EE 332 Lecture 12 2019.pdf - EE 332 Devices and Circuits II Lecture#12 ©UWECE TC Chen NONIDEAL BJT BEHAVIOR DIFFUSION CAPACITANCE Problem Find the

EE 332 Lecture 12 2019.pdf - EE 332 Devices and Circuits II...

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4/19/2019 1 ©UWECE TC Chen EE 332: Devices and Circuits II Lecture #12 ©UWECE TC Chen NONIDEAL BJT BEHAVIOR: DIFFUSION CAPACITANCE Problem : Find the diffusion capacitance Given : 𝐼 ? = 2.0 mA; 𝜏 ? = 2 × 10 −10 s ? ? = 𝐼 ? 𝑉 ? 𝜏 ? = 2 × 10 −3 0.025 2 × 10 −10 = 16 pF ANSWER
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4/19/2019 2 ©UWECE TC Chen EARLY EFFECT In a practical BJT, output characteristics have a positive slope in forward- active region; collector current is not independent of v CE . Early effect: When output characteristics are extrapolated back to point of zero i C , curves intersect (approximately) at a common point v CE = - V A which lies between 15 V and 150 V. (V A is named the Early voltage) Converge to a point ©UWECE TC Chen EARLY EFFECT while v CE increases, v BC decreases, the reverse-bias across collector- base junction increases, width of the collector-base depletion layer increases and width of the base decreases (base-width modulation). W b decreases Diffusion current also increases since concentration gradient increases E B C W BE W b W BC v CE v BC i C i B
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