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Electron Explorer Aalborg University Copenhagen Medialogy MED4 2007 Sensors Technology Basic electronic elements – transistor Smilen Dimitrov Basic electronic elements – transistor 1 Contents 1 Introduction............................................................................................................... 3 2 Transistor....................................................................................................................5 3 Bipolar junction transistor (BJT) .............................................................................. 6 3.1 Hydraulic analogy of a transistor....................................................................... 13 4 Transistor (BJT) as an electronic element ...............................................................16 4.1 Measuring (testing) transistors.......................................................................... 25 5 FET transistors..........................................................................................................27 6 Transistor construction ........................................................................................... 28 7 Basic circuits ............................................................................................................ 29 7.1 Common emitter amplifier.................................................................................29 7.2 Current source and current mirror ....................................................................36 7.3 Astable multivibrator .........................................................................................38 7.4 Differential amplifier.......................................................................................... 41 8 Sensing application.................................................................................................. 44 8.1 Phototransistor...................................................................................................44 9 PE Questions............................................................................................................ 45 Basic electronic elements – transistor 2 1 Introduction We have previously introduced the model that conceptualizes the focus we have in ST: In these parts of the lectures, we focus on the hardware – electronics part of our sensor- based interaction input system: The understanding of this part of the process requires in essence two things: understanding of the conversion from a given physical parameter into an electric parameter – which is the sensing process: process of electric measurement that the sensor performs; and understanding concepts in electrical circuits which are used to perform signal conditioning. Both of these require understanding of electrical properties of matter. Basic electronic elements – transistor 3 So far, we have discussed circuit theory analysis of resistive circuits - and among them, sensor resistive circuits as well. We have also looked at the capacitor as a non-ohmic element, its influence in an electric circuit and the possibilities to use it in a sensor context. We have also looked at the diode (PN junction) as a semiconductor element. We now continue introducing basic electronic elements - this lecture is concerned with discussing the transistor. The transistor too is a semiconductor element. Special types of transistors also can be applied as sensor devices - these will be briefly introduced as well. Basic electronic elements – transistor 4 2 Transistor Like the diode, the transistor is a semiconductor device as well - however, it commonly has three terminals, so it is a bit more complex to understand. The main purpose of a transistor is to control large current flow through two of those terminals, by smaller current/voltage applied on the third terminal. At this level, we should understand that there are two basic types of transistors, which differ in construction and usage (but not in the main purpose) - these are bipolar junction transistors (BJT) and field effect transistor (FET). “Transistors are divided into two main categories: bipolar junction transistors (BJTs) and field effect transistors (FETs). Transistors have three terminals where, in simplified terms, the application of voltage to the input terminal increases the conductivity between the other two terminals and hence controls current flow through those terminals. The physics of this 'transistor action' are quite different between the BJT and FET… In analog circuits, transistors are used in amplifiers, audio amplifiers, radio frequency amplifiers, regulated power supplies, and in computer PSUs, especially in switching power supplies. Transistors are also used in digital circuits where they function similarly to electrical switches. Digital circuits include logic gates, RAM (random access memory) and microprocessors.[1]” Note however, that the division between BJT and FET is not the only categorisation of transistors - “Transistors are categorized by: • Semiconductor material: germanium, silicon, gallium arsenide, silicon carbide • Type: BJT, JFET, IGFET (MOSFET), IGBT, "other types" • Polarity: NPN, PNP, N-channel, P-channel • Maximum power rating: low, medium, high • Maximum operating frequency: low, medium, high, radio frequency (RF), microwave • Application: switch, general purpose, audio, high voltage, super-beta, matched pair • Physical packaging: through hole metal, through hole plastic, surface mount, ball grid array Thus, a particular transistor may be described as: silicon, surface mount, BJT, NPN, low power, high frequency switch. [1]” In this lecture, we will mostly focus on describing the principle of work of a BJT transistor, provide a simplified circuit theory model of it, and discuss application of an Basic electronic elements – transistor 5 NPN BJT (as the calculations with it are easier) in several basic circuits. We will not go in further details of transistor design and analysis, which in itself is a rather complex area; we are only briefly going to mention FETs. 3 Bipolar junction transistor (BJT) The following excerpt gives a good introduction to BJTs: “The bipolar junction transistor (BJT) was the first type of transistor to be commercially mass-produced. Bipolar transistors are so named because conduction channel uses both majority and minority carriers for main electric current. The terminals are named emitter, base and collector. Two p-n junctions exist inside the BJT, collector-base junction and base-emitter junction. It is commonly described as a current operated device because the collector current is controlled by the current flowing between base and emitter terminals. [1]” Note that in a P semiconductor, majority carriers are holes, and minority carriers are free electrons; in a N semiconductor, majority carriers are free electrons, and minority carriers are holes. As an introduction, let’s include the following excerpt: “A BJT consists of three differently doped semiconductor regions, the emitter region, the base region and the collector region. These regions are, respectively, p type, n type and p type in a PNP transistor, and n type, p type and n type in a NPN transistor. Each semiconductor region is connected to a terminal, appropriately labeled: emitter (E), base (B) and collector (C). Figure 1. Simplified cross-section of an npn BJT (left), closeup of a transistor (right) (Ref. [2]) The base is physically located between the emitter and the collector and is made from lightly doped, high resistivity material. The collector surrounds the emitter region, making it almost impossible for the electrons injected into the base region to escape Basic electronic elements – transistor 6 being collected, thus making the resulting value of a very close to unity, and so, giving the transistor a large ß. The bipolar junction transistor, unlike other transistors, is not a symmetrical device. This means that the interchange of the collector and the emitter makes the transistor leave the forward active mode, starting to operate in the reverse mode. Small changes in the voltage applied across the base-emitter terminals causes the current that flows between the emitter and the collector to change significantly. This effect can be used to amplify the input current. BJTs can be thought of as voltage- controlled current sources, but are usually characterized as current amplifiers due to the low impedance at the base. [2]” As a discussion of the principle of operation of a BJT, the following long excerpt is included: " A Bipolar Transistor essentially consists of a pair of PN Junction Diodes that are joined back-to- back. This forms a sort of a sandwich where one kind of semiconductor is placed in between two others. There are therefore two kinds of Bipolar sandwich, the NPN and PNP varieties. The three layers of the sandwich are conventionally called the Collector, Base, and Emitter. The reasons for these names will become clear later once we see how the transistor works. Figure 2.Illustration of BJT transistor as two PN junctions (Ref. [6]) Some of the basic properties exhibited by a Bipolar Transistor are immediately recognisable as being diode-like. However, when the 'filling' of the sandwich is fairly thin some interesting effects become possible that allow us to use the Transistor as an amplifier or a switch. Basic electronic elements – transistor 7 To see how the Bipolar Transistor works we can concentrate on the NPN variety. Figure 3 shows the energy levels in an NPN transistor when we aren't externally applying any voltages. We can see that the arrangement
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