PNPN型器件

PNPN型器件,由PNPN四层半导体材料构成的二端器件或三端器件。二端器件称PNPN闸流二极管,一般用来产生电流脉冲,用途和闸流二极相似。三端器件一般增加一个P门极,用以触发器件进入导通状态,这种器件又称可控闸流管或可控硅。从可控硅器件中派生出来的另一种新型PNPN四层三端器件是把传统的P门极改为N门极,这种器件的触发灵敏度高,在使用上可以代替单结晶体管,因此也称为程控单结晶体管。
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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