Chapter 4 (lec 11-13)

Question 1

What is carrier injection

Answer 1

The process of introducting excess carriers

Question 2

What are the three methonds for carrier injection

Answer 2

Optical
Electron bombardement
Biasing

Question 3

What are the two type of carrier recombination

Answer 3

Direct recombination ( band to band )
Indirect recombination (through trap centers )

Question 4

Explain the three ways indirecet recombination can ocurr

Answer 4

1. An electron in Ec looses energy and falls into trap level
   - a hole is then generated in Et
   - The electron combines with the hole, an EHP is annihilated
2. An electron from Et drops to Ev to create a hole in Et
   - An electron from Ec drops to Et to combine with the hole and EHP is annihilated
3. An electron drops from Ec to Et
   - The same electron drops from Et to Ev to combine with a pre existing hole in the valance band

Question 5

What are the two type of excess carrier analysis

Answer 5

1. Transient Analysis
   -Momentary turning on and off of the source
   - After generation the excess carriers recombine to decay their concentrations and after a certain time no more excess carriers are there
2. Steady state analysis
   - Continous generation and recombination
   - Excess carriers always present

Question 6

What are the quasi-fermi level and what does it indicate

Answer 6

Fn and Fp are the steady state analog equilibrium fermi level when excess carriers are present
- The positions of Fn and Fp indicate how far the electron and hole concentrations are from their equilibrium values No and Po after excess carrier generation

Question 7

Explain the process of diffusion

Answer 7

If there is a gradient (gradual change) in carrier concentration in the medium (substrate) carriers will move from a region of higher concentration to a region of lower concentration due to thermal velocity

Question 8

What is diffusion current

Answer 8

The motion of the carrier due to the diffusion process create a current

Question 9

What implies no net diffusion current

Answer 9

Uniform carrier concentration - implies no net motion wich means no diffusion current