4.1.1(a) Diodes
Application of Semi-conductor P-N Junction Diodes – Diodes in Series
Figure 1: DIODES IN SERIES
When diodes are connected in series to a known load then it must be remembered that the current will be the same and the maximum forward current must not be exceeded for each diode. Because each diode has a small forward resistance there will be a volts drop across each diode, which will depend on each diode's characteristics. These individual volts drops will subtract from the supply voltage to leave a certain voltage across the load (see later notes on rectifiers).
Figure 2: DIODES IN PARALLEL
Diodes in Parallel
Where current supplied by one rectifier would exceed its maximum forward current, or exceed its maximum operating temperature, it is possible to connect two or more diodes in parallel. The current, therefore, will be divided between the diodes.
The voltage across each diode will be the same and the current distribution between the diodes will depend on the characteristics of the diodes (again, for further information on rectifiers see later notes in this series).
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TESTING OF DIODES
It is essential the diode is connected the correct way round in a circuit, so a coloured band or spot usually marks the cathode (k) end.
Figure 3: TESTING OF DIODES
If it is necessary to verify the connections in the absence of any marking then a test meter is used. Using the old AVO-meter it should be remembered, as with any ohmmeter, that the BLACK (NEGATIVE) terminal becomes the positive output and RED (POSITIVE) terminal is the negative. When a 'FLUKE' is used it has a switch selection to test diodes.
The meter displays the forward voltage drop (VF) up to 2 volts and beeps briefly for one diode drop (VF < 0.7V) for the forward bias test. For reverse bias or open circuit the meter displays OL, and if there is a short circuit the meter emits a
continuous tone.
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4.1.1(b) Diodes
SEMICONDUCTOR MATERIALS
Figure 4 shows the structure of the germanium and silicon atoms, two very important elements in the manufacture of diodes and transistors
Figure 4: ATOMIC STRUCTURE
Bear in mind that the diagrams are only two-dimensional and that in reality the orbiting electrons do not rotate in perfect circles or rotate in a flat plane.
From figure 4 it can be seen that each atom has four electrons in its outer shell, these electrons are called VALENCE ELECTRONS, they are farthest from the nucleus and therefore are least tightly bound (less attractive force). It is the valence electrons that play the active part in electrical conduction.
Silicon and germanium are crystalline substances and the valence electrons of the individual atom link up and arrange themselves with the valence electrons in adjacent atoms to form CO-VALENT BONDS. Every atom has a half-share in eight valence electrons. This gives a very stable arrangement of a regularly repeating three dimensional structure called a crystal lattice. Figure 5 shows the two dimensional effect of the covalent bonding. Pure silicon and germanium are therefore very good insulators.
At room temperatures the atoms are vibrating sufficiently in the lattice for a few bonds to break, setting free some valence electrons, leaving a "hole" where the electron was. Free electrons are attracted to the hole as the atom, short of an electron is now positively charged.
Figure 5: CO-VALENT BONDS
If a battery is placed across a pure semiconductor, electrons are attracted to the positive terminal. These free electrons travel through the semiconductor topping' from one hole to another, and it therefore appears that the positive holes are moving towards the negative terminal. This current flow is very small and is called INTRINSIC CONDUCTION.
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To understand the concept of electrons moving one way and holes moving the other is not easy but it can be likened to an empty seat at the end of a row in a cinema. Assume the vacant seat to be at the right hand end of the row. If the first person next to the seat moves into it, then he/she has moved to the right, but the vacant seat has moved one place to the left. If each person in the row does the same (i.e. moves to the empty seat to his/her right) as soon as it becomes empty, the vacancy (hole) appears to have moved along the row in one direction while the occupants (electrons) have move in the opposite direction.
If the temperature is raised more bonds break down and conduction increases i.e., resistance decreases, this means more heat is generated, and more conduction occurs, resistance decreases further, more heat is generated - and so on. This is called thermal runaway and will eventually destroy the crystal structure.
Semiconductors have a negative temperature coefficient. In other words their resistance decreases with an increase in temperature. We need now to look at how we can change the basic insulator into a conductor. This is achieved by mixing (doping) a very small quantity of a selected impurity atom into the semiconductor material. (Typically 1 part in 1010) The material now becomes an extrinsic semiconductor.
There are two types of extrinsic semiconductors:
1. N-Type semi-conductor material.
2. P-Type semi conductor material.
N-Type Semi-conductor Material
Doping impurities such as phosphorus or arsenic are used. These have five (pentavalent) electrons in the outermost orbit. When introduced into the basic material, four of the electrons join up with the co-valent bonding, whilst one electron is left 'free'. (The number of free electrons can be strictly controlled by this doping).
The free electrons can migrate through the inter-atomic space and can therefore act as current carriers when a (very low) voltage is applied.
Figure 6: N-TYPE SEMI-CONDUCTOR
Note: Although extra electrons have been inserted, it must be remembered that each impurity atom is itself neutral and so the whole of the N-type material is also neutral.
MAJORITY CARRIER - ELECTRONS (NEGATIVE)
[N = N-TYPE] MINORITY CARRIER - HOLES (due to intrinsic conduction)
Figure 7: ELECTRON FLOW IN AN N-TYPE SEMI-CONDUCTOR
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P-Type Semi-conductor Material
In this material, impurities such as Indium or Aluminium are used. These have three (trivalent) electrons in the outermost orbit. When introduced into the basic material, all three electrons link into the crystal structure but this leaves a 'hole' in the structure. This hole is looking for an electron to fill it and so it is a form of positive current carrier. If a (very small) voltage is applied, electrons will move to fill in the holes but this forms fresh holes and so there is a general drift of holes through the material from positive to negative (in the opposite sense to the electron flow in the N-type material). Again, the material is neutral.
Figure 8: P-TYPE SEMI-CONDUCTOR MAJORITY CARRIER - HOLES (POSITIVE)
[P = P-TYPE] MINORITY CARRIER - ELECTRONS (due to intrinsic conduction)
Figure 9: ELECTRON FLOW IN A P-TYPE SEMI-CONDUCTOR
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THE P-N JUNCTION
Imagine a piece of N-type material being brought into contact with a piece of P-type material. Both pieces are, up to the instant of contact, neutral.
Remembering that the holes are looking for electrons to complete the lattice network, it can be seen that electrons will migrate across the junction to fill in the holes as soon as the two materials are brought together.
Figure 10: P-N JUNCTION BEFORE CONTACT
As electrons leave the N-type material, it will become positively charged. As electrons fill holes in the P-type material, it will become negatively charged.
A BARRIER POTENTIAL is built up at the boundary, forming what is known as the Depletion Layer (figure 8). This build-up in potential will eventually be strong enough to stop further migration of electrons across the junction.
Figure 11: P-N JUNCTION
The Barrier Potential is approximately 0.2V for Germanium and 0.6V for Silicon. It must be remembered that the barrier potential is always present at a P-N junction - even if it is sitting in a storage bag on a shelf.
If an external supply is connected +ve to the P-type material and -ve to the N-type, it will oppose the barrier potential. If it is bigger than the barrier potential, the barrier potential will be overcome and current will flow, electrons moving from supply negative to positive and holes moving in the opposite direction, as shown in figure12. This is known as FORWARD BIASING the junction.
Figure 12: FORWARD BIAS P-N JUNCTION
The intrinsic conduction, (covalent bonds breaking down at normal temperature) produces minority carriers and thus small current flows in the same direction as the majority carriers i.e., it adds to it.
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If the external supply is connected in the other sense, +ve to the N-type and -ve to the P-type, it will reinforce and increase the barrier potential and therefore no current will flow, except for any slight leakage current (see below). The depletion layer will be enlarged as shown in figure 13. This is known as REVERSE BIASING the junction.
Figure 13: REVERSE BIAS P-N JUNCTION
At first sight it might appear that there is no current flow, but due to intrinsic conduction, which produces minority carriers, which causes a tiny current to flow across the junction this is known as the LEAKAGE CURRENT.
Raising the temperature of the P-N junction causes a rapid increase in the generation of minority carriers, and therefore leakage current increases. At room temperature each 10°C increase roughly doubles the rate of generation for germanium.
For silicon the doubling rate is 5°C. It might appear from this that germanium would be used for higher temperature conditions, however, although the rate of increase is greater for silicon, its actual value is considerably less than that of germanium, so silicon is used where high temperatures are encountered.
RECTIFIER ACTION
If an ac supply is applied to a P-N junction then when 'P' is made positive to 'N' then the positive half cycle will flow through the junction as it is forward biased. On the negative half cycle of the ac 'P' is negative to 'N'.
This is the reversed bias mode and the junction will not conduct on this half of the cycle.
Figure 14: ACTION OF A DIODE
The junction passes current through R only when the P material is positive.
Therefore an output voltage is produced only on the positive input half cycle.
Figure 15: DIODE SYMBOL
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Figure 16: DIODES
The P-N junction is acting as a rectifier and is known as a SEMICONDUCTOR DIODE. The symbol is as shown in figure 15.
It is important to note that the arrow points in the direction of CONVENTIONAL current flow and the two connections are known as the ANODE (A) and CATHODE (K). The cathode (negative end) is often marked with a band as shown in figure 16.
Diode Characteristics
Typical characteristic curves for silicon and germanium diodes at 25°C are shown in figure 17. When forward biased, a voltage is required to overcome the barrier voltage before the diode current increases; this is typically 0.2V for germanium and 0.6V for silicon. After this, current rises rapidly as the applied voltage increases.
Figure 17: DIODE CHARACTERISTIC CURVES
The left-hand side of the origin of the characteristic curve is where the voltage is reversed, i.e. reverse biased. As can be seen the current is extremely small, this is the leakage current due to minority carriers. Note that the voltage scale is not linear, with the larger divisions on the negative axes of the graph.
As the voltage is increased at a certain point the current increases rapidly to a high value. This is known as AVALANCHE BREAKDOWN and will cause permanent damage to the diode if it is allowed to occur.
It occurs because as the reverse voltage becomes too great, the minority carriers are accelerated to a point where they heat up the diode and collide with atoms in the depletion layer. This will dislodge further electrons, thus creating more minority carriers and this effect 'avalanches' to cause a rapid rise in current. The breakdown voltage can have any value from a few volts up to 1000V for silicon and 100V for germanium depending on the construction of the diode and the level of doping.
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Diode Parameters
Diodes are manufactured in a wide range of voltage and current ratings. These must be taken into account when choosing a diode for a particular circuit.
Typical parameters considered are:
1. Maximum forward current 2. Peak inverse voltage Maximum operating temperature
The diode has a small forward resistance when it is conducting, so power must be dissipated as it conducts. This power dissipation causes heat at the junction, this local heating must be kept down, as excessive leakage current will occur. There is therefore a MAXIMUM FORWARD CURRENT so that the temperature is not reached which will cause deterioration of the structure of the diode.
The PEAK INVERSE VOLTAGE (PIV) is the maximum operating voltage appearing across the terminals of the diode acting in the reverse direction, and therefore represents the maximum reverse voltage that may be applied to the diode without reverse breakdown occurring. This may be written as Maximum Reverse Voltage instead of PIV.
MAXIMUM OPERATING TEMPERATURE is a maximum junction temperature above, which the structure of the diode deteriorates. The maximum forward current is so chosen that this temperature is not exceeded in the worst combination of circumstances.
However, it should be remembered that the maximum forward current will also depend on the temperature in which the diode is operating; and maximum forward current is usually quoted at two or more ambient temperatures.
We know as the temperature rises the leakage current increases and as a guide the leakage current doubles in value for each 10°C rise in temperature.
Depending on its use, frequency is also a parameter to be considered, but generally these are special diodes and will be discussed later.
Single Phase Half Wave Rectifier
With reference to figure 18, when terminal A is positive with respect to B the diode conducts, this causes a current to flow around the circuit and a voltage will be developed across RL. When the input polarity reverses terminal A will be negative with respect to B and the diode will switch off.
Figure 18: HALF WAVE RECTIFICATION
The voltage developed across RL is therefore half-sine-waves and is known as a half wave rectifier. The output being DC, albeit variable. The average value being half that of the supply, i.e. peak x 0.318. (assuming no losses). The output DC
‘ripples’ have a frequency equal to the input frequency of the AC supply, i.e. ripple frequency - supply frequency.
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Single Phase Full Wave Rectifier
As the name implies this uses both half cycles of the input wave form. Figure 19 shows diodes D1 and D2 used with a transformer, which is centre tapped at C. The point C can be considered as neutral with terminals A and B swinging alternately positive and negative about it.
When A is positive to C, Diode D1 conducts with D2 switched off. On the other half cycle of input, B is positive to C and D2 conducts with D1 switched off. The output is therefore undirectional, with both diodes alternately conducting, giving a full wave output across RL. The average output voltage is 0.637 x peak (assuming no losses), i.e. average of the supply.
Figure 19: FULL WAVE RECTIFICATION
The output DC 'ripple’ is therefore twice the input supply frequency. Having to use the double winding on the transformer makes this component more bulky in size and therefore more expensive.
A point to note about this circuit is that when D1 is conducting, the voltage across the load resistor RL is the peak voltage. With D2 cut off the voltage across C-B is in series with this voltage, so these two voltages combine to give a total of twice the peak voltage.
This will act as a reverse voltage across D2 so the peak inverse voltage for the diodes must be twice the peak voltage on either half of the secondary of the transformer.
Bridge Rectifier
This is also a single phase full wave rectifier, and has advantages over the previous circuit in that the transformer does not need to produce twice the voltage required and the secondary is in use all the time. Unlike the previous circuit where only half the secondary winding was used at any one time.
Figure 20 shows a bridge rectifier. Assume the top of the secondary winding of the transformer to be positive (positive half cycle), trace the current flow through the load using the arrows shown.
Figure 20: BRIDGE RECTIFIER - FIRST HALF CYCLE
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Figure 21: BRIDGE RECTIFIER - SECOND HALF CYCLE
On the next half cycle (figure 21) assume the bottom of the secondary is positive and trace the circuit through the load following the arrows. Note the direction of current through the load is the same during each half cycle, i.e. it is DC.
Note that in this circuit the two non-conducting diodes have twice the supply voltage across them, (load/supply voltage + supply voltage = twice supply voltage). However, this voltage is shared between the two non-conducting diodes in series, therefore the peak inverse voltage per diode is the supply voltage. As before the ripple frequency is twice the supply frequency.
Typically all four diodes are available in one package.
Three Phase Half Wave Rectification
In order to obtain three-phase half wave rectification a diode must be inserted into each of the supply lines to the load and the return from the load to the supply MUST be to the star point of the three-phase system.
Therefore this form of rectification can only be used where there is a star connection using a neutral line. Assume this star connection is the secondary of a three phase (DELTA-STAR) transformer as shown in figure 22.
Figure 22: DELTA STAR TRANSFORMER
Figure 23 shows the waveform of the three-phase supply and the resultant supply voltage to the load.
Figure 23: WAVEFORMS - THREE PHASE RECTIFIER
Note that the ripple frequency of this rectifier output is three times the supply frequency, with three DC output voltage 'blips' for one sequence of the three-phase AC supply.
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Three Phase Full Wave Rectification
This form of connection does not require a neutral line, so can be used on either Star or Delta connected systems. Figure 24 shows the diode circuit diagram.
Figure 24 FULL WAVE RECTIFIER CIRCUIT
The arrows show the time in the three phase cycle when phase A is maximum and passing peak current to the load (say 10 amps). After passing through the load, the current splits into two, of five amps each to return to the B and C lines back to the supply.
The arrows show the time in the three phase cycle when phase A is maximum and passing peak current to the load (say 10 amps). After passing through the load, the current splits into two, of five amps each to return to the B and C lines back to the supply.