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Transport characteristics of n+-i-n+ structures fabricated on a GaAs/AlGaAs HEMT wafer are presented. An insulating strip (i) between two-dimensional electron gases (n+) is created by a single line scan of a focused ion beam. At sufficiently large biases, the current–voltage (I–V) characteristics of the n+-i-n+ structures display highly linear increases of the currents as a function of the biases. The observed I–V’s are quantitatively explained by the space-charge-limited injection into the insulating region when the bias is large enough for the flatband condition. The charged defect density and the length of the insulating region are obtained from the I–V characteristics.