OUTLINE

1
Lecture 19

• OUTLINE
• pn junctions (contd)
• Charge control model
• Reading Finish Chapter 6.3

2
Minority-Carrier Charge Storage

• When VAgt0, excess minority carriers are stored in
  the quasi-neutral regions of a pn junction

3
Derivation of Charge Control Model

• Consider a forward-biased pn junction. The total
  excess hole charge in the n quasi-neutral region
  is
• The minority carrier diffusion equation is
  (without GL)
• Since the electric field is very small,
• Therefore

4
(Long Base Diode)

• Integrating over the n quasi-neutral region
• Furthermore, in a pn junction
• So

5
Charge Control Model

• We can calculate pn-junction current in 2 ways
• From slopes of ?np(-xp) and ?pn(xn)
• From steady-state charges QN, QP stored in each
  excess-minority-charge distribution

6
Charge Control Model for Narrow Base

• For a narrow-base diode, replace tp and/or tn by
  the minority-carrier transit time ttr
• time required for minority carrier to travel
  across the quasi-neutral region
• For holes on narrow n-side
• Similarly, for electrons on narrow p-side

7
Summary

• Under forward bias, minority-carrier charge is
  stored in the quasi-neutral regions of a pn
  diode.
• Long base
• Short base

8

• The steady-state diode current can be viewed as
  the charge supply required to compensate for
  charge loss via recombination (long base) or
  collection at the contacts (short base)
• Long base
• Short base
• where

Note that