Development of advanced technologies for the fabrication of III-V high electron mobility transistors

Boyd, Euan James (2004) Development of advanced technologies for the fabrication of III-V high electron mobility transistors. PhD thesis, University of Glasgow.

Abstract

Over the past 5 years there has been an increase in the number of applications that require
devices that operate in the millimetre range (30-300GHz). This demand has driven research into
" devices that will operate at frequencies above 100GHz. This performance has been achieved
using two main technologies, the Heterojunction Bipolar Transistor (HBT) and the High
Electron Mobility Transistor (HEMT). At present it is a HEMT device that holds the record for
the highest operating frequency of any transistor. It is this technology that this project
concentrates on.
In order to fabricate devices that operate at these frequencies two methods are commonly
employed. The first is to vary the material of the device, in particular, increasing the indium
content of the channel. The second method is to reduce the physical dimensions of the
transistors, including reducing the gate length of the device therefore reducing transit time and
gate capacitance. Reducing the separation of the source-drain ohmic contacts or employing a
self-aligned ohmic strategy reduces the associated parasitic resistances. This project will
concentrate on the scaling of the gate length in addition to the reduction of parasitic resistances
with the use of self-aligned ohmic contacts.This work includes the realisation of the first self-aligned 120nm T -Gate. GaAs pHEMT
fabricated at the University of Glasgow. These devices required the development of two key
technologies, the non-annealed ohmic contact and the succinic acid based selective wet etch.
The self-aligned devices showed good RF performance with a ft of 150 GHz and a fmax of 180
GHz which compares favourable with results o~ 120nm GaAs pHEMTs previously fabricated at Glasgow.
The investigation of gate length scaling to device performance included the development of two
lithographic process capable of producing HEMT with a gate length of 50nm and 30nm respectively in addition to a method ~f sample preparation that allows these devices to be
analysed using TEM techniques.
This work has lead to the realisation of SOnm T -gate metamorphic HEMTs using a
PMMAIcopolymer resist stack, these devices displayed an excellent yield, with over 95% of
devices working. The uniformity of the gate process was also high with a threshold voltage of -
0.44SV with a standard deviation of O.OOSV. The devices demonstrated an .it of 330GHz and a
fmax of 260GHz making these devices some of the fastest transistors that have ever been
fabricated on a GaAs substrate. The second lithography process was developed to realise T -gates with a gate length of less than
SOnm. This processed used a two stage "bi-lithography" process to minimise the effect of
forward s7attering through the resist. The gate footprint was transferred into a Si02 gate by a dry
etch process. This lithography process was integrated into a full process flow for lattice matched
InP HEMTs Using this process, HEMTs were fabricated with a T-gate of 2Snm. This is the
smallest T -gate device that has been fabricated at the University of Glasgow and is comparable
with the smallest HEMT devices in the world.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
• Colleges/Schools: College of Science and Engineering > School of Engineering > Electronics and Nanoscale Engineering
• Supervisor's Name: Thayne, Professor Iain
• Date of Award: 2004
• Depositing User: Ms Dawn Pike
• Unique ID: glathesis:2004-5381
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 11 Jul 2014 10:52
• Last Modified: 11 Jul 2014 10:52
• URI: https://theses.gla.ac.uk/id/eprint/5381