Ch8-Ch9-2slides

Ch8-Ch9-2slides - 2/22/2011 8 Bipolar Transistors Most...

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2/22/2011 1 8 Bipolar Transistors 1 Most LOGIC circuits in CMOS Bipolar = important for most Analog circuits Oscillators Timers Amplitude limiters Nonlinear signal processors Power switches Transient protectors Here, we shall cover topics that are important for Layout Design, but usually not covered in elementary texts. N P N Transistor Model 8.1 Operation EB junction = forward-biased n p diode CB junction = reverse-biased n p , current-controlled current source I c = b F I B I c = I s exp(V BE /V T ) V BE = V T ln(I c /I s ) 2 I B depends on exp(V BE /V T )
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2/22/2011 2 I s = Emitter saturation current V T = thermal voltage = 26 mV at 300K dV BE /dT = -2mV / C 1 deg in D T can induce 8% mismatch in V BE ! V BE = V T ln(I c /I s ) 3 (1)Beta Rolloff High I c b drops because injected minority majority in Base (so-called High-Level Injection) Low I c b drops because leakage current ≈ injected minority current (transistor current is overwhelmed by the uncontrolled leakage current) 4
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2/22/2011 3 NPN b drops at I c > 5mA/mil 2 = 8 m A/ m m 2 Due to high-level injection b drops at I c < 10nA/mil 2 = 15pA/ m m 2 Due to recombination in depletion region, recombination at oxide interface, shallow emitter effect, … BETA ROLLOFF: 5 Lateral-PNP : b of lateral PNP is much smaller than NPN. Why ? High-I C rolloff occurs earlier because high-level injection condition occurs earlier due to small base N Low-I C rolloff occurs earlier because current flows near the surface (thus higher recombination at oxide-Si interface) Lateral PNP: usually the High-I C rolloff and Low-I C rolloff meet already , and the peak beta is already in the high-level injection condition ! Tough to design. BETA ROLLOFF: 6
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2/22/2011 4 (2)Avalanche Breakdown How high a voltage can a BJT operate at ? V EBO = EBJ breakdown with C open V CBO = CBJ breakdown with E open V CEO = Breakdown with B open 7 NPN V EBO = 7V : hot carriers generated during avalanche produces recomb centers at the oxide-Si interface degrades NPN b rapidly The recomb centers degrade low-current b NPN V CBO = 20V 120 V As CBJ is at subsurface, nor oxide interface problem here. Thus, no effect on b . Lateral-PNP Both V EBO and V CBO are due to breakdown of Base-Epi junction. so, no effect on beta. 8 (2)Avalanche Breakdown How high a voltage can a BJT operate at ? V EBO = EBJ breakdown with C open V CBO = CBJ breakdown with E open V CEO = Breakdown with B open NPN V CEO = about 0.6 V CBO Called beta multiplication because impact ionization begins to occur at a voltage well below breakdown voltage
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2/22/2011 5 A 36V-rated BJT: V CES = Base shorted to Emitter, beta multiplication. When I
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Ch8-Ch9-2slides - 2/22/2011 8 Bipolar Transistors Most...

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