Completed Projects

Thanks to the excellent transport properties of the InP/GaInAs material system, InP HEMTs (High Electron Mobility Transistors) offer the best performance for low-noise and high-frequency applications (for example, in extremely low noise amplifiers).
Typical applications can be found in the fields of deep-space communications and radio astronomy, which impose stringent demands on the signal amplification and therefore require cryogenic operation of the transistors at temperatures of ~10K. In the Terahertz Electronics Group (formerly Microwave-Electronics), we have a leading tradition of working on the manufacturing process and cryogenic characterization of such transistors for collaborative projects with the European Space Agency (ESA). This work focuses on the characterization of InP/GaN HEMT devices.
At IFH the analysis and measurement environment “HEMT Measurement GUI” (developed at our institute) controls a Vector Network Analyzer HP8510c and a Parameter Analyzer HP4156b and is used to measure and extract the de-embedded S-Parameters and other basic HEMT key parameters. In this work it should be extended by the following measurement modules:

• breakdown voltage determination
• extraction of intrinsic device parameters
• device stress testing environment
• pulse measurement environment
• statistical data analysis
• temperature monitoring and control of cryostat

The work requires becoming familiar with the functionality of an HEMT device, the measurement equipment and the “HEMT Measurement GUI” software environment.
After becoming acquainted with the topic the student is expected to pick one measurement module by a time, investigate the most promising solution and implement it into the measurement environment.
Depending on the kind of thesis (Semester or Diploma thesis) and the number of students, the extent of this work will be adapted.

Work: theory (20%), Matlab programming (70%), measurements (10%)
Requirements: Matlab programming

Background

GaN-based semiconductor devices are currently the focus of great interest in academia and industry. These semiconductor alloys have a wide bandgap (> 3.4 eV) and high electric breakdown fields, which allow them to be used for the fabrication of short-wavelength (blue, UV) optical devices and high-power electronics. Our group focuses on the development of High Electron Mobility Transistors (HEMTs) using both the more established AlGaN/GaN system and the new AlInN/GaN layers. Recently, the Millimeter-Wave Electronics Group demonstrated AlGaN/GaN HEMTs on silicon substrates with cutoff frequencies of more than 100 GHz, and established a new world record with 205 GHz HEMTs using AlInN/GaN layers grown on silicon carbide (SiC).

Motivation

Given the origin of the two-dimensional electron gas (2DEG) channel created at the interface between two undoped materials by piezoelectric and spontaneous polarization, the modeling of GaN –based HEMTs still presents many challenges. Another challenge arises from the peculiar form the velocity-field characteristics, displaying negative differential mobility and quasi-saturation behavior. Moreover, surface states have a major influence in the electron density of the 2DEG, and their control is of the utmost importance for transistor performance. Obtaining a good physical model which agrees with experimental data constitutes a fundamental objective, because it would prove extremely useful in transistor design by helping the understanding and improvement of some crucial aspects limiting high-speed and highpower operation.

Goals

The thesis work will focus on the modeling and analysis of crucial factors in the performance of GaN-based HEMTs. Among these, the student will work to identify the most relevant electron transport mechanisms in the device, to quantify the effective gate length with different material systems, and to simulate the effect of traps at the transistor surface and in the bulk. Models will be immediately related/compared to experimental data from our HEMTs, and the student will attempt to optimize models so they better reproduce measured results. Given the great interest of GaN-based devices, a successful thesis in this field will constitute a worthy addition to the curriculum of a students interested in microelectronics, semiconductor device engineering/physic and/or microwave electronics.

Prerequisites

Prerequisites for this thesis are knowledge of semiconductors basics, basic notions of solid state physics. The modeling will be through a simulation performed with a commercial software package. Some basics of programming language are a bonus, but are not strictly required. Depending on the student`s aptitude and motivation, the work might include some experimental work. This proposal, limited to the development of a single phenomenology, can also be carried out in a single semester, for a semester thesis work.

Background

In the MWE-laboratory we have a leading tradition of working on the manufacturing process and cryogenic characterization of InP-transistors for collaborative projects with the European Space Agency (ESA). The experimental determination of the influence of different parameters (layer thickness, material composition, doping, etc.) on the DC and RF characteristics of transistors always requires considerable time and financial resources.

Goal

For this reason, this Semester or Diploma thesis focuses on the development of a two-dimensional HEMT simulation model which should be elaborated using the commercially available simulation software external page Sentaurus Device from Synopsis. The work requires becoming familiar with the functionality of an HEMT device and the simulation software (tutorials and a detailed manual are available).

Follow up

In the following project phase a simulation model based on an existing HEMT structure should be developed and the results compared with measurements of an actual device. If the validity of the model is verified, an optimization of the layer parameters with respect to reducing the parasitic elements or enhancing the cutoff frequency can be done.

Work: theory (30%), simulation (60%), measurements (10%)

Requirements: Basics of semiconductor physics