. The Bell System technical journal . (C) IMBEDDED EMITTER - DIFFUSED SURFACE LAYERFig. 4 — Low-base resistance structures. r-X-I-1 AND X-P-I-\ JUNCTION TRANSISTOR TKIODES 525 These advantages are, however, balanced in part ])y an increase in the(Miiitter depletion rojiion capacitance associated with the low resistivitybase material. DESICX TIIKORY General The piiiicipal objectives in the initial i)-n-i-p desif>;ii liave been highalpha cntoff lre(inency, low collector capacitance, and low ohmic baseresistance. The eqnivalent circnit employed is shown in Fig. 5. Theoutput and feedback admitt
. The Bell System technical journal . (C) IMBEDDED EMITTER - DIFFUSED SURFACE LAYERFig. 4 — Low-base resistance structures. r-X-I-1 AND X-P-I-\ JUNCTION TRANSISTOR TKIODES 525 These advantages are, however, balanced in part ])y an increase in the(Miiitter depletion rojiion capacitance associated with the low resistivitybase material. DESICX TIIKORY General The piiiicipal objectives in the initial i)-n-i-p desif>;ii liave been highalpha cntoff lre(inency, low collector capacitance, and low ohmic baseresistance. The eqnivalent circnit employed is shown in Fig. 5. Theoutput and feedback admittances which are important in earlier jnnc-. Ypo — qlekT inh 2 Fig. 5 — Equivalent circuit of the p-u-i-p transistor. tion triodes are omitted, since the space charge layer widening factor (Hi or Hec, ■— —F7-) is very small. ^Fhe transfer admittance is shownqw dVc as a current generator (aie) with cutoff fre(|uency (| a | ^3 db down) of/a because this gives explicit recognition to base region diffusion transit time Tb and allows it to be combined with space charge layer transit time Tc . Km)tier Region Design Emitter region acceptor concentration should be very laige (10 — 10atoms/cc) in order to keep the injection ratio y close to unity at bothlow and high frequencies.^ At low frequencies, y is determined by emitterresistivity and carrier life path or diffusion length, base resistivity and width, as 1 7 = To = 1 + (TbW OeLne 526 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 At high frequencies, 7 is determined by the ratio of acceptor densityin the emitter to donor density in the base as 1 7hf = 1 4- ^ /I Obviously, s
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