Figure 17: Symbol for Light-Emitting Diode'''
Reference diodes were developed to replace Zener diodes in certain applications because of the Zener’s temperature instability. Reference diodes provide a constant voltage over a wide temperature range. Besides VZ, the important characteristic of this device is Tminand Tmax, which specifies the range over which an indicated temperature coefficient is applicable. The temperature coefficient is expressed as a percent of change of reference (VZ) per degree centigrade change in temperature.
P-N junctions exhibit capacitance properties because the depletion area represents a dielectric, and the adjacent semiconductor material represents two conductive plates. Increasing reverse bias decreases this capacitance, while increasing forward bias increases it. When forward bias is large enough to overcome the barrier potential, high forward conduction destroys the capacitance effect, except at very high frequencies. Therefore, the effective capacitance is a function of external applied voltage. This characteristic is undesirable in conventional diode operation but is enhanced by special doping in the varactor or variable capacitance (varicap) diodes. Application categories of the varactor can be divided into two main types: tuning and harmonic generation. Different characteristics are required by the two types, but both use the voltage-dependent junction capacitance effect. '''Figure 18''' shows the voltage capacitance relationships.
'''Figure 18: Typical Voltage Capacitance Relationship in Varactor'''
As a variable capacitor, the varactor is rugged, small, not affected by dust or moisture, and is ideal for remote control and precision fine-tuning. The current uses of tuning diodes span the spectrum from A.M. radio to microwaves. The most significant parameters of a tuning diode are the capacitance ratio, "Q," series resistance, nominal capacitance, leakage current, and breakdown voltage.
The capacitance ratio, which defines the tuning range, is the amount of capacitance variation over the bias voltage range. It is normally expressed as the ratio of the low-voltage capacitance divided by the high-voltage capacitance. For example, a typical specification that reads C4/C40=3 indicates that the capacitance value at 4 volts is 3 times the capacitance value at 40 volts. The high voltage in the ratio is usually the minimum breakdown voltage specification.
A 4-volt lower limit is quite common, since it describes the approximate lower limit of linear operation for most devices. The capacitance ratio of tuning diodes varies in accordance with construction. "Q" is inversely proportional to frequency, nominal capacitance to frequency, nominal capacitance, and series resistance. Ideally, tuning diodes should have high "Q," low series resistance, low reverse leakage, and high breakdown voltage at any desired capacitance ratio; however, as might be expected, these parameters are not unrelated, and improving one degrades another so that a compromise often must be reached. As a rule, diodes with low capacitance values have the highest "Q." Various schematic symbols are used to designate varactor diodes, as shown in '''Figure 19'''.
'''Figure 19: Schematic Symbols of Varactors'''
A diode that makes use of the charge carrier tunneling is the tunnel diode. It is very heavily doped so that there are many majority carriers and ions in the semiconductor sections. Because of the large number of carriers, most are not used during the initial recombination that produces the depletion region. As a result, the depletion region is very narrow, producing a thin junction that is easily crossed by electrons.
Because of the large number of carriers, there is much drift activity in the P and N sections, causing many valence electrons to have their energy levels raised closer to the conduction region. Therefore, it takes only a small applied forward voltage to cause conduction. As forward bias is first increased, diode current rises rapidly; after many carriers start participating in current flow, the random activity of the free electrons filling holes is reduced considerably, so there is much less tendency for valence electrons to be raised in energy to the conduction band. Therefore, the tunnel effect is reduced, and majority carriers make up most of the current flow. '''Figure 20''' shows the operation curve of the tunnel diode and '''Figure 21''' shows the circuit symbol.
'''Figure 20 – Tunnel Diode Operational Curve'''
'''Figure 21: Tunnel Diode Symbol'''
The sharp reduction of the available minority carriers reduces the overall current flow, so that current flow starts to decrease as diode-applied voltage is increased. As the voltage is further increased, the tunneling effect plays less and less of a part, until a valley is reached when the P-N unit starts to act as a normal semiconductor diode; the current then rises with voltage. The area after the tunneling current reaches its peak is called the ''negative resistance region,'' because current goes down as voltage is raised. Because of the tunneling effect, the tunnel diode has a quick response and can be used as a good electronic switch between the peak and valley of current. Also, the negative resistance region allows the diode to be used as an oscillator.
By its nature, the tunnel diode has a rather high reverse current, but operation under this condition is generally not used.