Transient Behaviour of the Transistor

[Music] in a pnp transistor with no external voltage applied the concentration graphs of majority and minority carriers in the neutral parts of the p and n regions are horizontal no current flows in the external circuit the forward voltage applied to the emitter base junction together with the reverse voltage applied to the collector-based junction cause a large current to flow in the collector circuit and a very small current to flow in the base circuit we will now consider the concentration curves of the minority carriers plotted in a linear graph with no voltages applied the concentration curves are horizontal but when voltages are applied to both the emitter base junction and the collector base junction the concentration curve in the base region will be at an angle to the horizontal axis the base current or input current is proportional to the difference between the surface area of the indicated triangle and that of the original rectangle this difference is in fact a measure of the number of additional recombinations per second the collector current or output current is a function of the angle formed by the concentration curve of holes in the base if in addition to the forward bias we apply a stepped voltage pulse to the emitter base circuit we can examine the variations in the collector current as a function of time at this stage the collector current has a certain low value but when the pulse is applied the collector current reaches a higher value what happens to the collector current when this pulse is applied this sharp rise occurring at the leading edge of the applied voltage pulse causes the emitter base barrier region to become narrower simultaneously the number of holes in the base near the barrier increases consequently the whole current flowing from the emitter into the base will be temporarily higher causing the concentration curve to straighten again the variations in the angle of this concentration curve near the collector base junction are directly related to the variations in the collector current the time taken for this current to attain its new constant value is called the rise time t tr when the pulse is cut off the current decreases what happened at this trailing edge of the house the width of the emitter base barrier region increased and the concentration of the holes near the junction followed this change immediately after which the concentration curve straightened out again thus the changes in the angle of the concentration curve near the collector base junction are directly related to the variations of the collector current the time taken for this current to fall to its original value is called the fall time tf so far we have studied the effect resulting from the direct application of a stepped voltage pulse this technique known as voltage steering may however lead to very high input currents in practical circuits therefore this input current will always be limited by an input resistor because of this limitation on the value of the input current it is said that the transistor is current controlled consequently there will be a considerable increase in the times necessary to bring about a change in the concentration curves of holes and electrons in the base thus tr and tf will also increase up to now we have considered the time taken for small current variations to occur which in fact means that the transistor is acting as a pulse amplifier when a higher input voltage is applied the output current also becomes higher but the rise and fall times remain the same as before let us now examine the possibilities of using a transistor as a switching element with input and output resistors but first of all let us refer to a series of typical output characteristics with load line and working point again if we apply a small variable voltage the variations of the input current ib will follow exactly the variations of the input voltage but the variation in the output current ic will not be the same in this example the time taken for these current variations to occur will be rather long compared with the pulse duration if we now apply a much higher input current the collector current will be limited by the saturation value minus i cs so tr will now be much shorter this effect can also be explained from the concentration curves consider first that the transistor is working in the normal state as soon as the stepped voltage pulse is applied the transistor will continue to operate in the normal state and the collector current will increase as a function of the angle formed by the whole concentration curve but because this current flows through the output resistor the voltage drop over this resistor increases and consequently the voltage drop over the collector base junction decreases to such an extent that this junction becomes biased in the forward direction the transistor is then saturated and the collector current remains constant the whole concentration curve rises still further until the number of recombinations per second becomes equal to the base current which is of course the input current if however the transistor had not reached the saturation state the collector current would have attained a much higher value so in order to reduce the time taken for the current to become stable it is necessary that the transistor be brought rapidly into saturation in the same way we can expect that tf can be reduced by bringing the transistor rapidly into cut off to obtain an overall picture of the switching action we must start with the transistor biased at cut off and then apply a very high positive stepped voltage pulse the total input voltage vi shown as a function of time will now be of this form initially because the transistor is at cut off the collector current is zero both junctions are reverse biased and therefore the whole concentration curve is horizontal and of a very low value what happens then when this high stepped voltage pulse is applied the application of the pulse sets up an input current but the electrons entering the base do not disappear due to recombination because the concentration of holes in the base is still very low therefore the electrons in the base and the holes in the emitter which contribute to the input current are absorbed in creating a narrower emitter base barrier region this continues until such time as the barrier becomes biased in the forward direction after which the holes start to flow into the base the time taken for this space charge region to become neutralized is called the turn on delay time t d from then on the emitter base junction is biased in the forward direction and the concentration curve starts to rise and with it the collector current part of this current is consumed in reducing the width of the collector base barrier region this action continues until the collector base junction is biased in the forward direction the concentration curve continues to rise but the collector current remains at a fixed value now let us see what happens when the transistor is again switched from saturation to cut off the electrons are drawn out of the base and the surplus holes quickly disperse the level of the concentration curve drops to a point where the collector base junction again becomes biased in the reverse direction however as the angle has remained constant throughout the collector current has retained its original value the time taken during which the collector current remains constant after the pulse has been switched off is called the saturation time t s from then on the angle decreases until the cutoff situation is again reached simultaneously the collector current decreases towards zero summing up when the transistor is switched from the cut off state to the saturation state and vice versa four steps on the collector current curve can be distinguished one td which is the time needed to neutralize the emitter base junction two t r the time during which the transistor is operating in the normal state three t s the time during which the transistor remains in the saturation state and for tf the period during which the transistor passes through the reverse state to the cutoff state so to reduce tr it is sufficient to increase the saturation voltage but then simultaneously ts will increase because the amount of holes stored in the base has also increased in the same way to reduce tf the transistor must be brought rapidly to cut off but in this case because the emitter base barrier region has become wider the delay time td increases [Music] you