Basic Of Electronics Engineering (Bipolar Junction Transistors)

Bipolar Junction Transistors (BJT)

Introduction
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

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. To see how the Bipolar Transistor works we can concentrate on the NPN variety

Figure 1 shows the energy levels in an NPN transistor when we aren't externally applying any voltages. We can see that the arrangement looks like a back-to-back pair of PN Diode junctions with a thin P-type filling between two N-type slices of 'bread'. In each of the N-type layers conduction can take place by the free movement of electrons in the conduction band. In the P-type (filling) layer conduction can take place by the movement of the free holes in the valence band. However, in the absence of any expernally applied electric field, we find that depletion zones form at both PN-Junctions, so no charge wants to move from one layer to another

Consider now what happens when we apply a moderate voltage between the Collector and Base parts of the transistor. The polarity of the applied voltage is chosen to increase the force pulling the N-type electrons and P-type holes apart. (i.e. we make the Collector positive with respect to the Base.) This widens the depletion zone between the Collector and base and so no current will flow. In effect we have reverse-biassed the Base-Collector diode junction. The precise value of the Base-Collector voltage we choose doesn't really matter to what happens provided we don't make it too big and blow up the transistor! So for the sake of example we can imagine applying a 10 Volt Base-Collector voltage

Now consider what happens when we apply a relatively small Emitter-Base voltage whose polarity is designed to forward-bias the Emitter-Base junction. This 'pushes' electrons from the Emitter into the Base region and sets up a current flow across the Emitter-Base boundary. Once the electrons have managed to get into the Base region they can respond to the attractive force from the positively-biassed Collector region. As a result the electrons which get into the Base move swiftly towards the Collector and cross into the Collector region. Hence we see a Emitter-Collector current whose magnitude is set by the chosen Emitter-Base voltage we have applied. To maintain the flow through the transistor we have to keep on putting 'fresh' electrons into the emitter and removing the new arrivals from the Collector. Hence we see an external current flowing in the circuit.The precise value of the chosen Emitter-Base voltage isn't important to our argument here, but it does determine the amount of current we'll see. For the sake of example we've chosen a half a volt. Since the Emitter-Base junction is a PN diode we can expect to see a current when we apply forward voltages of this sort of size. In practice with a Bipolar transistor made using Silicon we can expect to have to use an Emitter-Base voltage in the range from around a half volt up to almost one volt. Higher voltages tend to produce so much current that they can destroy the transistor!It is worth noting that the magnitude of the current we see isn't really affected by the chosen Base-Collector voltage. This is because the current is mainly set by how easy it is for electrons to get from the Emitter into the Base region. Most (but not all!) the electrons that get into the Base move straight on into the Collector provided the Collector voltage is positive enough to draw them out of the Base region. That said, some of the electrons get 'lost' on the way across the Base. This process is illustrated in Figure 4

Some of the free electrons crossing the Base encounter a hole and 'drop into it'. As a result, the Base region loses one of its positive charges (holes) each time this happens. If we didn't do anything about this we'd find that the Base potential would become more negative (i.e. 'less positive' becuase of the removal of the holes) until it was negative enough to repel any more electrons from crossing the Emitter-Base junction. The current flow would then stop.To prevent this happening we use the applied Emitter-Base voltage to remove the captured electrons from the Base and maintain the number of holes it contains. This have the overall effect that we see some of the electrons which enter the transistor via the Emitter emerging again from the Base rather than the Collector. For most practical Bipolar Transistors only about 1% of the free electrons which try to cross Base region get caught in this way. Hence we see a Base Current, IB, which is typically around one hundred times smaller than the Emitter Current, IE
Bipolar transistors, having 2 junctions, are 3 terminal semiconductor devices. The three terminals are emitter, collector, and base. A transistor can be either NPN or PNP. See the schematic representations below

Note that the direction of the emitter arrow defines the type transistor. Biasing and power supply polarity are positive for NPN and negative for PNP transistors. The transistor is primarily used as an current amplifier. When a small current signal is applied to the base terminal, it is amplified in the collector circuit. This current amplification is referred to as HFE or beta and equals Ic/Ib.
As with all semiconductors, breakdown voltage is a design limitation. There are breakdown voltages that must be taken into account for each combination of terminals. i.e. Vce, Vbe,and Vcb. However, Vce(collector-emitter voltage) with open base, designated as Vceo, is usually of most concern and defines the maximum circuit voltage.
Also as with all semiconductors there are undesireable leakage currents, notably Icbo ,collector junction leakage; and Iebo, emitter junction leakage. A typical collector characteristic curve is shown below:

Their work led them first to the point-contact transistor and then to the bipolar junction transistor. Since then, the technology has progressed rapidly. The development of a planar process yielded the first circuits on a chip and for a decade, bipolar transistor operational amplifiers and digital TTL circuits were the workhorses of any circuit designer.
The spectacular rise of the MOSFET market share during the last decade has completely removed the bipolar transistor from center stage. Almost all logic circuits, microprocessor and memory chips contain exclusively MOSFETs.
Nevertheless, bipolar transistors remain important devices for ultra-high-speed discrete logic circuits such as emitter coupled logic (ECL), power-switching applications and in microwave power amplifiers.
In this chapter we first present the structure of the bipolar transistor and show how a three-layer structure with alternating n-type and p-type regions can provide current and voltage amplification. We then present the ideal transistor model and derive an expression for the current gain in the forward active mode of operation. Next, we discuss the non-ideal effects, the modulation of the base width and recombination in the depletion region of the base-emitter junction.
Structure and principle of operation
A bipolar junction transistor consists of two back-to-back p-n junctions, who share a thin common region with width, wB. Contacts are made to all three regions, the two outer regions called the emitter and collector and the middle region called the base. The structure of an NPN bipolar transistor is shown in Figure 1 (a). The device is called "bipolar" since its operation involves both types of mobile carriers, electrons and holes.