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