. The Bell System technical journal . he junctiontransistor. Fig. 2(a) shows an impuiity density profile for a p-n-i-palong an axial line running through emitter, base, collector space-chargelayer, and collector. Similar profiles for step junction (alloy) and gradedjunction (grown crystal) p-n-ps are sho^^^l in Figs. 2(b) and (c). The emitter and collector regions of the p-n-i-p have very high im-l)urity concentrations (low resistivities), while the impurity density inthe l)ase is moderately high and the depletion layer is almost free ofimpurities. The high acceptor density in the emitter forc
. The Bell System technical journal . he junctiontransistor. Fig. 2(a) shows an impuiity density profile for a p-n-i-palong an axial line running through emitter, base, collector space-chargelayer, and collector. Similar profiles for step junction (alloy) and gradedjunction (grown crystal) p-n-ps are sho^^^l in Figs. 2(b) and (c). The emitter and collector regions of the p-n-i-p have very high im-l)urity concentrations (low resistivities), while the impurity density inthe l)ase is moderately high and the depletion layer is almost free ofimpurities. The high acceptor density in the emitter forces most of theemitter current to flow as holes, giving an injection ratio (7) close tounity. The high densitj^ in the collector gives a low^ collector body re-sistance and fixes the position of one face of the collector depletion layer. a 1 W ^ Xm -> P 0 — INTRINSIC < 3 X 103 D ,06_|o17 EMITTER B AS E DEPLETION LAYER COLLECTOR (a) STEP-BASE p-n-L-p lOS-109 108-io5 EMITTER COLLECTOR (b) STEP (alloy) p-n-p ,10-108. COLLECTOR (c) GROWN (graded) p-n-p Fig. 2 — Impurity } profiles. 522 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 The high donor concentration in the base region leads to low ohmicbase resistance (r^) and fixes the position of the base face of the deple-tion layer. In the depletion layer, the concentration of impurities is solow that the field region (space-charge layer) extends from the n-typebase to the p-type collector at low voltages. Depletion Layer The properties of the depletion layer which are important at highfrequencies are the capacitance across it (Ce) and the carrier transittime through it {tc). These are determined primarily by the impuritydensity, the thickness of the region, and the base-to-collector and field distributions in the depletion layer for both small and FIELD DISTRIBUTION POTENTIAL DISTRIBUTION DEPLETION REGION (a) Nd=Na
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