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N_BJT_Sedra_N9 - Bipolar Junction Transistors(BJTs(09 1...

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1 Bipolar Junction Transistors (BJTs) (09)
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2 BIPOLAR JUNCTION TRANSISTORS (BJT’s) § 5.2 Operation of the npn Transistor in Active Mode 1. Let us look at the physical operation of the transistor in ‘Active Mode’. Two external voltage sources, V BE and V CB , are used, to establish the required bias conditions, for ‘Active Mode’ operation. Figure 5.3 Current flow in an npn transistor biased to operate in the active mode. ( Reverse current components due to drift of thermally generated minority carriers are not shown .)
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3 BIPOLAR JUNCTION TRANSISTORS (BJT’s) § 5.2 Operation of the npn Transistor in Active Mode (continue) 1. The voltage V BE sets the base (p-type) to be higher in potential than the emitter (n-type) ----- forward biasing the EB junction, (EBJ). The voltage V CB causes (n-type) collector to be at a higher potential than the base (p-type) ------ reverse biasing CB junction, (CBJ). Current Flow 1. Only the diffusion current components are considered in our description of current flow, since it is pre-dominant. Usually, very small minority-carrier-current has been neglected , for the time-being. 2. We know the diffusion current consists of the majority carriers. Thus it has two components; current made up of ‘Electrons’ and the current component made up of ‘Holes’.
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4 BIPOLAR JUNCTION TRANSISTORS (BJT’s) 1. The design of the base of the transistor is such that it is lightly doped and has lesser width (of the base region). Therefore, majority carriers relevant to the base region are designed to be lesser than the majority carriers of emitter region (heavily doped). 2. Coming back to our diagram and current flow, forward bias on the emitter base junction causes the current to flow across this junction (EBJ). This current will have two components: a) Electrons injected from emitter into base b) Holes injected from the base into the emitter. Emitter (E) Collector (C) Base (B)
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5 BIPOLAR JUNCTION TRANSISTORS (BJT’s) 1. It is highly desirable that the current component made up of electrons (majority carrier of the emitter) from emitter to base is at a much higher level than the second component (Holes from the base to emitter) ---- this is accomplished by fabricating the device with heavily doped emitter and a lightly doped base. 2. The current flowing across EBJ is shown as i E in the diagram. The direction of i E is out of the emitter lead. --------- in the direction of ‘Hole’ current and opposite to the direction of ‘Electron’ current. i E is the sum of ‘Electron’ current and the ‘Hole’ current; Electron current is much larger than the Hole current ------- (for npn transistor) Emitter (E) Collector (C) Base (B)
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6 BIPOLAR JUNCTION TRANSISTORS (BJT’s) 1. The base is usually very thin, the electrons injected from the emitter into the base will be minority carriers when in the p-type base region.
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