GaAs/AlAs ASPAT Diodes for Millimetre and Sub-Millimetre Wave Applications

  • Mohd Abdullah

Student thesis: Phd


The Asymmetric Spacer layer Tunnel (ASPAT) diode is a new diode invented in the early 90s as an alternative to the Schottky barrier diode (SBD) technology for microwave detector applications due to its highly stable temperature characteristics. The ASPAT features a strong non-linear I-V characteristic as a result of tunnelling through a thin barrier, which enables RF detection at zero bias from microwaves up to submillimetre wave frequencies. In this work, two heavily doped GaAs contact layer on top and bottom layers adjacent to lightly doped GaAs intermediate layers, enclose undoped GaAs spacers with different lengths sandwiching an undoped AlAs layer that acts as a tunnel barrier. The ultimate ambition of this work was to develop a MMIC detector as well as a frequency source based on optimized ASPAT diodes for millimetre wave (100GHz) applications. The effect of material parameter and dimensions on the ASPAT source performances was described using an empirical model for the first time. Since this is a new device, keys challenges in this work were to improve DC and RF characteristic as well as to develop a repeatable, reproducible, and ultimately manufacturable fabrication process flow. This was investigated using two approaches namely air-bridge and dielectric-bridge fabrication process flows. Through this work, it was found that the GaAs/AlAs heterostructures ASPAT diode are more amenable to the dielectric-bridge technique as large-scale fabrication of mesa area up to 4×4µm2 with device yields exceeding 80% routinely produced. The fabrication of the ASPAT using i-line optical lithography which has the capability to reduce emitter area to 4×4µm2 to lower down the device capacitance for millimetre wave application has been made feasible in this work. The former challenge was extensively studied through materials and structural characterisations by a SILVACO physical modelling and confirmed by comparison with experimental data. The I-V characteristic of the fabricated ASPAT demonstrated outstanding scalability, demonstrating robust processing. A fair comparison has been made between ASPAT and SBD fabricated in-house; indicating ASPAT is extremely stable to the temperature. The RF characterisations were carried out with the aid of Keysight ADS software. The DC characteristic from fabricated GaAs/AlAs ASPAT diodes were absorbed into an ADS simulation tool and utilized to demonstrate the performance of MMIC 100GHz detector as well as 20GHz/40GHz signal generators. Zero bias ASPAT with mesa area of 4×4µm2 with video resistance of 90KΩ, junction capacitance of 23fF and curvature coefficient of 23V-1 has demonstrated detector voltage sensitivity above 2000V/W, while the signal source conversion loss and conversion efficiency are 28dB and 0.3% respectively. An estimate noise equivalent power (NEP) for this particular device is 18.8pW/Hz1/2.
Date of Award1 Aug 2018
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorMohamed Missous (Supervisor) & Robin Sloan (Supervisor)


  • Detector
  • Millimetre and Sub-Millimetre Wave
  • Tunnelling DIode
  • Zero bias ASPAT
  • Asymmetric Spacer layer Tunnel
  • Physical Modelling
  • MMIC
  • DC and RF characterisations
  • GaAs/AlAs heterostructures ASPAT
  • GaAs/AlAs
  • Frequency Multiplier
  • Semiconductor Fabrication

Cite this