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 behaviour 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 coloured 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 transisor. 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 behaviour of all Bipolars is essentially the same as described in these pages.
Content and pages maintained by: Jim Lesurf (firstname.lastname@example.org)
using HTMLEdit3 on a StrongARM powered RISCOS machine.
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.