Patent Number: 043483518
Section: description

The following examples relate to the method according to the invention utilizing specific numerical values in the preceding mathematical relationships. Specifically, a commercially desired neutron doped silicon material specification is listed hereinbelow with example I illustrating the shortcomings of art taught selection while example II illustrates the method according to the invention and application of the mathematical selection process listed hereinabove. EXAMPLES A commercially meaningful neutron transmutation doped single crystal silicon material would require resistivity of 50 ohm-cm.+-.15%. The resistivity limits are hence 42.5-57.5 ohm-cm. Converting resistivities to dopant concentrations according to the mathematical definition, we have; ##EQU2## EXAMPLE I Example I utilizes the prior art taught selection method wherein resistivity limits for starting materials provide maximum dopant concentration in starting material equal to 0.1 times average concentration in final product which is equal to 0.1 times 1.92 which is equal to 0.192 ppb (parts per billion dopant impurity), Minimal resistivity for "N" type starting material would equal to 500 ohm-cm while minimal resistivity for "P" type starting material would equal 1300 ohm-cm and no starting material would be utilized with p-n junctions. EXAMPLE II Resistivity limits for starting material under the method according to the invention utilizes maximum dopant difference in starting material equal to .DELTA.C.sub.s equal to C.sub.D max-C.sub.D min=0.59 ppb. Homo geneity factor as defined herein and for the purposes of this invention of the final product is a.sub.D equals .DELTA.C.sub.D /C.sub.D equals 0.59/1.92 equals 0.307 and homogeneity factor for starting material as defined for the purposes of this invention is represented by a.sub.s and equals 0.600. The homogeneity factor of starting material is based on the results of measurements on many pieces of starting material, and is the maximum inhomogeneity likely to be encountered. The maximum average dopant concentration in the starting material is equal to C.sub.s which equals a.sub.D /a.sub.s times C.sub.D equal to 0.307/0.600 times 1.92 equal to 0.98 ppb "N" or "P" type. Minimal resistivity equals 98 ohm-cm "N" type or 255 ohm-cm "P" type. The selection of material containing p-n junctions will be illustrated with two examples, the first example showing an acceptable piece, the second example having a rejectable piece. Donor concentrations will be considered positive numbers and acceptor concentrations as negative numbers. EXAMPLE III Max donor concentrations=0.3 ppb Max acceptor concentrations=0.1 ppb Max difference=0.3-(-0.1)=0.4 ppb which is less than 0.59 ppb Material is acceptable. EXAMPLE IV Max donor concentrations=0.3 ppb Max acceptor concentrations=-0.4 ppb Max difference=0.3-(-0.4)=0.7 ppb which is more than 0.59 ppb; thus, the material must be rejected.