Post on 05-Jan-2016
transcript
22
Carrier DriftObserve that the text uses “e” instead of “q” as a symbol for a unit of charge
Drift current densityddrift vJ
Current density due to the holesep
Drift velocity of holes Ev pdp
EenJ ntdrifn
Current density due to the holes
Current density due to the electrons
EepJ ptdrifp
Drift velocity of electrons Ev ndn
Total drift current density EnpeJ npdrift )(
Charge density:
Charge density: en
33
Definition of Carrier Mobility
Ev pdp Drift velocity of holes
Drift velocity of electrons Ev ndn
p : mobility of holes
n : mobility of electrons
Mobility: relates the average drift velocity of a carrier to the electric field
99
Conductivity
EEnpeJ npdrift )(
)( np npe Conductivity:
Resistivity: (Ω-cm)-1
)(
11
np npe
N-type semiconductor:ndnnp eNennpe )(
P-type semiconductor:papnp eNepnpe )(
Intrinsic semiconductor: )()( npinp ennpe
(Ω-cm)-1
1414
Non-equilibrium Excess Carriers in Semiconductors (Chapter 6)
When external field (electric, thermal, optical) is applied on the semiconductor, the semiconductor is operating under non-equilibrium conditions
Excess electrons in the conduction band and excess holes in the valence band may exist in addition to the thermal equilibrium condition
Excess carriers’ movements: generation, recombination, drift, and diffusion
1515
Total Diffusion Current
dx
dneDJ ndifnx ,
Electron diffusion current
Hole diffusion current
dx
dpeDJ pdifpx ,
Total diffusion current
)(dx
dpD
dx
dnDeJ pndif
Dn: electron diffusion coefficient (m2/s)
Dp: hole diffusion coefficient (m2/s)
1616
Low-Level Injection and High-Level Injection
Low-level injection: excess carrier concentration is much less than thermal equilibrium majority carrier concentrations
• n-type material: no>>po, δn(t) << no
• p-type material: po>>no, δn(t)<<po
High-level injection: excess carrier concentration is comparable to or greater than the thermal equilibrium majority carrier concentrations
• n-type material: no >> po, δn(t) >= no
• p-type material: po >>no, δn(t)>= po
1818
Carrier Generation and Recombination
Mechanisms of generation and recombination: band-to-band, traps (recombination centers)
19
Excess Carrier Generation and Recombination
When external force (electric, optical, thermal) is applied, excess electrons and holes are create in pairs
pn RR
With generation of excess carriers, concentration of electrons and holes are increased
pn gg
Electrons and holes are recombined at the same time of generation are equal
nnn 0 ppp 0
2020
Carrier Generation and Recombination in Equilibrium (Band-to-Band)
For direct band-to-band generation, the electrons and holes are created in pairs
00 pn RR
For direct band-to-band recombination, the electrons and holes are combined in pairs
00 pn GG
In thermal equilibrium, the generation and recombination rates are equal
0000 pnpn RRGG