ANNOUNCEMENTS

1
Lecture 28

• ANNOUNCEMENTS
• For Problem 2 of HW9, use A 10-7 cm2
• The Term Project will be posted online today
• Due on Friday, April 27
• You may work with one partner
• OUTLINE
• The PNPN thyristor
• Reading Chapter 13

2
The Silicon Controlled Rectifier (Thyristor)

• The SCR is a PNPN device
• Used in high-power switching applications

3
SCR Operation

• The SCR can be modeled as 2 interconnected BJTs

4
Forward blocking state

• Few holes injected into N region
• Thermal R-G current only

5
Conducting State

• Holes injected into N
• Due to wide N-P depletion region, holes are swept
  into P, turning on PNP device
• This turns on NPN device
• This regenerative effect maintains conduction
  even at low voltage, down to a minimum holding
  current.

6
Quantitative Analysis

• We can write
• Normally, (a1a2) lt1
• However, as VAK ?, base width narrowing causes
  (a1a2) ?1, IAK?