Introduction to Transistors

Transcription

1 Introduction to Transistors Transistors come in two general types. The first is the Bipolar Junction Transistor (BJT) which is the topic of the material that follows. The second type is the Field Effect Transistor (FET) which will be looked at in another part of the course. Bipolar Junction Transistor (BJT) This transistor is a semiconductor device consisting of 2 P N junctions the Base-Emitter junction and the Base-Collector junction. There are two different types of BJT referred to as NPN and PNP transistors. Diagrams illustrating their construction are shown. NPN Transistor As the name suggests this transistor is constructed from 2 sections of N type semiconductor material and 1 of P material creating 2 PN junctions. PNP Transistor This transistor also has 2 PN junctions but is constructed from 2 sections of P type semiconductor material and 1 of N material. What is the Difference? The most significant difference between these 2 types of transistors relates to how they are used. NPN transistors are used with positive supply voltages called +V CC and the PNP uses a negative supply voltage referred to as V CC. We will focus almost exclusively on the NPN type of transistor. Transistor Terminals A BJT transistor has 3 leads that are referred to as the Base (B), Collector (C) and Emitter (E). Both the physical and schematic symbols below show these terminals.

2 Transistor Schematic Symbols The standard symbols for use in a schematic diagram are shown. A slightly different approach to viewing these symbols is shown as well. These alternate diagrams are consistent with the idea that the terminal pointing up is usually connected to the supply voltage (either +V CC or V CC ) and the terminal pointing down is connected to ground. Transistor Electron Flow The 3 regions of the transistor are not manufactured exactly the same. The emitter area is a large heavily doped N type region with lots of free electrons, the base is a small, thin lightly doped (low density of holes) P type area and the collector is a large moderately doped N type area. An NPN transistor is shown with 2 bias voltages V CC and V BB. The supply voltage V CC has its negative terminal connected to the Emitter. This will repel free electrons into the Emitter region and through to the lightly doped base region where a small percentage of these electrons will combine with holes and flow out of the base terminal by the attraction of the V BB voltage as valence electrons. The rest of the electrons injected the base will continue on into the Collector region creating a large electron flow from Emitter to Collector in the transistor.

3 We can attempt to quantify the number of electrons moving through each region of the transistor. If the assumption is that 100 electrons flow through the emitter into the base, the design of the transistor is such that approximately 1 electron will flow out of the base and the other 99 will continue to the collector region. Transistor Conventional Current Flow Conventional current as previously learned flows in the opposite direction to electron flow. The diagram below shows the 3 transistor currents; Base Current (I B ), Collector current (I C ) and Emitter current (I E ) with their correct direction of flow. Transistor Current Relationships From the diagram above its can be written that I E = I B + I C This is just an application of KCL. From the description and quantification of electron flow above it can be predicted that relatively speaking the Base current in a transistor is small and the Collector and Emitter currents are large. So the expression above can often be approximated as I E I C

4 Transistor Current Block Diagram From a current point of view the block diagram represents the input and output currents of the transistor. The Base current I B is the Input current and the Collector current I C is the Output current. Transistor Parameters There are a number of transistor parameters that can be considered. Current Gain - β The Current Gain - β - is defined as the ratio of Collector to Base currents. β of course has no units. β = I C /I B From this definition we can write that I C = β x I B The outcome of this expression is important. It tells us that the collector current is β times larger than the base current. In fact it can be written that the smaller Input Base current I B controls the larger Output Collector current I C. It is often written that the BJT is a current controlled device an input current controls an output current. From the expression I C = β x I B the following can be derived I E = (β + 1) x I B

5 Variability of Current Gain - β Values of β can be measured or found on a data sheet for a transistor. A more advanced term h FE is also used to describe current gain. However for any given transistor the value of β is highly variable. In what ways does it vary? 1. β varies from device to device. For example: A 2N 4124 is a small signal transistor. From the data sheet the value of β varies from 120 to 360. A diagram of this transistor is shown with a TO-92 casing. A 2N 3055 is a power transistor. From the data sheet its β varies from 20 to 70. Diagrams of this transistor are shown with a TO-3 and a TO-220 casing. 2. β also varies within devices as can be seen above. There is at least a 100 % variation in the values of β for these devices. 3. β also varies with the value of collector current. 4. β also varies with temperature.

6 Alpha Parameter α The Alpha parameter is the ratio of the number of electrons that leave the Collector region to the number of electrons that enter the Emitter. So in terms of current flow α = I C /I E From this expression it can be shown that I C = α I E and I B = I E - I C = I E - α I E so I B = (1 α) I E Also α and β can be related α = β/(β + 1) and β = α/(1 - α) Example 1. If I E = 20 ma and β = 200. Find I B, I C, and α I B = I E /(β + 1) = 20 ma/(200 +1) = 100 µa I C = I E - I B = 20 ma 0.1 ma = 19.9 ma α = I C /I E = 19.9 ma/20 ma = If I C = 50 ma and β = 400. Find I B, I E, and α I B = I C /β = 50 ma/400 = 125 µa I E = I C + I B = 50 ma ma = ma α = I C /I E = 50 ma/ ma = Note: the value of α for this question indicates also that if 1000 electrons enter the emitter region, 4 leave the base region and 996 continue to the collector. Transistor Testing Transistors can be tested using an Ohmmeter in a similar way to testing a diode.

7 Back to Back Diode Model of Transistor This model is helpful to see the 2 PN junctions in the transistor and how they can be tested. NPN Transistor PNP Transistor For this simple model the transistor is tested as if it was a pair of diodes. The Base Emitter junction is one diode and the Base Collector junction is the other. Use the ohmmeter and connect the leads to each anode and cathode of the equivalent PN junctions. The results for working transistor are shown. Polarity of Connection On or Off Resistance B (+) E (-) On Low R B (-) E (+) Off High R B (+) C (-) On Low R B (-) C (+) Off High R C (+) E (-) Off High R C (-) E (+) Off High R

8 PNP Transistor Testing The testing for a PNP transistor is very similar. The table shows the results for a functional transistor. Polarity of Connection On or Off Resistance B (+) E (-) Off High R B (-) E (+) On Low R B (+) C (-) Off High R B (-) C (+) On Low R C (+) E (-) Off High R C (-) E (+) Off High R