PNPN型器件

1 . Lecture #20 ANNOUNCEMENT No Coffee Hour today / OUTLINE • The BJT: final comments • PNPN thyristor Reading: Chapter 13 Spring 2003 EE130 Lecture 20, Slide 1 Schottky-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 base Spring 2003 EE130 Lecture 20, Slide 2 1 

2 . Reducing τB for 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 increased Example: Add recombination centers (Au atoms) in the base Spring 2003 EE130 Lecture 20, Slide 3 The Silicon Controlled Rectifier (Thyristor) • The SCR is a PNPN device – Used in high-power switching applications Spring 2003 EE130 Lecture 20, Slide 4 2 

3 . SCR Operation • The SCR can be modeled as 2 interconnected BJTs Spring 2003 EE130 Lecture 20, Slide 5 Forward blocking state • Few holes injected into N region • Thermal R-G current only Spring 2003 EE130 Lecture 20, Slide 6 3 

4 . Conducting State 1. Holes injected into N 2. Due to wide N-P depletion region, holes are swept into P, turning on PNP device 3. This turns on NPN device • This regenerative effect maintains conduction even at low voltage, down to a minimum “holding current”. Spring 2003 EE130 Lecture 20, Slide 7 Quantitative Analysis • We can write: I AK = α1I AK + I R 01 + α 2 I AK + I R 02 I R 01 + I R 02 I AK = 1 − (α1 + α 2 ) • Normally, (α1+α2) <1 • However, as VAK ↑, base width narrowing causes (α1+α2) →1, IAK↑ Spring 2003 EE130 Lecture 20, Slide 8 4