Semiconductors

Question File Number 11



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Devices

The diode is used extensively in radio equipment. It is a two terminal device that passes current in only one direction.

Transistors are three-terminal devices. We look at the junction transistor followed by the field effect transistor.

Materials

In Electronic Fundamentals we discussed the atomic structure of various materials.

Materials composed of atoms whose outer electrons are loosely bound to the nuclei are classified as conductors because their electrons can be moved about easily by an applied potential, and so produce an electric current. Materials with strong bonds between electrons and nuclei are classified as insulators because their electrons cannot be moved about easily by an applied potential, to produce an electric current. Between these two extremes there is a category of materials which can have their properties controlled to vary between conductors and insulators. This class of material is called semiconductors. In electronics, silicon, germanium, and gallium are the semiconductor materials that are of major interest.

Atomic structure

The peculiar properties of semiconductors are due to their crystal structure. Each atom has four outer electrons, and each of these electrons, called valence electrons, is bonded to one of the neighbouring atoms.

This diagram shows the crystal form and a typical bonding through linking lines.

lattice

At very low temperatures the crystal structure is complete and the resistivity of the material is very high. As the temperature increases, an increasing number of electrons acquire enough energy to break the bond, and they become free and able to contribute to a current flow. The resistivity decreases as temperature is increased, therefore, the material has a negative temperature coefficient. It is important to remember that this effect can, if left unchecked, result in permanent damage to a semiconductor device.

When an electron breaks away from its parent atom, a hole is left behind. Since this is caused by the loss of a negative charge, the hole can be considered to be a positive charge. The hole will be filled by another electron, and another hole created. As this happens throughout the crystal lattice, there is not only conduction by electrons in one direction, but also an apparent conduction of holes (positive charges) in the opposite direction. This diagram shows a free electron and a hole in a crystal lattice.

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Impurity Additives

To ensure control of conduction, the materials silicon and germanium are refined to a very high degree of purity. Other materials called impurities are carefully added, in a process called doping, to the silicon or germanium so that they behave as required for use in electronic circuits.

As already noted, both germanium and silicon have four valence electrons. If a material having five valence electrons is added as the impurity, it will lock in to the crystal structure, leaving one electron free. There will be a surplus of negative charge carriers, and the germanium or silicon so treated is known as N-type material.

The atoms which produce the additional electrons are called donors. The donors most used are arsenic, antimony, and phosphorus. It is important to remember that, the material as a whole is electrically neutral because the spare electron is balanced by its own atomic nucleus, which carries a positive charge.

If a material having only three valence electrons is added to the germanium or silicon there will be the absence of a bond between two atoms. This creates a hole, which is a positive charge carrier. Such material is known as P-type and conduction is mainly by holes, in contrast to N-type in which conduction is mainly by electrons.

N-Type Semiconductor

ntype

In each type of material there will be some conductors of the opposite type caused by thermal effects. These conductors are known as minority carriers. Atoms which produce P-type material are called acceptors and are aluminium, gallium, and indium.

This diagram shows N-type semiconductor material.

depletionexpl

Many semiconducting devices are formed by mixing donor and acceptor atoms in a crystal by themselves. The commonest of these is Gallium Arsenide, used in many transistors, including RF power transistors, UHF RF amplifying transistors, and light emitting diodes.

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Diodes

This diagram shows the state of the junction between a piece of P-type and a piece of N-type material for four conditions.

(A) shows the state of the junction when the pieces of semiconductor material are not together.

(B) shows a junction formed by doping the opposite ends of a crystal of semi-conductor material with acceptor atoms and donor atoms. When there is no external voltage applied the free electrons and holes near the junction drift across and combine. The N-type material loses electrons and is positive, while the P-type gains electrons and is negative. This process sets up a field in the region of the junction which prevents any further carriers from crossing the junction. The area immediately on either side of the junction is reduced of carriers by this field, and is known as the depletion region. Some current flows through the junction because temperature effects produce electron-hole pairs in the junction area. This current is called leakage current.

(C) shows the condition when a positive voltage is applied to the N-type material and a negative one to the P-type, the depletion layer is reinforced, and the width of the depletion is increased. Under these conditions, the diode is reverse-biased and very little current can flow.

(D) shows the condition when a negative voltage is applied to the N-type material and a positive one to the P-type, the depletion layer is reduced, and the width of the depletion is decreased.

Diodes

If the potential is high enough the junction will conduct. This is known as forward bias. The voltage required to forward bias the junction depends on the material, and is about 0.6 volt for silicon and 0.2 volt for germanium.

The ideal diode would be one that appeared to be at zero ohms when forward biased, and infinite ohms when reverse biased. In practice, leakage current always flows and typical values are 1 ľA for silicon and 50 ľA for germanium.

This diagram shows the symbol used for the diode and the names given to the terminals. Conventional current flows from the anode to the cathode.

The arrowhead on the circuit symbol shows the direction of conventional current flow, so this indicates flow from positive to negative potentials.

Point Contact Diodes

Junction diodes have high internal capacitance and are restricted to relatively low frequency use. The point contact diode has a capacitance of only a few picofarads and can be used at frequencies in the VHF and UHF ranges for such purposes as signal monitoring and for signal detection.

Varicap Or Varactor Diodes

When a junction diode is reversed biased, the depletion region of a diode is cleared of carriers, and the depletion region acts as an insulator. The reverse-biased diode therefore forms a capacitor, and by varying the width of the depletion layer by varying the reversed biasing voltage, the capacitance of the junction can be made to vary. Varicap or varactor diodes are used as frequency modulators or to vary the tuning of resonant circuits. Oscillators

Diode Characteristics

DiodeChar

This diagram shows the characteristics of typical semiconductor diodes.

The curves are for signal diodes. For power diodes the reverse voltage is within the limits of 50 and 1200 volts.

Diodes have three characteristics of importance, which can be obtained from manufacturers' data sheets. They are:

1. Peak Inverse (or reverse) voltage (PIV): the maximum voltage in the reverse direction that the diode can withstand before breaking down.

2. Maximum forward current: usually two values are given one for non-repetitive peak current, such as a switch-on surge, and the other for repetitive peaks.

3. Average forward current: is accepted as the working current with natural cooling.

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The Zener Diode

The zener diode is a diode with special doping to use the reverse voltage characteristics for voltage-regulation applications. See Regulated Supplies

Diode Packages

Diodes come in a variety of shapes and sizes, depending on the application. Their physical size is large for power diodes and small for low power applications. The main problem is how to identify the leads.

Usually, the cathode is marked with a ring or band, a dot or a red end. Sometimes, a diode symbol is marked on the diode to indicate the connections. If a diode is unmarked, an ohm-meter can be used to find out which end is which. The diode is reversed biased when the meter shows a high resistance and forward biased when the meter shows a low resistance. Do not forget that most multimeters used for measuring resistance have voltage on the probes of opposite polarity to that marked, for example, a negative voltage on the red probe. If in doubt, check with a known diode.

Transistors

The name transistor is derived from the words transfer and resistor.

In this diagram, (A) shows the transistor symbol and the block representation of an NPN transistor.

Transistors

The emitter-base junction is forward biased with about 0.6 V so that it has a low resistance. The base-collector junction is reverse biased with about 6 V and has a high resistance. The centre connection, the base, is very thin and most of the current entering it goes right through to the collector circuit.

When the base-emitter junction is forward-biased electrons flow from the emitter into the base. Because of the thinness of the base region and the attraction of the higher collector voltage, most of the electrons go right through the base into the field of the reverse-biased collector-base junction. Here, they are swept through the depletion layer and out the collector terminal.

(B) shows the symbol and the block representation for a PNP transistor.

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Transistor Circuits

TransCct

There are three methods of connecting transistors into circuits. The diagram shows the three type of circuits as common emitter, common base, and common collector. They are identified by the terminal which is common to both input and output circuits.

Common emitter circuit: is the one that is most used because of its moderate input impedance and high power gain (see (a)).

Common base circuit: is where most of the emitter current flows through to the collector, and the input current is high. Current gain is always less than one. Because of the high current in the input, the input impedance is low. Output impedance is high (see (b)).

Common collector or circuit: has a high input impedance and low output impedance. Because the base to emitter voltage is constant (0.6 V for silicon, 0.2 V for germanium), the emitter voltage closely follows any variations in base voltage. Voltage gain is always less than one, but there is a current gain in this circuit (see (c)).

Transistor Operation

TransAmps2

This diagram shows a transistor in a circuit where the voltage supplying the base can be adjusted. If the base is held at the bottom end of the potentiometer R1, point A, the base-emitter voltage will be zero and no current will flow from emitter to collector. There will be no voltage across the load resistor RL, and the collector voltage will equal the supply voltage. The transistor, in this condition, is said to be cutoff.

If the base voltage is increased by moving the potentiometer towards point B, base current will start to flow once it gets over the threshold voltage for the material (approximately 0.2 V for germanium and 0.6 V for silicon). Base current will produce a corresponding collector current, and there will be a drop in collector voltage. As the base voltage is further increased towards point C, the collector voltage will continue to fall until it reaches a voltage just above zero. The transistor is now said to be saturated. Any further increase in base voltage will not increase the collector current.

transchar

Transistor manufacturers provide characteristic curves which show information about the transistor. The commonest curve is one relating collector current to collector voltage for different fixed values of base current. Typical curves for the common-emitter collector characteristics for a small-signal silicon NPN transistor are shown in this diagram.

Another important characteristic curve for an active device is the transfer curve. The transfer curve relates the input voltage or current to the output voltage or current.

transfer

This diagram shows the transfer curve for a small-signal NPN transistor. It shows how the collector current changes with a change in base input voltage.

When the base voltage is zero no collector current flows. This is the base voltage bias point for collector current cutoff. If the base voltage is increased, collector current flows. A point is marked on the graph where 10 mA of collector current flows for about 0.7 base voltage.

The base voltage and the corresponding collector current is important in amolifier applications.

Methods Of Biasing

Biasing of transistors is the process of setting up the DC operating conditions for the application intended.

transamps

This diagram shows three common biasing methods. The first point to note is that supply polarity is opposite between PNP and NPN transistors.

The PNP transistor requires a positive emitter supply, (see arrow on symbol) with the base and collector at a more negative potential.

The NPN transistor requires a negative emitter supply, with the base and collector at a more positive potential.

The main problem to be considered in biasing a transistor is compensation for the change in transistor operating conditions with a change in temperature. This is called the thermal drift of a transistor.

At high temperatures, a certain number of electron-hole pairs form in the collector-base junction, and the resulting leakage current in the base circuit is amplified just as a signal current would be. The collector current is then increased and the transistor gets hotter. The process is self-perpetuating and is known as thermal runaway. Germanium transistors are more likely to be damaged in this way, but silicon devices are not immune. To avoid these effects, bias circuits have been devised to compensate for the change in transistor operating conditions with a change in temperature.

(a) shows the simplest bias circuit using current biasing. A resistor between the supply rail and base is used to set base current to the required value. This circuit has no provision to compensate for the increased collector current that causes thermal runaway.

(b) shows the circuit when the base resistor is connected to the collector rather than to the supply. The increase in collector current causes the collector voltage to fall thus reducing the base voltage, and bringing the collector current back to near the original value. This type of circuit is termed a collector feedback circuit using current biasing.

(c) shows the circuit that is most frequently used.

The base bias is obtained from a voltage divider arrangement that is arranged to keep the base voltage at a constant value. The small resistor in the emitter lead provides voltage feedback. An increase in collector current increases the emitter voltage and because the base voltage is fixed by the base voltage divider, the base-to-emitter voltage is reduced thus reducing the collector current back towards its original value. This is known as voltage biasing with emitter feedback.

Bipolar junction transistors are called bipolar devices because both major and minor carriers are involved in their operation.

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Field Effect Transistors

Fetdiag

Field effect transistors are uni-polar as only one form of carrier is involved, this carrier is the majority carrier for the material used. These are junction FETs and known as JFETs. This diagram shows the principle of operation.

The depletion regions between the gate layers and channel causes a variable width of the channel, depending on the gate-to-source voltage. The channel acts as a voltage-controlled variable resistor with a very-high input resistance. The two ends of the channel are known as the source and the drain. The source corresponding to the emitter of a transistor, and the drain the collector.

For HF and VHF use, the usual circuit configuration is the common source, and for UHF the common gate circuit. JFETs are available in both P-channel and N-channel types.

MOSFETs

The diagrams (a) and (b) show the circuit symbols for two types of JFET. Both are depletion types.

The channel conducts when there is no gate potential, and as the gate is reverse-biased, the channel becomes depleted until the device is cutoff. In the N-type, the channel is of N-type material, and the carriers are electrons. In the P-type, the channel is of P-type material, and the carriers are holes. In N-type, the drain is positive with respect to the source, and the drain current decreases as the gate is made more negative with respect to the source. In the P-type device the drain is at a positive potential with respect to the source.

For the examples shown in (c), (d), and (e), the full name for these are metal oxide semiconductor field effect transistor, MOSFET. Various symbols are used to indicate MOSFETs and are shown in (c), (d), and (e). They are made in both N-channel and P-channel, and also in depletion and enhancement types. The enhancement types pass little or no current if the gate is unbiased, and the gate must be forward biased to increase conduction.

The gate is insulated from the channel, and this gives a very high input resistance. For a JFET the input resistance is up to 1000 megohm for silicon types. For the MOSFET it is 109 to 1016 ohms. Because of the very high input resistance, static voltages can build up to a very high value in MOSFETs, and can destroy the oxide film between gate layers and channel. Once damaged it can never be repaired. To overcome this problem many types of MOSFETs have built-in gate-protective zener diodes. Without this provision the gate insulation may be perforated easily by small static charges on the user's hands or the application of excessive voltages. The protective diodes are connected between the gate(s) and the source of the MOSFET.

These devices are used at HF, VHF, and UHF as amplifiers, oscillators, product detectors, and mixers. They have excellent temperature characteristics, and have lower internal noise than bipolar devices.

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FET Biasing

FetBias

This diagram shows curves for various FETs.

Characteristic curves can be drawn for FETs as for bipolar transistors. The main one is a plot of drain current versus drain voltage for different fixed values of gate voltage.

As the drain voltage is increased, the drain current first rises rapidly, then steadies and remains at an almost constant level. This is due to pinch-off , a point where the full width of the channel is almost totally depleted. The sudden rise in the right-hand side of the curves of (a) is because a breakdown voltage has been reached and the JFET looses its normal operating characteristics. The value of the drain-source voltage at which this occurs is typically 25 to 50 volts.

Bias control in FETs is normally done by using a source resistor. With the depletion mode FET the gate must have the opposite polarity to the drain. Biasing therefore would appear to require two supplies of opposite polarities for this device. One a positive supply for the drain and a negative supply for the gate.

In practice, this is rarely done except in special cases. For small-signal applications, the source current is used to provide the bias voltage.

FetAmps

By inserting a resistor in series with the source lead as shown in this diagram, the source current generates a voltage which holds the source a few volts positive over the common or negative supply lead.

The gate is maintained at the common potential by the gate resistor RG and as no current flows in the gate circuit the gate is effectively at a few volts negative relative to the source.

FETs can be connected in three circuit configurations, in much the same way as transistors can. The three are:

1. Common source,

2. Common gate, and

3. Common drain or source follower.

Properties of the three modes are similar to those of the equivalent transistor connection.

Semiconductors

ic

It would be nice to be able to have standardised connections for the leads of semiconductors. In practice pinouts vary widely and the manufacturer's data must be consulted.

Integrated circuits (IC's) are a network of semiconductors manufactured on the one chip. There are many varieties, both analog and digital. They are as complicated as anyone can make them.

Their drawings vary and many different styles of presentation are possible. A specification sheet may - or may not - give a block diagram of its interior workings. This diagram here is for illustration only and shows just one very simple component - six inverters in the one package.

Practical Considerations

Excessive heat damages semiconductors. The manufacturer's limits of voltage, current, and power dissipation should always be observed to keep the heat of the device to safe limits. When soldering, keep the contact time as short as possible.

Applications For These Semiconductor Devices

Typical applications are in amplifiers, oscillators, modulators and demodulators, too numerous to mention.

See Oscillators and Amplifiers



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Compiled Sun Nov 28 2010 at 8:41:44pm


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