I 



PROPERTIES AND APPLICATIONS OF W-/)-W TRANSISTORS 557 



exactly the same for all electrons, the effect would be simply to delay the 

 output signal with respect to the input and there would be no effect on fre- 

 quency response. But there is a certain amount of dispersion in transit time 

 which means that the electrons corresponding to a particlar part of the 

 input signal wave do not all arrive simultaneously at the collector. When 

 this difference in time of arrival amounts to an appreciable part of a cycle 

 there is a tendency for some of the electrons to cancel the effect of others 

 so that the frequency response begins to fall off. As the signal frequency 

 increases beyond this point, the effect becomes more and more pronounced 

 and the response continues to fall with increasing frequency. 



In terms of the equivalent circuit, this dispersion in transit time means 

 that beyond a certain frequency, r^ (and hence a) begins to decrease with 

 increasing frequency and so the transistor may be said to have a certain 

 a-cutoff which we will call fca ■ 



Shockley has shown that fca is inversely proportional to the square of 

 the />-layer thickness and hence increases rapidly as the p layer is made 

 thinner. For n-p-n transistors now available, this cutoff should occur at fre- 

 quencies between five and twenty megacycles. 



Another limitation on frequency response comes about from the fact that, 

 at sufficiently high frequencies, the emitter junction fails to behave as a pure 

 resistance and is, in effect, shunted by a capacitance. In terms of the equiva- 

 lent circuit, this means that r^ is shunted by a capacitance. 



The effect which this has on frequency response can be reduced by re- 

 ducing the impedance of the source from which the emitter is driven. But 

 so far as the emitter junction is concerned, r^ is always in series with the 

 source impedance and so it is the value of ri, which ultimately determines 

 the emitter cutoff frequency. 



This capacitative reactance should begin to become appreciable with re- 

 spect to emitter resistance at a frequency which may be of the same order 

 as /ca . If Th is high, the emitter cutoff frequency /c<, will then be of the same 

 order of magnitude as fca and will increase as r^ is decreased. 



A third cause for limited frequency response is the capacitance of the 

 collector junction. The w-type germanium on one side of the junction be- 

 haves as one plate of a parallel-plate condenser and the />-type germanium 

 on the other side behaves as the other plate. Since the transition from n to p 

 type germanium may be made in an exceedingly small fraction of an inch, 

 the plates of the condenser are very closely spaced and the capacitance may 

 be appreciable. 



Collector capacitance also depends on collector voltage, decreasing with 

 increasing voltage. Theoretically, the capacitance should be in proportion 

 to the negative one-third power of Vc . 



