An Introduction to Bipolar Junction Transistors. Prepared by Dr Yonas M Gebremichael, 2005

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1 An Introduction to Bipolar Junction Transistors

2 Transistors Transistors are three port devices used in most integrated circuits such as amplifiers. Non amplifying components we have seen so far, such as resistors, capacitors, inductors and diodes are passive components. In comparison transistors are active components. Two basic types: The bipolar or junction transistor its operation depends on the flow of both majority and minority carriers The Unipolar or field effect transistor (FET) in which the current is due to majority carriers only (either electrons or holes)

3 Bipolar Junction Transistors In the construction, a very thin slice of lightly doped p or n type semiconductor (the BASE, B) is sandwiched between two thicker heavily doped materials of the opposite type constituting the COLLECTOR, C and the EMMITOR, E. npn are more common while pnp are not so commonly used as npn ones - due to the properties of the materials used, pnp are more expensive to manufacture and they do not perform so well at high frequencies. Schematic representation of a junction transistor : pnp or npn

4 Junction Transistors-theory of operation In normal use as an amplifier, the p-n junction (diode) between collector and base is reversed biased and the p-n junction between emitter and base forward biased. When the base-emitter junction is about +0.6V, the majority carriers (electrons) in the n-type cross the junction into the base. At the same time holes flow from base to the emitter, but since the p type is lightly doped and also thin (~1um), the flow of holes is small compared to electrons. The electrons from the emitter are swept across the junction to the collector because they are attracted by the by the positive voltage on the collector, very few of which recombine with holes (the base current) before reaching the collector. Thus, the base current is very small, typically about 1% of the emitter current.

5 Transistor Symbols The arrow on the emitter indicates the direction of conventional current flow under normal bias conditions.

6 Junction transistor as an amplifier Three configurations are use in practice - In each case one terminal is connected to signal common (though not always directly) and the input and output signals are taken between common and the other two terminals. These are common emitter, common collector and common base. In the circuit shown (common emitter), npn transistor, the collector is typically fed from a +12V dc supply, normally denoted V cc A fairly high value resistor is placed in series with this, say 2k. c-b junction is reverse biased A further dc supply is connected between base and emitter, to forward biases the base-emitter junction

7 Junction transistor as an amplifier The small base current I B flows and turns on a larger collector current I C. I C is zero until the I B flows, a junction transistor is thus current operated device. A transistor may be thought of as an electronic tap able to control a large flow of electrons with only small variations of the 'handle'. The 'handle' in the case of a transistor is called the "base". The in and out 'pipes' are called the "emitter" and the "collector". Voltage changes at the base of the transistor result in changes to the flow of electricity through the transistor.

8 Junction transistor as an amplifier Typically I C is 100 to 1000 times greater than I B depending on the type of the transistor. If we think of I B as an input and I C as an output, then the transistor is a current amplifier. The current gain is h FE. h FE I I C B In the symbol h FE F indicates forward current and E shows transistor connected as common emitter. For the circuit shown earlier, I C =1mA and I B =10µA, h FE = 1mA/10µA = 100. You should also note that since the current flowing out of the transistor must equal the current flowing into it (By Kirchhof s Law), the current I B +I C = I E. Since, with a very thin base region, I C >> I B, I E will very nearly equal I C. Where I E is the emitter current.

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11 Summary Junction transistor To summarise: I C is zero until I B flows the collector current I C is proportional to the base current I B, with a current gain h FE = I C /I B, of around 100 to I B is zero until V BE is about 0.6V V BE remains close to 0.6V for a wide range of I B. For a fixed base current, the collector current is almost independent of the collector-emitter voltage down to voltages of the order of 0.5V (V CEsat ) and so looking into the collector we see a high resistance current source. Transistors can be used as a switch (saturation region) or amplifiers (constant current region)

12 Transistor as a switch So far we have looked at transistors as amplifying devices. However another important application is their use as a switch. The advantages of transistor switches over mechanical switches are: (i) very high speed of operation -present day transistors can operate at up to 10 9 times a second (ii) very high reliability (iii) electronically-controlled operation (iv) low cost (v) small size Their disadvantages are: (i) the switch is not a true open circuit in the OFF condition, a small but finite current still flows (ii) the switch is not a true short circuit in the ON condition, there is a finite voltage drop across it, typically about 0.1 V

13 Transistor as a switch However, in many practical cases the disadvantages are of little significance. In many applications the use of transistor switches provides great improvements in operation over alternative methods, such as relays and other mechanical switches. One of the most important applications is in the control of logic levels in digital circuits. For instance, modern digital computers rely almost exclusively for their operation on the use of (FET) transistor switches. The operation of a transistor as a switch is the basics of switching type (digital) circuits. In digital circuits, the outputs and inputs involve only two levels of voltage HIGH or LOW. I.e. two state circuits. HIGH logic level is referred as logic level 1 which is usually for voltages near the supply (e.g. +5V) LOW is referred as logic level 0. Transistors are cheap, reliable, no moving parts with almost indefinite life and can switch millions of times a second. A perfect switch would have no resistance when ON and zero resistance when OFF with no power consumption

14 Transistor as a switch