Arsenic radiotracer diffusions have been performed both in intrinsic and in homogeneously doped extrinsic silicon single crystals. Under intrinsic conditions, the tracer diffusion coefficient may be represented by the expressionDi=60 exp (−4.20 eV/kT) cm2/sec. Measured in extrinsic silicon, in the absence of a concentration gradient, the tracer diffusion coefficient conforms with the relationshipD=Di(n/ni), in which (n/ni) in the ratio of the free electron concentration in the extrinsic sample to that of intrinsic silicon at the diffusion temperature.