Effectiveness of a Ge fraction modulated spacer in hole resonant tunneling diodes (RTDs) with Si/strained Si-1 Ge-x(x) heterostructures epitaxially grown on Si(1 0 0) was investigated to improve the electrical characteristics at higher temperatures. Electrical characteristics measured for 30 RTDs, with the modulated spacer at higher Ge fraction (x = 0.48) on a single wafer, show that the deviation of the peak current and voltage at the resonant peak falls in ranges of +/- 25% and +/- 10%, respectively. For the RTDs, negative differential conductance (NDC) characteristics are obtained even at higher temperatures around 230 K than that for the RTDs with x = 0.42. The result indicates that the introduction of higher Ge fraction is effective for NDC in RTD at higher temperature. (C) 2008 Elsevier B.V. All rights reserved.