518 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



prescribed by structure and material. Further extension of frequency 

 range and, to a lesser degree, of power capability must be sought in 

 new materials or in improved structures. The p-n-i-p* transistor em- 

 ploys a new structure which in theory promises to increase the useful 

 frequency range of junction triodes by a factor of at least ten. In the 

 p-n-i-p, the n region of the base and the p region of the collector are 

 separated by a relatively thick region of i-type (i.e., intrinsic or near-in- 

 trinsic, almost free of donor and acceptor centers) semi-conductor. This 

 permits establishment of a thick collector depletion layer at relatively 

 low voltages, thus producing low collector capacitance and several other 

 desirable features. 



The advantages of the new structure may be seen by study of the 

 limitations of previous triode structures. In general, high frequency 

 performance of conventional units, such as p-n-p alloy transistors, is 

 improved by making the base region thinner to increase the alpha cutoff 

 frequency (/„), by using lower resistivity base material to reduce the 

 ohmic base resistance (n'), and by decreasing the area of emitter and 

 collector junctions to reduce the collector capacitance (Cc). These equiva- 

 lent circuit parameters are of nearly equal importance as may be seen 

 from the gain-bandwidth expression discussed below. 



The design changes required to improve the parameters involve con- 

 flicts, and compromises are necessary. For example, the decrease of base 

 thickness which increases alpha cutoff frequency also increases (less 

 rapidly) the ohmic base resistance. f The decrease in base resistivity 

 which reduces base resistance also increases (again, less rapidly) the 

 collector capacitance and decreases the collector breakdowTi voltage, 

 thus decreasing power capacity. The reduction of junction area which 

 decreases collector capacitance reduces the current rating and thereby 

 the possible power rating. For transistors having circular electrodes, it 

 may also increase the ohmic base resistance. 



For these reasons, conventional junction triodes designed for high 

 frequency application tend to be very small and to have very low voltage, 

 current, and power ratings. Ultimately, the decrease of collector reverse 

 breakdown voltage sets a lower limit to usable base resistivity and 

 thereby to the thickness of the collector depletion region. This sets a 

 lower limit on base region thickness, since average base layer thickness 

 should be two or more times depletion layer thickness. For base layers 

 thinner than this, irregularities in thickness or in impurity distribution 

 may permit the depletion layer to contact the emitter, producing the 



* And its homologue, the n-p-i-n. 



t In the junction tetrode, this increase of base resistance is overcome by crowd- 

 ing the minority carrier emission close to one of the base contacts, thus producing 

 low ohmic base resistance. See Reference 2. 



