PHYSICAL MODELLING OF TUNNEL DIODES FOR TERAHERTZ FREQUENCY APPLICATIONS

Abstract

It has been over a half-century since Leo Esaki in the early 1960 reported the first tunnel diodes. These first quantum electron tunnelling devices were named Esaki diode in his honour. This was a testament to quantum transport in a semiconductor which was subsequently demonstrated to be extremely stable. The work reported in this thesis probed into the functionality of three new and different types of tunnel diodes, namely a Single Barrier Asymmetric Spacer Tunnel (ASPAT), a Double Quantum Well Asymmetric Spacer Tunnel (QASPAT), and a Double Barrier Quantum Well Resonant Tunnelling Diode (RTD), with the aim to investigate the capabilities of each tunnel device. Adorned with the status of a maturing tunnel diode within the semiconductor arena, the ASPAT diode appears to be an exceptionally promising candidate for high-frequency applications, due to its highest theoretical cut-off frequency which can reach up to 2 THz. Its asymmetric spacer structure leads to a unique asymmetrical I-V characteristic that offers significant improvement over Schottky and Planar Doped Barrier (PDB) diodes without sacrificing sensitivity or dynamic range aspects. Such stimulating feature is indeed paramount for implementations of high-speed detectors, especially for operations in the mm-wave/THz regime. In this research, an original and novel idea to tweak the ASPAT structure has led to a most remarkable feature that reflects a dual-function device in a single diode. Originating from the ASPAT structure, the I-V characteristic has zero-bias turn-on feature but showed a most interesting feature of negative differential resistance (NDR) region in reverse bias enabling this device to function as both a detector and an oscillator, depending on bias voltage. This device is temperature independent as its I-V characteristics does exhibit insensitivity to temperature over a wide range of temperature variations. It displays less than ~ 5 % current change when compared to the I-V characteristics at selected temperatures (from 77 K to 400 K), to the I-V at room temperature. Quantum based devices, particularly RTD, appears to be promising devices that possess the capability of providing close to ~ 1 mW of RF power in the millimetre wave and THz regions of the electromagnetic spectrum. This feature is mainly attributed to their unique NDR feature showcased in the I-V characteristic of the RTDs reported here. Following modelling and validation of ASPAT and QASPAT diodes with high indium-rich material profiles, the study was then extended to quantum modelling of advanced double barrier In0.8Ga0.2As/AlAs RTD. The work culminated in an accurate physical model which was exploited to minimise fabrication costs in developing experimental devices. The two-dimensional (2D) simulation for all devices under study demonstrated outstanding agreement with the measured DC and RF characteristics validating the models used.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Mohamed Missous (Supervisor) & Robin Sloan (Supervisor)