Rare-earth elements present in the growth melt of the LPE process possess the ability to chemically bind atoms responsible for shallow impurity levels in III-V semiconductor compounds. In continuation of our recent theoretical work on this subject, we present a diagrammatical analysis of equations governing this phenomenon. The causal picture of the dependence of free donor concentration versus rare-earth content in the growth melt contains two loops of negative feedback which play a major part in shaping this relation. Mathematical apparatus accompanying the diagrams is used to numerically evaluate various elements of this representation for realistic values of the model parameters, thus allowing insights into mechanism of the purification effect. In particular, the domain of efficiency for the impurity removal is established.
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