Zener diodes can be distinguished from ordinary diodes by their code and breakdown voltage which are printed on them (there are exceptions).
Zener diode codes usually begin with BZX or BZY. Th eir breakdown voltage is printed with V in place of a decimal comma, so 4V7 means 4,7 V.
Zener diodes are rated by their breakdown voltage and maximum power:
Th e minimum voltage available is 2,4 V.
Power ratings of 400 mW and 1,3 W are the most common.
1N4728A – 1N4752A Series One Watt Zeners
Absolute Maximum Ratings* TA = 25°C unless otherwise noted Tolerance: A = 5%
Device VZ(V) ZZ (W) @ IZT (mA)
ZZK(W) @ IZK(mA) VR (V) @ IR(mA) ISURGE (mA)
IZM (mA) 1N4728A 3.3 3.6 3.9 400 1 1 100 1,380 276 1N4729A 4.3 10 10 400 1 1 100 1,26 252 1N4730A 9.0 9.0 76 400 1 1 50 1,19 234 1N4731A 69 64 58 400 1 1 10 1,07 217
Figure 8.9: Data sheet for certain Zener diodes
Transistors
Th e transistor is a small semiconductor device that has revolutionised the electronic industry. Transistors are available in diff erent shapes and sizes.
Th e design of a transistor allows it to function as an amplifi er or a switch. Th is is accomplished by applying a small amount of electricity on the base to regulate current fl ow through the device, much like turning a tap to control a supply of water. When the transistor is used as an electronic switch, it is mostly used in digital and computer circuits. When it is used as an amplifi er, it amplifi es very small
Zener diode – is a specially constructed semiconductor device with unique reverse- bias characteristics.
Forward region
Reverse region
For example, when a person speaks into a microphone, the sound waves (audio) are converted into an electrical signal. However, this electrical signal is too small to drive a speaker. It must fi rst be made bigger, and this is achieved by feeding the electrical signal to the base of a transistor. Th e transistor will enlarge the signal and enable us to hear it through the speaker.
Construction
Transistors are composed of three parts – a base, a collector and an emitter. Th e base is the “valve or tap” that will control fl ow. Th e collector is the supply, and the emitter is the outlet. By controlling the base, fl ow can be switched on or off (electronic switch) or increased and decreased (amplifi er). In this way, a very small amount of current may be used to control a large amount of current, as in an amplifi er.
Figure 8.10: Block diagrams and symbols of PNP and NPN transistors
Th ere are two types of bipolar transistors. If the middle layer is P-type, the outside layers must be N-type. Such a transistor is an NPN transistor. One of the outside layers is called the emitter, and the other is known as the collector. Th e middle layer is the base. Th e places where the emitter joins the base and the base joins the collector are called junctions or depletion layers.
Th e layers of an NPN transistor must have the proper voltage connected across them. Th e voltage of the base must be more positive than that of the emitter. Th e voltage of the collector, in turn, must be more positive than that of the base. Th e voltages are supplied by a battery or some other source of direct current.
Emitter fi rst developed by John Robert Woodyard
In a PNP bipolar transistor, the emitter and collector are both a P-type
semiconductor material and the base is N-type. A PNP bipolar transistor works on the same principle as an NPN transistor, but it diff ers in one respect. Th e main fl ow of current in a PNP transistor is controlled by altering the number of holes rather than the number of electrons in the base. Also, this type of transistor works properly only if the negative and positive connections to it are the reverse of those of the NPN transistor.
Depletion regions
Th e two drawings below clearly show the formation of the depletion regions (layers) in the diff erent types of transistors. It must be remembered that the collector and emitter regions are heavily doped and therefore have a much lower resistance than the base region which is lightly doped. It must also be remembered that the base region is much thinner compared to the collector and the emitter. (On the diagrams below it is drawn bigger to make the illustration easier.)
Figure 8.12: Formation of depletion regions
From the drawing it can be seen that the depletion region is wider in the base region than in the collector and emitter regions. Th e outer regions are more heavily doped, resulting in the penetration of the electrons deeply into the base region for the PNP transistor, and the penetration of holes into the base region for the NPN transistor.
Th e depletion layers are formed because of the diff usion that takes place between the N- and P-type materials. In a PNP transistor, with no bias voltage applied, it can be seen that the barrier potential of the P-region near the P-N junction is more negative and the base region is more positive near the P-N
junction. Th e barrier potentials will be opposite in the NPN transistor.
Construction
Th ese bipolar transistors are three layer semiconductors that consists of one piece of N-type material which is sandwiched between two P-type materials, or a P-type material that is sandwiched between two N-type materials. Th ey are the NPN type and the PNP type. Further, they are made from two types of semiconductors: silicon and germanium. For the purposes of this book, the focus will be on the silicon type.
Emitter Base Collector
Depletion regions
Depletion regions Emitter Base Collector
While it is not necessary to know the actual construction of the transistors, one does need to know how they work and what the applications for these components are. In fi gure 8.13 one can see two deconstructed transistors, showing the wafer on the inside.
Figure 8.13: The actual wafers are shown inside the transistors Transistor operation
For the transistor to be operated as an amplifi er (in the linear or active region), the base-emitter junction must be forward biased in the case of a NPN transistor. Th e diagram below indicates how the bias voltages must be connected to the transistor for proper operation.
Figure 8.14: Forward biasing of NPN transistor
Because the transistor has two P-N junctions, the following must happen in order for it to operate as an amplifi er;
• the base-emitter junction must be forward biased (low resistance).
• the base-collector junction must be reversed biased (high resistance).
• Electrons will now fl ow from the emitter to the base because of the bias of the battery.
Did you know?
Th e wafer is the part that contains the pad n materials. Th e rest is just the “housing”
to hold it, and so that we can handle the component.
• Because the electrons are negative, they are attracted across the junction by the higher positive potential of the collector voltage.
• From this one can see that the electrons are the majority carriers and the holes the minority carriers in the NPN transistor.
Th e PNP transistor operates in exactly the same manner as the NPN transistor. Th e only diff erence is that the majority carriers are holes in the PNP transistor. Once again
• the base-emitter junction is forward biased.
• the depletion region becomes narrower.
• holes start to fl ow from the emitter to the base.
• Although the depletion region is widened between the collector-base region, the holes are attracted by the negative voltage of the battery connected to the collector of the transistor.
Figure 8.15: Forward biasing of PNP transistor Current fl ow in a transistor (for NPN)
Th e relationship between the diff erent currents fl owing through a transistor is always the same. Th e largest current is the emitter current IE,with the smallest being the base current IB. Th e collector current IC is the diff erence between the emitter and the base current.
Th e current fl ow in a transistor can thus be indicated as follows:
IE = IB + IC or IC = IE – IB
IE= emitter current IB= base current IC= collector current.
Please note that the arrows indicating the currents, indicate the conventional current fl ow through the transistor.
Figure 8.16: Block representation of both transistors
Figure 8.17: Schematic symbols of both transistors Regions in which a transistor operates
A transistor can be operated in three basic regions. Th ese regions can be clearly indicated on the graph below, which is a graph of the output characteristics of a common emitter amplifi er. Th e three regions are as follows:
• Saturation region
• Active region
• Cut-off region
Figure 8.18: Transistor characteristic curve Th e saturation region
Th is is when the transistor is completely on (fully on). Both the base-emitter and base-collector junctions must be forward biased. In this area, the transistor can be used as a switch that is closed.
Linear or active region
In this region the transistor is used between the CUT-OFF and the SATURATION region. Th e base-emitter junction must be forward biased and the base-collector
Current fl ow directions
lc
VCE Take note
Conventional current is the movement of holes from positive to negative.
Cut-off region
In this region, both the base-emitter and base-collector junctions are reversed biased and the transistor is thus in the off state. Only a small leakage current is fl owing. In this area the transistor can be used as a switch that is open.
Values (coding)
Transistors come in diff erent packages and each one is designed for a certain purpose. Hence they have various current, voltage and frequency ratings and, when selecting a transistor for a certain application, one would have to keep these in mind.
When purchasing a semiconductor from an electronics shop, one must ask for the pin identifi cation of the purchase. Th ey will consult a data book for the information.
Alternatively, one can visit the datasheet archive on the Internet. Below are some of the pin connections for certain transistor packages:
Figure 8.19: Identifi cation of the terminals for some casings
Th e current and voltage specifi cations are given in a table shown below:
NPN transistors BC107 NPN TO18 100 mA 45 V 110 300 mW Audio, low power BC182 BC547
BC109 NPN TO18 200 mA 20 V 200 300 mW Audio (low
noise), low power BC184 BC549
BC182 NPN TO92C 100 mA 50 V 100 350 mW General purpose,
low power BC107 BC182L BC547B NPN TO92C 100 mA 45 V 200 500 mW Audio, low power BC107B 2N3053 NPN TO39 700 mA 40 V 50 500 mW General purpose,
low power BFY51
come from the old valve radio technology, where the terminal that was giving off electrons was called the emitter, and the terminal receiving the electrons was called the collector.
Valves are still used in some circuits today,
Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data.
PNP transistors BC178 PNP TO18 200 mA 25 V 120 600 mW General purpose,
low power BC478
TIP32A PNP TO220 3 A 60 V 25 40 W General purpose,
high power TIP32C
TIP32C PNP TO220 3 A 100 V 10 40 W General purpose,
high power TIP32A
Th e shapes of transistors vary, and are said to come in certain packages. Th ese usually start with a TO followed by a number. Some packages are shown below.
TO-3 – Transistor Outline Package, Case Style 3
TO-18 – Transistor Outline Package, Case Style 18
TO-39 – Transistor Outline Package, Case Style 39
TO-92 – Transistor Outline Package, Case Style 92
TO-220 – Transistor Outline Package, Case Style 220 Figure 8.20: Diff erent casings for diff erent transistors.
Th e voltage bias conditions for the three regions are shown below.
Transistor switched on fully Current is maximum
Saturation Vbe ≥ 1,2 V Transistor switching on more or less
One can control amount of current fl owing.
Active 0,6 V ≤
As a switch the transistor operates in the cut-off and saturation regions.
When the base-emitter voltage of a transistor is less than 0,6 V, the transistor is said to be off , and no current can fl ow from the collector through to the emitter. When the base-emitter voltage is above 1,2 V, the transistor is conducting fully and the maximum permissible current is fl owing. Even if the bias voltage is increased, the Remember operate properly. If it is hot when you touch it, switch the circuit off .