Low-Frequency Noise in TFETs

Abstract

Nanowire tunnel field-effect transistors (TFETs) were investigated by carrying out noise measurements and low-temperature DC measurements. The TFET tunnelling junction was realised by a GaSb/InAs heterojunction resulting in a broken band gap. TFET noise currents were measured at frequencies between 10 Hz and 1 kHz. The results imply that noise in TFETs at the current state of development is dominated by generation-recombination processes caused by traps in the gate oxide. Trap densities between 10^20 cm^-3 eV^-1 and 10^22 cm^-3 eV^-1 were extracted from the noise measurements. The temperature-dependent DC measurements show that the TFETs' off-current is sensitive to the temperature, with lower off-currents at lower temperatures. This indicates that it is not only the tunnelling junction which is governing the off-current. It is concluded that in the devices' off-state electrons can still tunnel into the channel area through the broken band gap but require additional thermionic excitation over the bent channel conduction band to constitute a current.

Popular Abstract

The ever-growing demand for electronic devices in all areas of our lives can only be satisfied due to the constant development of today’s most essential of all active electronic devices – the metal-oxide-semiconductor field-effect transistor (MOSFET). However, its further development is drawing to an end, so coming up with alternatives is of utmost importance. One of the most promising ones is the tunnel field-effect transistor (TFET).
A MOSFET basically is an electrical switch. The current between two of the device’s contacts – source and drain – can be controlled by a third contact, the gate. In digital applications, such as all our computers and smartphones, MOSFETs only switch between an on- and an off-state, meaning flowing current or almost no current, respectively. The ability to switch between these two states as fast as possible is what governs a MOSFET’s speed and energy-efficiency.
Over the last 50 years MOSFETs have undergone constant development to increase these measures. Due to the underlying physical principles that MOSFETs are based on, this development is drawing to an end. In a MOSFET electrons have to overcome an energy barrier to establish a current. With the gate contact this barrier can be raised (off-state) or lowered (on-state). This principle establishing the current is at the same time the principle which limits further scaling of MOSFETs as there are always a few electrons which can overcome the barrier – even in the device’s off-state.
The idea for TFETs to overcome this limit is to control the current by opening or closing a narrow gap in the energy structure of the device. Instead of overcoming a barrier the electrons have to tunnel through it. In contrast to the MOSFET structure the electrons on the source side of the TFET structure face a restriction from the top which reduces the off-current.
In my thesis I contribute to the development of TFETs as successors of or complements to MOSFETs by examining electrical noise and the current temperature dependence in TFETs. A well-known form of electrical noise is noise which finds its way into an audio signal (e. g. buzzing speakers). However, noise is present in all electrical signals and examining the noise in TFETs gives information about which parts of the devices require particular improvement to finally lead to industrially applicable TFETs benefitting the broad public.

• author: Hellenbrand, Markus
• supervisor: Lars-Erik Wernersson
• organization: Department of Physics
• course: FYSM60 20151
• year: 2015
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Tunnel Field-Effect Transistor, Steep-Slope Devices, III-V, InAs, GaSb, Broken Band Gap
• language: English
• id: 5435992
• date added to LUP: 2015-06-08 18:39:42
• date last changed: 2015-06-08 18:39:42