Ch8-Ch9-2slides - 8 Bipolar Transistors Most LOGIC circuits...

Info icon This preview shows pages 1–6. Sign up to view the full content.

View Full Document Right Arrow Icon
2/22/2011 1 8 Bipolar Transistors 1 Most LOGIC circuits in CMOS Bipolar = important for most Analog circuits Voltage & current amplifiers Voltage & current references 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 )
Image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
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
Image of page 2
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
Image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
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 because B & C are lightly doped. 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
Image of page 4
2/22/2011 5 A 36V-rated BJT: V CES = Base shorted to Emitter, beta multiplication.
Image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 6
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern