Solid State Devices
3n.1 Understand that doping of semiconductor material (silicon and germanium) produces p-type (electron deficient) and n-type (electron rich) semiconductors.
In modern electronic equipment today there are many components that are in the "family" called "semiconductor".
The name "semiconductor" correctly implies that a material is neither a conductor nor an insulator. An insulator has high resistance to the passage of electrons and a conductor has low resistance to the passage of electrons. The resistance of the "semiconductor" lies between those two extremes.
The two materials in general use that form semiconductors are SILICON and GERMANIUM.
Just for a moment we have to split the atom of silicon and germanium. The atoms of these two elements have a nucleus and respectively 3 and 4 rings of electrons. It is the outer rings that we are concerned with as these both have 4 electrons. Both of these outer rings have the capability of joining with an adjacent atom to form a crystal lattice.
What happen in the other rings is of no consequence with regards to this course. It is important that you understand that each atom has 4 electrons in the outer ring and that those in the silicon outer ring being nearer to the nucleus which has a positive charge has a greater hold than those in the outer ring of the germanium atom simply because they are that little bit further away from the nucleus and thus the attraction between them is less.
This distance from the nucleus is important as in silicon the outer electrons are NOT FREE to move from the lattice when they have combined with other atoms of silicon (and are thus an insulator) where as those in the germanium being further away can become detached (and so they have a tendency to being a conductor).
Making semiconductor material
This complex process takes both silicon and germanium (separately) and refines them to high standards of purity and then after all that hard work makes them impure again but in very controlled conditions called "doping".
This "doping" process introduces atoms which have either 5 or 3 electrons in this outer ring. The material where there are 5 electron in the outer ring is given the name N - Type Material as it is and it appears to be NEGATIVELY charged due to the "extra" electron (electron rich) where as if there is 3 electrons in the outer ring the material is given the name P - Type Material and appears to be positively charged due to an apparent absence of electrons (electron deficient).
Both Silicon and germanium can be doped to form N or P type material.
Understand current flow in terms of electron and hole movement. Understand the formation and effect of the depletion layer.
We will take these two parts of the syllabus together.
Let us look at the situation where we have a two pieces of semiconductor material, one piece N-Type and the other P-Type that are fused together. Now we have the situation, just like a party where free spirited boys and girls arrive in different cars from different directions and enter the hall via different doors but they all have one aim look for partners.
You have learned in the ILC that a flow of electrons is a current thus as there is a movement of electrons and holes there is in essence a current flow but it is not sustained.
Understand how the p-n junction forms a semiconductor diode.
At the same time this will leave holes in the N material which will be filled by more electrons entering the circuit from the negative terminal. A flow of electrons will then continue with the rate of the flow restricted by the resistor in the circuit. This circuit is said to forward bias the diode. It was said earlier that there needed to be energy applied to the circuit for current to flow and it is found that no current flows until a pressure (voltage) of about 0.6V for silicon and 0.3V for germanium diodes had been applied. This voltage is called the "barrier voltage" and is the energy that needs to be applied to help the electrons through the depletion layer.
CONVENTIONAL CURRENT FLOW ------- Current Flow from Positive to Negative
You may recall the diagrams above from your ILC course. Circuit A represents a reverse biased diode whilst Circuit B represents a forward biased diode.
If the diode is reversed biased, (as in circuit "A" above) no (or negligible) current flow will occur and electrons will build up at the battery end of the N material and Holes at the Battery end of the P material. This condition is called reverse bias and as a generalization other than "leakage current" no current flows.
Peak Inverse Voltage PIV
Diodes when used in home construction must be rated properly for the use to which they are to be put.
You must consider :-
Peak Inverse Voltage ( or Peak Reverse Voltage ) is the maximum voltage that a diode can withstand in the reverse direction without failing and starting to conduct. If you exceed the PIV the diode may be destroyed. Thus the diodes must have a PIV rating that is higher than the maximum voltage that will be applied to them when reverse biased.
In a DC only circuits, diodes should have a Peak Inverse Voltage rating greater than the highest voltage to which diode will be exposed.
In an AC circuits, such as power supplies, diodes should have a Peak Inverse Voltage rating up to 2.8 times the maximum RMS voltage (RMS is 0.707 of the peak voltage) of the transformer's secondary winding (depending upon the rectifier design).
Maximum Average Forward Current is the average forward current that a diode can conduct without being damaged.
In DC only circuits the Maximum Average Current is considered to be the current that the diode will continuously conduct.
In AC circuits such as power supplies the Maximum Average Current Rating of a diode should be twice the DC current that the supply will deliver at full load. For example; If a power supply can deliver 1 amp the rectifier diodes should have at least a 2 amp current rating.
3n.2 Recall that a Zener diode will conduct when the reverse bias potential is above its designed value and identify its V/I characteristic curve.
In the standard diode we have established that only a negligible current flows when the diode is reverse biased the "leakage current", but if the voltage is increased then it can reach a value when the diode just cannot prevent a flow of current and the diode can fail dramatically.
With the ZENER diode, as the reverse bias voltage is increased from zero it acts the same as any other diode and resists the passage of all but leakage current. Then when the voltage rises to its designed value, the depletion layer allows current to flow and the voltage remains at a stable level. So long as the current passing through the device does not exceed its rated handling capability the ZENER continues to function. This is achieved "somewhere" in the circuit with a current limiting resistor. However if the current passing is too great then the zener will suffer from failure.
3n.3 Understand that the depletion layer in a reverse biased diode forms the dielectric of a capacitor and that the magnitude of the reverse bias affects the width of the layer and the capacitance.
This part of the syllabus is referring to a type of diode called the Variable Capacitance diode or "varicap diode".
All diodes to a greater or lesser extent exhibit the phenomenon of an increasing width of the depletion layer when a reverse bias is applied. However Diodes that are made to be especially susceptible to the widening of the depletion layer, which in turn varies the capacitance associated with the diode.
The depletion layer can be thought of as the plates of a capacitor, and just like a capacitor by widening the gap between the plates the capacitance drops. With the Varactor diode as the reverse bias is increased so the depletion layer widens and the capacitance decreases and by reducing this applied reverse bias the capacitance then increases.
The depletion layer in the diode is acting as not only the plates of a capacitor but also the dielectric of a capacitor.
3n.4 Understand the 3 layer model of the transistor (npn and pnp) and the channel model of the FET.
The 3 layer model of the transistor
As the drawings show the layer which forms the middle of the sandwich is called the BASE, the others are called EMITTER and COLLECTOR respectively.
Bipolar transistors are constructed such that the junctions are so close together that electrons flowing across the base/emitter junction will control the flow of electrons in the collector/emitter junction. The result being that a small amount of current flowing in the base/emitter junction will control a much larger current flowing in the collector/emitter junction.
When we were discussing the diode a junction is said to be forward biased when the P type material is connected to the positive supply and the N type material is connected to the negative supply.
No current can flow between the collector/base junction because it too is reversed biased (or turned off) the transistor behaves as if it where 2 diodes connected back to back.
When the transistor is forward biased or turned on (0.6V for silicon, 0.3V for germanium), current flows across the base/emitter junction, but because the collector/emitter junction is physically so close, current flows across this junction also. With both junctions conducting most of the current flows across the collector/emitter junction since this is the path of least resistance, hence the base current is less than the collector/emitter current.
The transistor now no longer behaves like 2 diodes because the base current makes the collector current flow despite being reverse biased. The current flowing between the collector/emitter is much greater than the current flowing through the base/emitter. (Typically 25 to 800 times greater) and standards are improving all the time.
Understand the channel model of the FET.
The N type FET (Field Effect Transistor)
Both N and P type devices can be made. When the voltage applied at the gate has the effect of cutting down the current flow in the channel, the operation is said to be IN THE DEPLETION MODE
The gate is therefore normally forward biased with respect to the source. The increased gate voltage is used to increase current flow, the operation of the FET is said to be IN ENHANCEMENT MODE.
In most cases, ENHANCEMENT MODE devices are made without the conducting channel.
FET'S can be used in circuits similar to bipolar transistors but they give LOW VOLTAGE GAIN, and are only used when their peculiar advantages are required.
Dual gate mosfets are used as mixers and RF amplifiers in FM receivers. The shape of their characteristic also gives less distortion in power amplifiers, and HIGH POWER FET'S are now commonly used in HI FI and also find application in the power output stages of transmitters.
FET's are devices that depend on a junction action different to that of bipolar transistors.
JUNCTION FET'S are usually operated with their single junction Reverse biased.
MOSFET'S have almost infinite gate resistance, and the leads should not be touched unless first shorted.
FET'S are used in applications where their high input resistance, good switching characteristics and low noise factor outweigh their poor voltage gain.
3n.5 Understand the basics of biasing bipolar and FET transistors (including dual gate devices).
BIASING OF TRANSISTORS.
There are commonly 3 types of bias systems for transistors they are: -
2. CURRENT FEEDBACK TYPE
3. FIXED VOLTAGE TYPE
The purpose of biasing a transistor is to set its output current to a value which permits the best use of its transfer characteristic.
For a linear amplifier having a resistive load, the most useful bias setting is when the collector voltage is close to half the supply, (Class A).
The biasing method chosen must be stable, and thermal runaway must not occur.
Bias failure can be caused by either a short circuit or open circuit bias components. Either will greatly affect the working of the transistor as an amplifier.
DUAL GATE FET
The FET can also be made as a dual gate device. The original diagram of the FET is shown above (left) and the Dual Gate FET shown above (right). Note in the dual gate that the signal is on one gate and the main bias is on the other gate.
3n.6 Identify different types of small signal amplifiers (e.g. common emitter (source), emitter follower and common base) and explain their operation in terms of input and output impedances, current gain, voltage gain and phase change.
CURRENT GAIN hfe ( previously it was )
The amount of current flowing between the collector and the emitter of a BIPOLAR transistor is much greater than the current flowing between the base and the emitter, but the amount of current flowing in the collector is controlled by the base current. The ratio COLLECTOR CURRENT TO BASE CURRENT varies according to the collector current flowing. (An undesirable characteristic!) All comments regarding hfe in the remainder of this section must be understood in relation to this unwanted effect!
The constant is commonly known as current gain and the symbol used to indicate current gain is hfe ( previously it was ). A low gain transistor might have a gain of around 20 - 50, Power transistors sometimes have a gain of only 10, a high gain transistor might have a gain of 300 - 800 or even more.
The equation for the calculation of gain is
The current flowing in the collector = the hfe (gain) times the current flowing in the base.
Tolerance values of hfe are very large, so that transistors of the same type or even the same batch may have widely different hfe's. Published figures of transistor gains are only typical values. If an exact gain is wanted then the transistor will have to be tested. The secret is not to design a circuit where the maximum gain is required from a transistor, but to design such that many different devices can be used for the same circuit.
FIELD EFFECT TRANSISTORS.
To be strictly correct, the so-called FIELD EFFECT TRANSISTOR is not a transistor at all, as the word TRANSISTOR is derived from TRANSFER RESISTOR and the FET doesn't work like that at all. The FET relies upon the presence and the effects of an electric field.
There are 2 types of FET - The JUNCTION FET and the METAL OXIDE SILICON FET or MOSFET.
Both work by controlling the flow of current carriers in a narrow channel of silicon. The main difference between them lies in the way the flow is controlled.
Firstly the JUNCTION FET. A tiny bar of N or P type silicon has a junction formed near to one end. Connections are formed at either end of the silicon bar (see drawing) and also to the junction material (p type for N type FET).
The P type connection is called the GATE, the end of the bar nearest the gate is called the SOURCE, and the connection at the other end is called the DRAIN.
A junction FET is normally used with the junction reverse biased (it has a negative voltage on it for an N channel as opposed to what you might expect a positive one) so that a few moving carriers are around the junction (keeping it turned off) making the bar of silicon itself a poor conductor.
With less reverse bias (or less negative volts) on the junction the silicon bar will conduct better, and so on as the amount of reverse bias on the junction decreases the FET conducts better.
When a VOLTAGE is connected across the SOURCE and DRAIN the amount of current flowing between them depends on the amount of reverse bias (or negative volts) on the GATE and the ratio SOURCE - DRAIN CURRENT/GATE VOLTAGE is called the MUTUAL CONDUCTANCE the symbol for which is Gm. This quantity is a measure of the effectiveness of the FET as an amplifier of current flow.
Because the GATE is REVERSE BIASED, practically NO GATE CURRENT FLOWS, so that the RESISTANCE between GATE and SOURCE is VERY HIGH, much HIGHER than a BASE EMITTER junction of a BIPOLAR transistor, This uncommonly high resistance is put to good use, for instance in voltage measuring circuits NO LOAD is put on the circuit being measured.
3n.7 Recall the characteristics and typical circuit diagrams of different classes of amplifiers (i.e. A, B, A/B and C).
Classes of amplification
Several methods exist for biasing transistors; typically class A, B, C.
3n.8 Understand the concept of the efficiency of an amplifier stage and be able to estimate expected RF output power for a given DC input power, given the stage's efficiency.
When dealing with a power amplifier stage it is possible to estimate the expected output RF power from the DC input power given the stage's efficiency.
If the DC input power is 100 watts with a known efficiency of 60%, then 60% of 100 will be the RF output power (60 watts of output).
The remainder of the input power is converted into heat, which must be safely removed from the output device by a heat sink (for transistors) or air flow (for thermionic valves).
Thyristors or to put it another way, SILICON CONTROLLED RECTIFIERS are like a diode in that they have an anode and cathode or positive and negative end. They only conduct when a pulse is received at the gate, and do not stop conducting until the current flowing through them is zero. They have many uses in electronics, typically power supply protection and motor speed control circuits.