Strained Si/SiGe heterostructures for device applications

Abstract

A review is given of the most recent developments in the field of strained Si/SiGe heterostructures for transistor applications. Main topics are the pseudomorphic SiGe-base heterobipolar transistor (HBT) and modulation-doped field effect transistors (MODFET) utilizing strain-adjusting SiGe buffer layers. Si/SiGe HBTs have recently demonstrated transit frequencies in excess of 100 GHz, thus improving the high frequency cut-off of the best Si bipolar junction transistors (BJT) by a factor of two. They are now on their route from mere lab-demonstrators into the production lines. Si/SiGe MODFETs are at an earlier state, having just solved one of the most stringent material problems, namely the epitaxial growth of a relaxed SiGe buffer layer with low defect densities. This allowed the realization of n-type and p-type quantum well structures with unprecedentedly high carrier mobilities. p-type MODFETs with pure Ge channels have been fabricated on Si substrates recently. These are very promising devices for complementary applications in combination with Si-channel n-type MODFETs because of the almost perfectly matched carrier mobilities. Beside basic material aspects and physical properties, d.c.-results of test devices demonstrating superior transconductances will be treated.