Upload
megan-flowers
View
215
Download
0
Embed Size (px)
Citation preview
NEEP 541Ionization in Semiconductors - II
Fall 2002Jake Blanchard
Outline Radiation effects in
Semiconductors Ionization Recombination Trapping Junctions Example
Ionization Due to Radiation
When semiconductor is subjected to ionizing radiation (photons or charged particles) with energy greater than the bandgap, electrons are excited from the valence band to the conduction band
This creates an electron hole pair This increases electrons in conduction band
(excess carriers) If transferred energy exceeds the bandgap, the
excess goes to heating 3.7 eV needed to create electron hole pair in Si
(bandgap is about 1.1 eV) This is not sensitive to temperature
Ionization effects (continued) One effect of the excess carriers is
an increase in the electrical conductivity
There does not seem to be any difference between electrons and holes produced by ionizing radiation and those which are naturally present in the semiconductor
Recombination When the radiation stops, the electrons
and holes will begin to recombine and revert to their equilibrium densities
Recombination mechanisms Direct reaction between electron and hole
(this is how LEDs emit light) – not important in Si and Ge, but important in GaAs
In Si and Ge, an intermediate (defect) energy level captures either a hole or an electron, then captures the opposite type of carrier
Trapping
When defect densities are large and one carrier type is more easily captured than the other, then the excess densities are no longer equal.
This is called minority carrier trapping This leads to very long-lived excess
electrical conductivity For example, if holes are captured, then the
excess electrons remain in conduction band for a long time
P-N junctions Ionizing radiation creates a current in a PN
junction (“primary photocurrent”) There is a prompt current that is coincident with
the radiation and a delayed current that occurs after the radiation stops
The prompt current is from electron hole pairs created in the depleted region of the junction
The delayed current is from the minority carriers produced in the bulk diffusing to the junction
Solar Cells A solar cell is a PN junction
designed for optimum collection of sunlight
Typically 0.5 microns of n-type on 250 microns p-type
Typical Example of Effects of Ionizing Radiation
Donor doping density=1015 /cm3
Electron mobility=1300 cm2/V-sec Hole mobility=480 cm2/V-sec Recombination center
density=1014/cm3
Hole thermal velocity=107 cm/s Hole capture cross section=10-15
cm2
Example (continued) Intrinsic carrier density=2x1010 /cm3
Equilibrium electrical conductivity=0.21 /ohm-cm
10 rad pulse (absorbed dose: 1 rad=0.01 J/kg)
Produces 4x1014 electron-hole pairs/cm3
Short Pulse: electrical conductivity=0.33 /ohm-cm
Conductivity Decay time ~1 micro-s