Oscillators are everywhere in electronics. They are a basic
building block upon which the whole structure of
electronics and computers is based. This lesson looks at the 3
basic types of multivibrators (MV). They are designed to
have zero, one or two stable states; the astable (the prefix
'a' means 'not') or free running MV, the monostable MV
(also called the one-shot) and the flip-flop or bistable ('bi'
means 2, bistable means 2 stable states.) In the flip-flop
MV a trigger pulse or control signal is required to change
from one state to the other. MV's use regenerative
(positive) feedback; the active components present within
the circuit operate as switches being alternately cutoff or
driven into saturation. However, a basic understanding of them is still essential
since they are still used in many circuits.
This kit builds each of these three circuits and allows you
to experiment with them. To understand how these circuits
work will also make sure you have an understanding of
resistors, capacitors, RC characteristics, the transistor as a
switch and the light emitting diode (LED).
A diode on the input protects the kit if the battery is
connected wrongly. When the 9V battery is connected, the
astable MV should flash from one LED to the other. One
LED should be on for about twice the time of the other.
The LED in the monostable MV should remain off. In the
RS flip flop one LED should turn on and stay on. Two
flying leads are provided, one on the 9V rail and the other
on the earth rail. Play with touching the flying wires to the
trigger, set & reset points. Try to follow what happens on
the circuit diagram when you touch a lead. What you see is
all to do with transistors acting as switches and capacitors
charging and discharging with a time constant determined
by an R and a C in the charge path. You cannot do any
harm to the components by playing with the flying wires
since each has a 1K resistor in it to limit the base current
through the transistor so that it cannot be damaged. If you
have access to a CRO look at the changes of the base/emitter voltages of the transistors as you touch trigger,
set & reset.
1. The Flip Flop/Bi-stable.
Computer memory elements (the group of circuit
components in a memory IC which stores each 'bit' - binary
digit) use the flip-flop principal. Play with the flying wires
onto the set and reset wires. You can very quickly see what
this circuit does; it remembers information about which
was the last LED to be make to be turned on or off. Of
course, you have to define a convention: which flying wire
you are using, which pin is called what. Eg, set, reset, Q,
/Q.
When the power is connected to the circuit one or other of
the two transistors will turn on. Both transistors will try to
turn on as the base of each tries to go high. But due to
slight differences in component values one will be quicker
than the other. Suppose it is Q1. This means that Q1 collector voltage is low (below .65V), which means that
the base of Q2 is also low (since the two are connected)
and Q2 is off. Now when the set lead is touched by the
positive rail, Q2 is turned on because its base potential is
raised over 0.6V. So Q2 turns on and its collector potential
drops which drops the base potential of Q1 to below 0.65V
and so Q1 turns off. The circuit has flipped into its other
state. Touch the reset with the positive lead and the circuit
flops back to Q1 turned on again.
We called one LED the set, and the other reset but these
names are arbitrary. The flying negative lead also causes
the LED's to turn on/off but in the opposite way to the
sequence caused by the positive flying lead. Study what is
happening with the schematic above as you touch the set
and reset.
2. The Monostable Multivibrator.
Now we introduce an RC network into the flip flop circuit
just described. An electrolytic capacitor replaces one of the
base bias resistors of the flip flop circuit. And the biasing
which was supplied by
this resistor is provided by a 56Kresistor to the positive rail. When the power is turned on
the circuit will settle into a stable state in which T4 is on
and T3 is off. Use a multimeter to measure the base/emitter
and collector/emitter voltages of T3 & T4 to show this.
The capacitor will have about 6V across it. It is capacitor in the circuit which determines that T3 will be
off, and T4 will be on in its stable state.
A high applied to the trigger point turns T3 on. Then after
a few seconds T3 turns off and returns to the stable state.
What has happened is that as soon as the trigger goes high,
T3 is turned on and the positive end of the capacitor is
taken to zero. This immediately drags the negative end of
the capacitor to below zero potential. This turns T4 off
since the negative of the capacitor is connected to the base
of T4. The LED turns on because T3 is on. But then after
the pulse is removed C3 starts to charge up with a time
constant determined by C3 and R3. Then when the voltage
on the negative end of C3 reaches 0.65V, T4 starts to turn
on, T3 turns off and so does the LED. The state is then
stable again.
Play with this over & over until you understand what is
happening. A low pulse delivered to the trigger point when
the LED is off does nothing because it is already almost at
the same ground potential.
2. The Astable Multivibrator or Free Running MV
In this MV we replace the second base bias resistor by an
RC network. You can think of it as two monostable MV's
connected together; the output of one feeds the input of the
other First one LED is turned on, then the other. The
output is a square wave. Its mark/space ratio and its
frequency is determined by the values of the R & C
components. The time that the collector of T2 is low (& T1
high) is determined by the time constant R2 & C2.
Similarly, the time the collector of T1 is low (& T2 high) is
determined by the time constant of R1 & C1. We made R1
about twice the value of R2 to highlight this. The time
constant is: t=0.693 RC. Work them out for yourself to
check what you observe
Light Emitting Diodes or simply LED´s, are among the most widely used of all the different types of semiconductor diodes available today and are commonly used in TV’s and colour displays.
They are the most visible type of diode, that emit a fairly narrow bandwidth of either visible light at different coloured wavelengths, invisible infra-red light for remote controls or laser type light when a forward current is passed through them.
The “Light Emitting Diode” or LED as it is more commonly called, is basically just a specialised type of diode as they have very similar electrical characteristics to a PN junction diode. This means that an LED will pass current in its forward direction but block the flow of current in the reverse direction.
Light emitting diodes are made from a very thin layer of fairly heavily doped semiconductor material and depending on the semiconductor material used and the amount of doping, when forward biased an LED will emit a coloured light at a particular spectral wavelength.
When the diode is forward biased, electrons from the semiconductors conduction band recombine with holes from the valence band releasing sufficient energy to produce photons which emit a monochromatic (single colour) of light. Because of this thin layer a reasonable number of these photons can leave the junction and radiate away producing a coloured light output.
LED Construction
Then we can say that when operated in a forward biased direction Light Emitting Diodes are semiconductor devices that convert electrical energy into light energy.
The construction of a Light Emitting Diode is very different from that of a normal signal diode. The PN junction of an LED is surrounded by a transparent, hard plastic epoxy resin hemispherical shaped shell or body which protects the LED from both vibration and shock.
Surprisingly, an LED junction does not actually emit that much light so the epoxy resin body is constructed in such a way that the photons of light emitted by the junction are reflected away from the surrounding substrate base to which the diode is attached and are focused upwards through the domed top of the LED, which itself acts like a lens concentrating the amount of light. This is why the emitted light appears to be brightest at the top of the LED.
However, not all LEDs are made with a hemispherical shaped dome for their epoxy shell. Some indication LEDs have a rectangular or cylindrical shaped construction that has a flat surface on top or their body is shaped into a bar or arrow. Generally, all LED’s are manufactured with two legs protruding from the bottom of the body.
Also, nearly all modern light emitting diodes have their cathode, ( – ) terminal identified by either a notch or flat spot on the body or by the cathode lead being shorter than the other as the anode ( + ) lead is longer than the cathode (k).
Unlike normal incandescent lamps and bulbs which generate large amounts of heat when illuminated, the light emitting diode produces a “cold” generation of light which leads to high efficiencies than the normal “light bulb” because most of the generated energy radiates away within the visible spectrum. Because LEDs are solid-state devices, they can be extremely small and durable and provide much longer lamp life than normal light sources.
Light Emitting Diode Colours
So how does a light emitting diode get its colour. Unlike normal signal diodes which are made for detection or power rectification, and which are made from either Germanium or Silicon semiconductor materials, Light Emitting Diodes are made from exotic semiconductor compounds such as Gallium Arsenide (GaAs), Gallium Phosphide (GaP), Gallium Arsenide Phosphide (GaAsP), Silicon Carbide (SiC) or Gallium Indium Nitride (GaInN) all mixed together at different ratios to produce a distinct wavelength of colour.
Different LED compounds emit light in specific regions of the visible light spectrum and therefore produce different intensity levels. The exact choice of the semiconductor material used will determine the overall wavelength of the photon light emissions and therefore the resulting colour of the light emitted.
Light Emitting Diode Colours
Typical LED Characteristics
Semiconductor Material
Wavelength
Colour
VF @ 20mA
GaAs
850-940nm
Infra-Red
1.2v
GaAsP
630-660nm
Red
1.8v
GaAsP
605-620nm
Amber
2.0v
GaAsP:N
585-595nm
Yellow
2.2v
AlGaP
550-570nm
Green
3.5v
SiC
430-505nm
Blue
3.6v
GaInN
450nm
White
4.0v
Thus, the actual colour of a light emitting diode is determined by the wavelength of the light emitted, which in turn is determined by the actual semiconductor compound used in forming the PN junction during manufacture.
Therefore the colour of the light emitted by an LED is NOT determined by the colouring of the LED’s plastic body although these are slightly coloured to both enhance the light output and to indicate its colour when its not being illuminated by an electrical supply.
Light emitting diodes are available in a wide range of colours with the most common being RED, AMBER, YELLOW and GREEN and are thus widely used as visual indicators and as moving light displays.
Recently developed blue and white coloured LEDs are also available but these tend to be much more expensive than the normal standard colours due to the production costs of mixing together two or more complementary colours at an exact ratio within the semiconductor compound and also by injecting nitrogen atoms into the crystal structure during the doping process.
From the table above we can see that the main P-type dopant used in the manufacture of Light Emitting Diodes is Gallium (Ga, atomic number 31) and that the main N-type dopant used is Arsenic (As, atomic number 33) giving the resulting compound of Gallium Arsenide (GaAs) crystalline structure.
The problem with using Gallium Arsenide on its own as the semiconductor compound is that it radiates large amounts of low brightness infra-red radiation (850nm-940nm approx.) from its junction when a forward current is flowing through it.
The amount of infra-red light it produces is okay for television remote controls but not very useful if we want to use the LED as an indicating light. But by adding Phosphorus (P, atomic number 15), as a third dopant the overall wavelength of the emitted radiation is reduced to below 680nm giving visible red light to the human eye. Further refinements in the doping process of the PN junction have resulted in a range of colours spanning the spectrum of visible light as we have seen above as well as infra-red and ultra-violet wavelengths.
By mixing together a variety of semiconductor, metal and gas compounds the following list of LEDs can be produced.
Types of Light Emitting Diode
Gallium Arsenide (GaAs) – infra-red
Gallium Arsenide Phosphide (GaAsP) – red to infra-red, orange
Gallium Indium Nitride (GaInN) – near ultraviolet, bluish-green and blue
Silicon Carbide (SiC) – blue as a substrate
Zinc Selenide (ZnSe) – blue
Aluminium Gallium Nitride (AlGaN) – ultraviolet
Like conventional PN junction diodes, light emitting diodes are current-dependent devices with its forward voltage drop VF, depending on the semiconductor compound (its light colour) and on the forward biased LED current. Most common LED’s require a forward operating voltage of between approximately 1.2 to 3.6 volts with a forward current rating of about 10 to 30 mA, with 12 to 20 mA being the most common range.
Both the forward operating voltage and forward current vary depending on the semiconductor material used but the point where conduction begins and light is produced is about 1.2V for a standard red LED to about 3.6V for a blue LED.
The exact voltage drop will of course depend on the manufacturer because of the different dopant materials and wavelengths used. The voltage drop across the LED at a particular current value, for example 20mA, will also depend on the initial conduction VF point. As an LED is effectively a diode, its forward current to voltage characteristics curves can be plotted for each diode colour as shown below.
Light Emitting Diodes I-V Characteristics.
Light Emitting Diode (LED) Schematic symbol and I-V Characteristics Curves showing the different colours available.
Before a light emitting diode can “emit” any form of light it needs a current to flow through it, as it is a current dependant device with their light output intensity being directly proportional to the forward current flowing through the LED.
As the LED is to be connected in a forward bias condition across a power supply it should be current limited using a series resistor to protect it from excessive current flow. Never connect an LED directly to a battery or power supply as it will be destroyed almost instantly because too much current will pass through and burn it out.
From the table above we can see that each LED has its own forward voltage drop across the PN junction and this parameter which is determined by the semiconductor material used, is the forward voltage drop for a specified amount of forward conduction current, typically for a forward current of 20mA.
In most cases LEDs are operated from a low voltage DC supply, with a series resistor, RS used to limit the forward current to a safe value from say 5mA for a simple LED indicator to 30mA or more where a high brightness light output is needed.
LED Series Resistance.
The series resistor value RS is calculated by simply using Ohm´s Law, by knowing the required forward current IF of the LED, the supply voltage VS across the combination and the expected forward voltage drop of the LED, VF at the required current level, the current limiting resistor is calculated as:
LED Series Resistor Circuit
Light Emitting Diode Example No1
An amber coloured LED with a forward volt drop of 2 volts is to be connected to a 5.0v stabilised DC power supply. Using the circuit above calculate the value of the series resistor required to limit the forward current to less than 10mA. Also calculate the current flowing through the diode if a 100Ω series resistor is used instead of the calculated first.
1). series resistor required at 10mA.
2). with a 100Ω series resistor.
We remember from the Resistors tutorials, that resistors come in standard preferred values. Our first calculation above shows that to limit the current flowing through the LED to 10mA exactly, we would require a 300Ω resistor. In the E12 series of resistors there is no 300Ω resistor so we would need to choose the next highest value, which is 330Ω. A quick re-calculation shows the new forward current value is now 9.1mA, and this is ok.
Connecting LEDs Together in Series
We can connect LED’s together in series to increase the number required or to increase the light level when used in displays. As with series resistors, LED’s connected in series all have the same forward current, IF flowing through them as just one. As all the LEDs connected in series pass the same current it is generally best if they are all of the same colour or type.
Connecting LED’s in Series
Although the LED series chain has the same current flowing through it, the series voltage drop across them needs to be considered when calculating the required resistance of the current limiting resistor, RS. If we assume that each LED has a voltage drop across it when illuminated of 1.2 volts, then the voltage drop across all three will be 3 x 1.2v = 3.6 volts.
If we also assume that the three LEDs are to be illuminated from the same 5 volt logic device or supply with a forward current of about 10mA, the same as above. Then the voltage drop across the resistor, RS and its resistance value will be calculated as:
Again, in the E12 (10% tolerance) series of resistors there is no 140Ω resistor so we would need to choose the next highest value, which is 150Ω.
LED Driver Circuits
Now that we know what is an LED, we need some way of controlling it by switching it “ON” and “OFF”. The output stages of both TTL and CMOS logic gates can both source and sink useful amounts of current therefore can be used to drive an LED. Normal integrated circuits (ICs) have an output drive current of up to 50mA in the sink mode configuration, but have an internally limited output current of about 30mA in the source mode configuration.
Either way the LED current must be limited to a safe value using a series resistor as we have already seen. Below are some examples of driving light emitting diodes using inverting ICs but the idea is the same for any type of integrated circuit output whether combinational or sequential.
IC Driver Circuit
If more than one LED requires driving at the same time, such as in large LED arrays, or the load current is to high for the integrated circuit or we may just want to use discrete components instead of ICs, then an alternative way of driving the LEDs using either bipolar NPN or PNP transistors as switches is given below. Again as before, a seri
Transistor Driver Circuit
The brightness of a light emitting diode cannot be controlled by simply varying the current flowing through it. Allowing more current to flow through the LED will make it glow brighter but will also cause it to dissipate more heat. LEDs are designed to produce a set amount of light operating at a specific forward current ranging from about 10 to 20mA.
In situations where power savings are important, less current may be possible. However, reducing the current to below say 5mA may dim its light output too much or even turn the LED “OFF” completely. A much better way to control the brightness of LEDs is to use a control process known as “Pulse Width Modulation” or PWM, in which the LED is repeatedly turned “ON” and “OFF” at varying frequencies depending upon the required light intensity of the LED.es resistor, RS is required to limit the LED current.
LED Light Intensity using PWM
When higher light outputs are required, a pulse width modulated current with a fairly short duty cycle (“ON-OFF” Ratio) allows the diode current and therefore the output light intensity to be increased significantly during the actual pulses, while still keeping the LEDs “average current level” and power dissipation within safe limits.
This “ON-OFF” flashing condition does not affect what is seen by the human eye as it “fills” in the gaps between the “ON” and “OFF” light pulses, providing the pulse frequency is high enough, making it appear as a continuous light output. So pulses at a frequency of 100Hz or more actually appear brighter to the eye than a continuous light of the same average intensity.
Multi-coloured Light Emitting Diode
LEDs are available in a wide range of shapes, colours and various sizes with different light output intensities available, with the most common (and cheapest to produce) being the standard 5mm Red Gallium Arsenide Phosphide (GaAsP) LED.
LED’s are also available in various “packages” arranged to produce both letters and numbers with the most common being that of the “seven segment display” arrangement.
Nowadays, full colour flat screen LED displays, hand held devices and TV’s are available which use a vast number of multicoloured LED’s all been driven directly by their own dedicated IC.
Most light emitting diodes produce just a single output of coloured light however, multi-coloured LEDs are now available that can produce a range of different colours from within a single device. Most of these are actually two or three LEDs fabricated within a single package.
Bi-colour Light Emitting Diodes
A bi-colour light emitting diode has two LEDs chips connected together in “inverse parallel” (one forwards, one backwards) combined in one single package. Bi-colour LEDs can produce any one of three colours for example, a red colour is emitted when the device is connected with current flowing in one direction and a green colour is emitted when it is biased in the other direction.
This type of bi-directional arrangement is useful for giving polarity indication, for example, the correct connection of batteries or power supplies etc. Also, a bi-directional current produces both colours mixed together as the two LEDs would take it in turn to illuminate if the device was connected (via a suitable resistor) to a low voltage, low frequency AC supply.
A Bi-colour LED
LED Selected
Terminal A
AC
+
–
LED 1
ON
OFF
ON
LED 2
OFF
ON
ON
Colour
Green
Red
Yellow
Tricoloured Light Emitting Diode
The most popular type of tricolour light emitting diode comprises of a single Red and a Green LED combined in one package with their cathode terminals connected together producing a three terminal device. They are called tricolour LEDs because they can give out a single red or a green colour by turning “ON” only one LED at a time.
These tricoloured LED’s can also generate additional shades of their primary colours (the third colour) such as Orange or Yellow by turning “ON” the two LEDs in different ratios of forward current as shown in the table thereby generating four different colours from just two diode junctions.
A Multi or Tricoloured LED
Output Colour
Red
Orange
Yellow
Green
LED 1 Current
0
5mA
9.5mA
15mA
LED 2 Current
10mA
6.5mA
3.5mA
0
LED Displays
As well as individual colour or multi-colour LEDs, several light emitting diodes can be combined together within a single package to produce displays such as bargraphs, strips, arrays and seven segment displays.
A 7-segment LED display provides a very convenient way when decoded properly of displaying information or digital data in the form of numbers, letters or even alpha-numerical characters and as their name suggests, they consist of seven individual LEDs (the segments), within one single display package.
In order to produce the required numbers or characters from 0 to 9 and A to F respectively, on the display the correct combination of LED segments need to be illuminated. A standard seven segment LED display generally has eight input connections, one for each LED segment and one that acts as a common terminal or connection for all the internal segments.
The Common Cathode Display (CCD) – In the common cathode display, all the cathode connections of the LEDs are joined together and the individual segments are illuminated by application of a HIGH, logic “1” signal.
The Common Anode Display (CAD) – In the common anode display, all the anode connections of the LEDs are joined together and the individual segments are illuminated by connecting the terminals to a LOW, logic “0” signal.
A Typical Seven Segment LED Display
Opto-coupler
Finally, another useful application of light emitting diodes is in Opto-coupling. An opto-coupler or opto-isolator as it is also called, is a single electronic device that consists of a light emitting diode combined with either a photo-diode, photo-transistor or photo-triac to provide an optical signal path between an input connection and an output connection while maintaining electrical isolation between two circuits.
An opto-isolator consists of a light proof plastic body that has a typical breakdown voltages between the input (photo-diode) and the output (photo-transistor) circuit of up to 5000 volts. This electrical isolation is especially useful where the signal from a low voltage circuit such as a battery powered circuit, computer or microcontroller, is required to operate or control another external circuit operating at a potentially dangerous mains voltage.
Photo-diode and Photo-transistor Opto-couplers
The two components used in an opto-isolator, an optical transmitter such as an infra-red emitting Gallium Arsenide LED and an optical receiver such as a photo-transistor are closely optically coupled and use light to send signals and/or information between its input and output. This allows information to be transferred between circuits without an electrical connection or common ground potential.
Opto-isolators are digital or switching devices, so they transfer either “ON-OFF” control signals or digital data. Analogue signals can be transferred by means of frequency or pulse-width modulation.
A PN-junction diode is formed when a p-type semiconductor is fused to an n-type semiconductor creating a potential barrier voltage across the diode junction
The effect described in the previous tutorial is achieved without any external voltage being applied to the actual PN junction resulting in the junction being in a state of equilibrium.
However, if we were to make electrical connections at the ends of both the N-type and the P-type materials and then connect them to a battery source, an additional energy source now exists to overcome the potential barrier.
The effect of adding this additional energy source results in the free electrons being able to cross the depletion region from one side to the other. The behaviour of the PN junction with regards to the potential barrier’s width produces an asymmetrical conducting two terminal device, better known as the PN Junction Diode.
A PN Junction Diode is one of the simplest semiconductor devices around, and which has the characteristic of passing current in only one direction only. However, unlike a resistor, a diode does not behave linearly with respect to the applied voltage as the diode has an exponential current-voltage ( I-V ) relationship and therefore we can not described its operation by simply using an equation such as Ohm’s law.
If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased.
By applying a negative voltage (reverse bias) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased. This has the effect of increasing or decreasing the effective resistance of the junction itself allowing or blocking the flow of current through the diodes pn-junction.
Then the depletion layer widens with an increase in the application of a reverse voltage and narrows with an increase in the application of a forward voltage. This is due to the differences in the electrical properties on the two sides of the PN junction resulting in physical changes taking place. One of the results produces rectification as seen in the PN junction diodes static I-V (current-voltage) characteristics. Rectification is shown by an asymmetrical current flow when the polarity of bias voltage is altered as shown below.
Junction Diode Symbol and Static I-V Characteristics
But before we can use the PN junction as a practical device or as a rectifying device we need to firstly bias the junction, that is connect a voltage potential across it. On the voltage axis above, “Reverse Bias” refers to an external voltage potential which increases the potential barrier. An external voltage which decreases the potential barrier is said to act in the “Forward Bias” direction.
There are two operating regions and three possible “biasing” conditions for the standard Junction Diode and these are:
1. Zero Bias – No external voltage potential is applied to the PN junction diode.
2. Reverse Bias – The voltage potential is connected negative, (-ve) to the P-type material and positive, (+ve) to the N-type material across the diode which has the effect of Increasing the PN junction diode’s width.
3. Forward Bias – The voltage potential is connected positive, (+ve) to the P-type material and negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the PN junction diodes width.
Zero Biased Junction Diode
When a diode is connected in a Zero Bias condition, no external potential energy is applied to the PN junction. However if the diodes terminals are shorted together, a few holes (majority carriers) in the P-type material with enough energy to overcome the potential barrier will move across the junction against this barrier potential. This is known as the “Forward Current” and is referenced as IF
Likewise, holes generated in the N-type material (minority carriers), find this situation favourable and move across the junction in the opposite direction. This is known as the “Reverse Current” and is referenced as IR. This transfer of electrons and holes back and forth across the PN junction is known as diffusion, as shown below.
Zero Biased PN Junction Diode
The potential barrier that now exists discourages the diffusion of any more majority carriers across the junction. However, the potential barrier helps minority carriers (few free electrons in the P-region and few holes in the N-region) to drift across the junction.
Then an “Equilibrium” or balance will be established when the majority carriers are equal and both moving in opposite directions, so that the net result is zero current flowing in the circuit. When this occurs the junction is said to be in a state of “Dynamic Equilibrium“.
The minority carriers are constantly generated due to thermal energy so this state of equilibrium can be broken by raising the temperature of the PN junction causing an increase in the generation of minority carriers, thereby resulting in an increase in leakage current but an electric current cannot flow since no circuit has been connected to the PN junction.
Reverse Biased PN Junction Diode
When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material.
The positive voltage applied to the N-type material attracts electrons towards the positive electrode and away from the junction, while the holes in the P-type end are also attracted away from the junction towards the negative electrode.
The net result is that the depletion layer grows wider due to a lack of electrons and holes and presents a high impedance path, almost an insulator and a high potential barrier is created across the junction thus preventing current from flowing through the semiconductor material.
Increase in the Depletion Layer due to Reverse Bias
This condition represents a high resistance value to the PN junction and practically zero current flows through the junction diode with an increase in bias voltage. However, a very small reverse leakage current does flow through the junction which can normally be measured in micro-amperes, ( μA ).
One final point, if the reverse bias voltage Vr applied to the diode is increased to a sufficiently high enough value, it will cause the diode’s PN junction to overheat and fail due to the avalanche effect around the junction. This may cause the diode to become shorted and will result in the flow of maximum circuit current, and this shown as a step downward slope in the reverse static characteristics curve below.
Reverse Characteristics Curve for a Junction Diode
Sometimes this avalanche effect has practical applications in voltage stabilising circuits where a series limiting resistor is used with the diode to limit this reverse breakdown current to a preset maximum value thereby producing a fixed voltage output across the diode. These types of diodes are commonly known as Zener Diodes and are discussed in a later tutorial.
Forward Biased PN Junction Diode
When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material. If this external voltage becomes greater than the value of the potential barrier, approx. 0.7 volts for silicon and 0.3 volts for germanium, the potential barriers opposition will be overcome and current will start to flow.
This is because the negative voltage pushes or repels electrons towards the junction giving them the energy to cross over and combine with the holes being pushed in the opposite direction towards the junction by the positive voltage. This results in a characteristics curve of zero current flowing up to this voltage point, called the “knee” on the static curves and then a high current flow through the diode with little increase in the external voltage as shown below.
Forward Characteristics Curve for a Junction Diode
The application of a forward biasing voltage on the junction diode results in the depletion layer becoming very thin and narrow which represents a low impedance path through the junction thereby allowing high currents to flow. The point at which this sudden increase in current takes place is represented on the static I-V characteristics curve above as the “knee” point.
Reduction in the Depletion Layer due to Forward Bias
This condition represents the low resistance path through the PN junction allowing very large currents to flow through the diode with only a small increase in bias voltage. The actual potential difference across the junction or diode is kept constant by the action of the depletion layer at approximately 0.3v for germanium and approximately 0.7v for silicon junction diodes.
Since the diode can conduct “infinite” current above this knee point as it effectively becomes a short circuit, therefore resistors are used in series with the diode to limit its current flow. Exceeding its maximum forward current specification causes the device to dissipate more power in the form of heat than it was designed for resulting in a very quick failure of the device.
