Modeling of the source resistance in modulation-doped FETs

• Published in: Solid-state electronics 1992-04, Vol.35 (4), p.489-499
• Main Authors: Fu, S.T., Das, M.B.
• Format: Article
• Language: English
• Subjects: Applied sciences; Electronics; Exact sciences and technology; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Transistors
• ISSN: 0038-1101; 1879-2405
• DOI: 10.1016/0038-1101(92)90110-X

A distributed multiple layer transmission line (MTL) model has been developed for the planar source ohmic contacts as encountered in modulation-doped FETs. Detailed current and voltage distributions under the MTL system have been obtained by numerical solutions of the coupled differential equations. The results clearly demonstrate that source resistance R S cannot be estimated simply from knowledge of the behavior of series resistance R NG in non-gated TLM test structures. The effects of the heavily-doped cap layer and the variation of the gate-source spacing on R NG and R S have been studied. Differences between our results based on the MTL model and that obtained from the lumped element contact end-resistance (LECR) model are explained satisfactorily. Our results show that the LECR method could be used to calculate the series resistance of the MODFET structures provided appropriate end elements are first determined by the distributed MTL model. Our results clearly indicate that attempts to reduce R S by increasing the cap layer thickness or decreasing the gate-source spacing would have only limited effectiveness. However, reduction of the interfacial contact resistivities is more effective in the reduction of R S , as demonstrated in the case of pseudomorphic structures. Graphical results are presented that clearly show the dependence of R S on the structural parameters of representative conventional and pseudomorphic test MODFETs.