|Institution:||University of Manchester|
|Keywords:||Resonant Tunneling Diode, RTD, THz, Fabrication, InGaAs, Oscillator|
|Full text PDF:||http://www.manchester.ac.uk/escholar/uk-ac-man-scw:261818|
The resonant tunneling diode (RTD) is the fastest electron device to-date in terms of its ability to generate continuous-wave terahertz frequency at room temperature, owing to its unique characteristic of negative differential resistance (NDR). In this work, a lattice-matched In0.53Ga0.47As (on InP) is used as the cladding layer, while a highly-compressive strained In0.8Ga0.2As is sandwiched between two tensile-strained pseudomorphic AlAs barriers to form the active double barrier quantum well RTD structure grown by Molecular Beam Epitaxy. The ultimate aim of this work was to integrate an optimised RTD into an oscillator circuit to enable a 100 GHz (W-band) MMIC RTD oscillator.One of the key challenges in this work was to improve the DC performance of the RTD, through extensive material and structural characterisations. Growing nano-scale epitaxial layers require a high degree of controllability with mono-layer precision. The dependencies of the NDR components, such as the peak current density, peak voltage and peak-to-valley current ratio (PVCR) towards variations in structural thickness were studied systematically. Through this work, it is found that the peak current density is strongly affected by monolayer variation in barrier thickness. The effect of quantum well thickness variation towards peak current density is relatively weaker. Interestingly, variation in spacer layer thickness has very little influence towards the magnitude of the peak current density.The fabrication of the RTD using a conventional i-line optical lithography created its own challenge. The process capability to reduce mesa active area down to sub-micrometer level to reduce device’s geometrical capacitance for high frequency, THz applications has been made feasible in this work. The conventional i-line optical lithography was combined with a newly developed tri-layer soft reflow technique using solvent vapour resulted in sub-micrometer RTDs. The DC characterisation of the fabricated RTDs showed excellent device scalability, indicating a robust processing. This novel sub-micron processing technique with high throughput and repeatability is a very promising low cost technique.A collaborative effort between the University of Manchester and Glasgow paved the way towards the realisation of an integrated W-band RTD MMIC oscillator. The circuit-combining topology was designed by the High Frequency Electronics Group in Glasgow while the mask-layout and oscillator fabrication took place in Manchester. An active RTD from sample XMBE#301 with peak current density of 1.4 x 105 A/cm2 and PVCR of 4.5 was integrated into a 100 GHz MMIC oscillator to successfully produce a measured frequency of 109 GHz with an un-optimised 5.5 μW output power at room temperature (mesa area = 4x4 μm2).