1EE130 Lecture 20, Slide 1Spring 2003Lecture #20ANNOUNCEMENTNo Coffee Hour today /OUTLINE• The BJT: final comments• PNPN thyristorReading: Chapter 13EE130 Lecture 20, Slide 2Spring 2003Schottky-Clamped BJT• When the BJT enters the saturation mode, the Schottky diode begins to conduct and “clamps” the C-B junction voltage at a relatively low positive value.Æ reduced stored charge in quasi-neutral base2EE130 Lecture 20, Slide 3Spring 2003Reducing τBfor Faster Turn-Off• The speed at which a BJT is turned off is dependent on the amount of excess minority-carrier charge stored in the base, and also the recombination lifetime τB– By reducing τB, the carrier removal rate is increasedExample: Add recombination centers (Au atoms) in the baseEE130 Lecture 20, Slide 4Spring 2003The Silicon Controlled Rectifier (Thyristor)• The SCR is a PNPN device– Used in high-power switching applications3EE130 Lecture 20, Slide 5Spring 2003• The SCR can be modeled as 2 interconnected BJTsSCR OperationEE130 Lecture 20, Slide 6Spring 2003Forward blocking state• Few holes injected into N region• Thermal R-G current only4EE130 Lecture 20, Slide 7Spring 2003Conducting State1. Holes injected into N2. Due to wide N-P depletion region, holes are swept into P, turning on PNP device3. This turns on NPN device• This regenerative effect maintains conduction even at low voltage, down to a minimum “holding current”.EE130 Lecture 20, Slide 8Spring 2003Quantitative Analysis• We can write:•Normally, (α1+α2) <1• However, as VAK↑, base width narrowing causes (α1+α2) →1,
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