All types of tunnel diodes, including Si/SiGe resonant interband tunnel diodes, make use of the quantum mechanical tunneling effect. Characteristic to the current-voltage relationship of a tunnel diode is the presence of one or more negative differential resistance region, which enables many unique applications. Tunnel diodes are also capable of high speed operation because the quantum tunneling effect is a fast process.
Compared to the Esaki tunnel diodes, which are essentially discrete diodes and therefore incompatible with modern Si integrated circuit technologies, the Si/SiGe resonant interband tunnel diodes are such that their structure and fabrication are suitable for integration with modern Si complemenraty metal-oxide-semiconductor (CMOS) and Si/SiGe heterojunction bipolar technology.
Resonant tunnel diodes and resonant interband tunnel diodes were first realized in III-V compound material systems utilizing hetrojunctions made up of various III-V compound semiconductors. Reasonably high performance III-V resonant tunnel diodes have been realized. But such devices have not entered mainstream applications yet because the processing of III-V materials is incompatible with Si CMOS technology and the cost is high.
Si/SiGe resonant tunnel diodes suffer from limited performance because of the limited conduction band and valance band offsets.
Si/SiGe resonant interband tunnel diodes are developed , which offer the advantage of being compatible with the mainstream Si CMOS technology.
The structure of a typical Si/SiGe resonant interband tunnel didoe is shown on the right. Also shown is the corresponding band diagram calculated by Gregory Snider's 1D Poisson/Schrodinger Solver
The five key points to the design are: (i) an intrinsic tunneling barrier, (ii) delta-doped injectors, (iii) offset of the delta-doping planes from the heterojunction interfaces, (iv) low temperature molecular beam epitaxial growth (LTMBE), and (v) postgrowth rapid thermal annealing (RTA) for dopant activation and point defect reduction.
A minimum PVCR of about 3 is needed for typical circuit applications. Low current density Si/SiGe RITDs are suitable for low-power memory applications, and high current density tunndel diodes are needed for high speed digital/mixed-signal applications. Si/SiGe RITDs have been engineered to have room temperature PCVRs up to 4.0 . The same structure was duplicated by another research group using a different MBE system, and PVCRs of up to 6.0 have been obtained. In terms of peak current density, peak current densities ranging from as low as 20 mA/cm2 and as high as 218 kA/cm2, spanning seven orders of magnitude, have been obtained. A resistive cut-off frequency of 20.2 GHz has been realized on photolithography defined SiGe RITD followed by wet etching for further reducing the diode size , which should be able to improve when even smaller RITDs are fabricated using techniques such as electron beam lithography.
In addition to the realization of integration with Si CMOS and SiGe HBT that is discussed in the next section, other applications of SiGe RITD have been demonstrated using breadboard circuits, including multi-state logic .
Integration of Si/SiGe RITDs with Si CMOS has been demonstrated .
Vertical integration of Si/SiGe RITD and SiGe HBT was also demonstrated, realizing a 3-terminal negative differential resistance circuit element with adjustable peak-to-valley current ratio.
These results indicate that Si/SiGe RITDs is a promising candidate of being integrated with the Si integrated circuit technology.