P-N-I-P AND X-P-I-N JUNCTION TRANSISTOR TRIODES 519 



ac collector-to-emitter short circuit effect called "electrical punch- 

 through." Lower limits of junction areas are set by desired operating 

 currents and by mechanical reasons. Diminishing returns are reached 

 for structiu'es a few mils in diameter and a fraction of a mil thick. 



To facilitate comparisons, the limitations described qualitatively 

 above have been interpreted quantitatively in terms of a gain-band 

 figure of merit,* 



in which To is low frequency available power gain in the common emitter 

 connection! and B is the frequency at which the gain is 3 db down from 

 its low frequency value. A reasonable upper limit on this (power gain) X 

 (bandwidth-squared) product is 4 X 10^^, which indicates that a 0-10 

 mcps video gain of 26 db may be obtained by improvement of conven- 

 tional triode structures. 



The same figure of merit, for a p-n-i-p of equal junction area, is ap- 

 proximately 10^ ^ Calculation shows that units may be designed to 

 produce 10 db or more gain at 1,000 mcps. Although many of its operat- 

 ing principles are similar to those of the p-n-p and the n-p-n, the p-n-i-p 

 differs from the earher triodes in that low collector capacitance is ob- 

 tained by means of a thick collector depletion (space-charge) layer of 

 intrinsic semi-conductor. The section view of a p-n-i-p in Fig. 1 illus- 

 trates its major features. The vade depletion layer (electric field region) 

 produces small collector capacitance (Cc) and gives a high reverse break- 

 down voltage, while the very thin base region of low resistivity gives 

 simultaneously a low ohmic base resistance (Vb) and a very high alpha 

 cutoff frequency (/a). The design with four regions, emitter, base, de- 

 pletion layer, and collector, increases the (powder gain) X (band-squared) 

 figure of merit (/a/25r6'Cc) about two decades, thus increasing the useful 

 frequency range about one decade. 



The thick collector depletion layer of intrinsic or near-intrinsic semi- 

 conductor provides advantages in addition to the reduction of the col- 

 lector capacitance. Because base layer resistivity does not limit collector 

 breakdown voltage as it does in previous structures, much lower base 

 resistivities may be used, thus producing lower ohmic base resistances. 

 Furthermore, the thick depletion region makes the structure much more 

 rugged for very high alpha cutoff units since the very thin base layer is 



* This figure of merit is essentially identical with one described bv R. L. Pri- 

 chard at the A.I.E.E. Winter Meeting in New York City, Jan. 22, 1954. 



t It is assumed that the input terminals of the transistor are shunted bj' an 

 external resistance which determines the input impedance and therefore the band- 

 width. Power gain decreases approximately 6 db per octave at frequencies greater 

 than B. 



