Spring 2007EE130 Lecture 19, Slide 1 Lecture #19 OUTLINE pn junctions (cont’d) – Charge control model Reading: Finish Chapter 6.3

Spring 2007EE130 Lecture 19, Slide 2 Minority-Carrier Charge Storage When V A >0, excess minority carriers are stored in the quasi-neutral regions of a pn junction:

Spring 2007EE130 Lecture 19, Slide 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 G L ): Since the electric field is very small, Therefore:

Spring 2007EE130 Lecture 19, Slide 4 Integrating over the n quasi-neutral region: Furthermore, in a p + n junction: So: (Long Base Diode)

Spring 2007EE130 Lecture 19, Slide 5 Charge Control Model We can calculate pn-junction current in 2 ways: 1.From slopes of  n p (-x p ) and  p n (x n ) 2.From steady-state charges Q N, Q P stored in each excess-minority-charge distribution:

Spring 2007EE130 Lecture 19, Slide 6 Charge Control Model for Narrow Base For a narrow-base diode, replace  p and/or  n by the minority-carrier transit time  tr –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:

Spring 2007EE130 Lecture 19, Slide 7 Summary Under forward bias, minority-carrier charge is stored in the quasi-neutral regions of a pn diode. –Long base: –Short base:

Spring 2007EE130 Lecture 19, Slide 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