الفهرس 
acknowledgmentOnly 14 pages are availabe for public view
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Abstract
Measurements of the longitudinal and transverse direct eutent magnetoresistance of moderately doped samples were carried out a function of (temperature, T, down and magnetic fied KG. Thee different conduction mechanisms were observed. The excess donor concentrations of the investigated samples were, The average spacing between the donors was estimated and found tobe in the order of the effective Bohr radius of this system The present concentrations, therefore, were assumed be of moderate type. At low temperatures, in absence and presence of weak magnetic fields, the behaviour ofthe conductivity was of a metallic - one, the conductivity o tends to a finite value as 0 K. However, a high magnetic flelds, a metal- insulaor uansition might oceur at a particular threshold value, Ne. This critical magnetic field depends on the concentration of the caries, H., indeed, was found to increase as the concentration was increased, The presence of the magnetic field leads to shrink in the electron wave functions associated with the different donors, The result is that at a particular field, H., the overlap of the adjacent wave-funetions will be negligibly small so thatthe donors may become isolated. Hence, metabinsulator uansition may eceur. High concentrations, then needs high magnetic fields to induce such eansiion, Measurements carried magnetic field in the ionized impurity seatering regime (low temperatures, below show that varies with where a This is less thaa the expected theoretical findings. This may be due to possible contribution.