Bipolar Junction Transistors

 


 

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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:

9b.JPG (10909 bytes)

     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:

9A.jpg (34925 bytes)

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.

Figure 1.: (a) Structure and sign convention of a NPN bipolar junction transistor. (b) Electron and hole flow under forward active bias, VBE > 0 and VBC = 0.
Since the device consists of two back-to-back diodes, there are depletion regions between the quasi-neutral regions w.
The sign convention of the currents and voltage is indicated on Fig 1(a). The base and collector current are positive if a positive current goes into the base or collector contact. The emitter current is positive for a current coming out of the emitter contact. This also implies the emitter current, IE, equals the sum of the base current, IB, and the collector current, IC:
(0)
The base-emitter voltage and the base-collector voltage are positive if a positive voltage is applied to the base contact relative to the emitter and collector respectively.
The operation of the device is illustrated with Fig 1 (b). We consider here only the forward active bias mode of operation, obtained by forward biasing the base-emitter junction and reverse biasing the base-collector junction. To simplify the discussion further, we also set VCE = 0. The corresponding energy band diagram is shown in Fig 2. Electrons diffuse from the emitter into the base and holes diffuse from the base into the emitter. This carrier diffusion is identical to that in a p-n junction. However, what is different is that the electrons can diffuse as minority carriers through the quasi-neutral region in the base. Once the electrons arrive at the base-collector depletion region, they are swept through the depletion layer due to the electric field. These electrons contribute to the collector current. In addition, there are two more currents, the base recombination current, indicated on Fig 2 by the vertical arrow, and the base-emitter depletion layer recombination current (not shown).

Figure 2. : Energy band diagram of a bipolar transistor biased in the forward active mode.
The total emitter current is the sum of the electron diffusion current, IE,n, the hole diffusion current, IE,p and the base-emitter depletion layer recombination current, Ir,d.
(1)
The total collector current is the electron diffusion current, IE,n, minus the base recombination current, Ir,B.
(2)
The base current is the sum of the hole diffusion current, IE,p, the base recombination current, Ir,B and the base-emitter depletion layer recombination current, Ir,d.
(3)
The transport factor, a, is defined as the ratio of the collector and emitter current:
(4)
Using Kirchoff's current law and the sign convention shown in Figure 1(a), we find that the base current equals the difference between the emitter and collector current. The current gain, b, is defined as the ratio of the collector and base current and equals:
(5)
This explains how a bipolar junction transistor can provide current amplification. If the collector current is almost equal to the emitter current, the transport factor, a, approaches one. The current gain, b, can therefore become much larger than one.
To facilitate further analysis, we now rewrite the transport factor, a, as the product of the emitter efficiency, gE, the base transport factor, aT, and the depletion layer recombination factor, dr.
(6)
The emitter efficiency, gE, is defined as the ratio of the electron current in the emitter, IE,n, to the sum of the electron and hole current diffusing across the base-emitter junction, IE,n + IE,p.
(7)
The base transport factor, aT, equals the ratio of the current due to electrons injected in the collector, to the current due to electrons injected in the base.
(8)
Recombination in the depletion-region of the base-emitter junction further reduces the current gain, as it increases the emitter current without increasing the collector current. The depletion layer recombination factor, dr, equals the ratio of the current due to electron and hole diffusion across the base-emitter junction to the total emitter current:
The forward active mode is obtained by forward-biasing the base-emitter junction. In addition we eliminate the base-collector junction current by setting VBC = 0. The minority-carrier distribution in the quasi-neutral regions of the bipolar transistor, as shown in Figure 3, is used to analyze this situation in more detail.

Figure 3. : Minority-carrier distribution in the quasi-neutral regions of a bipolar transistor (a) Forward active bias mode. (b) Saturation mode.
The values of the minority carrier densities at the edges of the depletion regions are indicated on the Fig 3. The carrier densities vary linearly between the boundary values as expected when using the assumption that no significant recombination takes place in the quasi-neutral regions. The minority carrier densities on both sides of the base-collector depletion region equal the thermal equilibrium values since VBC was set to zero. While this boundary condition is mathematically equivalent to that of an ideal contact, there is an important difference. The minority carriers arriving at x = wB - xp,C do not recombine. Instead, they drift through the base-collector depletion region and end up as majority carriers in the collector region.
The emitter current due to electrons and holes are obtained using the "short" diode expressions yielding:
(11)
and
(12)
It is convenient to rewrite the emitter current due to electrons, IE,n, as a function of the total excess minority charge in the base, DQn,B. This charge is proportional to the triangular area in the quasi-neutral base as shown in Fig 3 a) and is calculated from:
(13)
which for a "short" diode becomes:
(14)
And the emitter current due to electrons, IE,n, simplifies to:
(15)
where tr is the average time the minority carriers spend in the base layer, i.e. the transit time. The emitter current therefore equals the excess minority carrier charge present in the base region, divided by the time this charge spends in the base.
A combination of equations (11), (14) and (15) yields the transit time as a function of the quasi-neutral layer width, wB', and the electron diffusion constant in the base, Dn,B.
(16)
We now turn our attention to the recombination current in the quasi-neutral base and obtain it from the continuity equation:
(17)
In steady state and applied to the quasi-neutral region in the base, the continuity equation yields the base recombination current, Ir,B:
(18)
which in turn can be written as a function of the excess minority carrier charge, DQn,B, using equation (13).
(19)
The long minority-carrier lifetime and the long diffusion lengths in those materials justify the exclusion of recombination in the base or the depletion layer. The resulting current gain, under such conditions, is:
(21)
From this equation, we conclude that the current gain can be larger than one if the emitter doping is much larger than the base doping. A typical current gain for a silicon bipolar transistor is 50 - 150.
The base transport factor, as defined in equation (18), equals:
(22)
This expression is only valid if the base transport factor is very close to one, since it was derived using the "short-diode" carrier distribution. This base transport factor can also be expressed in function of the diffusion length in the base:
(23)
As the voltages applied to the base-emitter and base-collector junctions are changed, the depletion layer widths and the quasi-neutral regions vary as well. This causes the collector current to vary with the collector-emitter voltage as illustrated in Figure 4.

Figure 4. : Variation of the minority-carrier distribution in the base quasi-neutral region due to a variation of the base-collector voltage.
A variation of the base-collector voltage results in a variation of the quasi-neutral width in the base. The gradient of the minority-carrier density in the base therefore changes, yielding an increased collector current as the collector-base current is increased. This effect is referred to as the Early effect. The Early effect is observed as an increase in the collector current with increasing collector-emitter voltage as illustrated with Figure 5. The Early voltage, VA, is obtained by drawing a line tangential to the transistor I-V characteristic at the point of interest. The Early voltage equals the horizontal distance between the point chosen on the I-V characteristics and the intersection between the tangential line and the horizontal axis. It is indicated on the figure by the horizontal arrow.

Figure 5. : Collector current increase with an increase of the collector-emitter voltage due to the Early effect. The Early voltage, VA, is also indicated on the figure.
 
Now, to the heart of the matter!
We have an operating curve consisting of a fairly linear segment bounded by two nonlinear ends: cutoff and saturation. 

Operating in the Middle
The transistor will operate very nicely if one could insure that no input voltage, i.e., signal voltage--would cause the collector current to ever operate beyond either end of the linear portion of the operating curve. 

To further beat a point into the ground: if one increased the input signal beyond this level, the output signal would now start to "clip" and cause distortion (sine wave gets flat on top and bottom). If the bias point were set either too low or too high, then the sine wave would start to clip on the top before the bottom, or visa versa (asymmetric clipping). 

Effects of different bias settings 

Effects of different bias settings 

   
When (negative) feedback is introduced, most of these problems diminish or disappear, resulting in improved performance and reliability. There are several ways to introduce feedback to this simple amplifier, the easiest and most reliable of which is accomplished by introducing a small value resistor in the emitter circuit. The amount of feedback is dependent on the relative signal level dropped across this resistor, e.g., if the resistor value approached that of the collector load resistor, the gain would approach unity (Gv ~ 1). 

From the explanation of how a Bipolar Transistor works, we can expect the main characteristic of a Bipolar Transistor to be its Current Gain value. In practice this value isn't a 'universal constant' but depends on various factors: e.g. the transistor's temperature, the size and shape of its Base region, the way it's various parts were doped to make them into semiconductors, etc.

The above illustration shows how, for a 'typical' transistor, the Current Gain varies with the Collector Current level, IC. from this graph we can see that the proportion of electrons 'caught' by a hole whilst trying to cross the Base region does vary a bit depending on the current level. Note that the graph doesn't show the transistor's beta value, it shows a related figure called the transistor's Small Signal current gain, hfe. This is similar to the beta value, but is defined in terms of small changes in the current levels. This parameter is more useful than the beta value when considering the transistor's use in signal amplifiers where we're interested in how the device responds to changes in the applied voltages and currents.

 The second way we can characterise the behavior of a Bipolar Transistor is by relating the Base-Emitter voltage, VBE, we apply to the Base current, IB, it produces. As can expect from the diode-like nature of the Base-Emitter junction this voltage/current characteristic curve has an exponential-like shape similar to that of a normal PN Junction diode.

As with the previous curve, the graph shown here should only be regarded as a 'typical' example as the precise result will vary a bit from device to device and with the temperature, etc.

In most practical situations we can expect the Collector current to be set almost entirely by the chosen Base-Emitter voltage. However, this is only true when the the Base-Collector voltage we are applying is 'big enough' to quickly draw over to the Collector any free electrons which enter the Base region from the Emitter.

The above plot of characteristic curves gives a more complete picture of what we can expect from a working Bipolar Transistor. Each curve shows how the colletor current, IC, varies with the Collector-Emitter voltage, VCE, for a specific fixed value of the Base current, IB. This kind of characteristic curve 'family' is one of the most useful ones when it comes to building amplifiers, etc, using Bipolar Transistors as it contains quite a lot of detailed information.

When the applied VCE level is 'large enough' (typically above two or three volts, shown as the region in blue) the Collector is able to to remove free electrons from the Base almost as quickly as they Emitter injects them. Hence we get a current which is set by the Base-Emitter voltage and see a current gain value which doesn't alter very much if we change either the base current or the applied Collector potential.

However, when we reduce the Collector potential so that VCE is less than a couple of volts, we find that it is no longer able to efficiently remove electrons from the Base. This produces a sort of partial 'roadblock' effect where free electrons tend to hang about in the Base region. (cream colored region) These makes the Base region seem 'more negative' to any electrons in the Emitter and tends to reduce the overall flow of current through the device. As we lower the Collector potential to become almost the same as that of the Base and Emitter it eventually stops drawing any electrons out of the device and the Collector current falls towards zero.

The precise voltage at which the Collector ceases to be an effective 'collector of electrons' depends on the temperature and the manufacturing details of the transistor. In general we can expect most Bipolar Transistors to work efficiently provided that we arrange for a VCE value of at least two or three volts - and preferrably five volts or more. Such a device can be used as an effective amplifier. Lower voltages may prevent it from working correctly.

Note that the graphs shown on this page are only meant as a general guide. Some transistors can work with much higher currents, or have much higher current gains, etc. However, the general pattern of behavior of all Bipolars is essentially the same as described in these pages.


Unijunction Transistor

  The unijunction transistor (UJT) is a three terminal device with characteristics very different from the conventional 2 junction, bipolar transistor. It is a pulse generator with the trigger or control signal applied at the emitter . This trigger voltage is a fraction (n) of interbase voltage, Vbb.The UJT circuit symbol, junction schematic, and characteristic curve are shown below.

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     The emitter terminal does not inject current into the base region until its voltage reaches Vp. Once Vp is reached the base circuit conducts and a positive pulse appears at the B1 terminal and a negative pulse at B2. The UJT incorporates a negative resistance region, a low emitter current, and a high output pulse current at terminals B1 and B2, making it an ideal pulse trigger. A simple RC timer circuit using a UJT is shown below.

10B.jpg (14636 bytes)

 

 
 

04.18.2002

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© 2002 =Functional combinations in solid states= h.dr. V.Gavryushin, h.dr. A.Žukauskas