Asymmetric contacts or split gate geometries can be used to obtain rectification, electroluminescence (EL) and photocurrent from carbon nanotube field effect transistors. Here, we report devices with both split gates and asymmetric contacts and show that device parameters can be optimised with an appropriate split gate bias, giving the ability to select the rectification direction, modify the reverse bias saturation current and the ideality factor. When operated as a photodiode, the short circuit current and open circuit voltage can be modified by the split gate bias, and the estimated power conversion efficiency was 1×10-6. When using split gates and symmetric contacts, strong EL peaking at 0.86 eV was observed with a full width at half maximum varying between 64 and 120 meV, depending on the bias configuration. The power and quantum efficiency of the EL was estimated to be around 1×10-6 and 1×10-5 respectively.
The electrical properties of carbon nanotube FETs (CNTFETs) have been studied in detail. The conduction type of the
CNTFETs was dependent on the work function of the contact metal, which suggests that Fermi level pinning at the
metal/nanotube interface is not strong. Based on the two-probe and four-probe resistance measurements, it has been
shown that the carrier transport at the contact is explained by the edge contact model even in the diffusive regime. The
chemical doping using F4TCNQ was effective in reducing not only the channel resistance but also the contact resistance.
In the CNTFETs fabricated using plasma-enhanced (PE) CVD-grown nanotubes, the drain current of the most of the
devices could be modulated by the gate voltage with small OFF current suggesting the preferential growth of the
nanotubes with semiconducting behavior.
Multichannel top-gate CNTFETs with horizontally-aligned nanotubes as channels have been successfully fabricated
using CNT growth on the ST-cut quartz substrate, arc-discharge plasma deposition of the catalyst metal, and ALD gate
insulator deposition. The devices show normally-on and n-type conduction property with a relatively-high ON current of
13 mA/mm. CNTFETs with nanotube network have also been fabricated by direct growth on the SiO2/Si substrate using
grid-inserted PECVD and using catalyst formed on the channel area of the FETs. The uniformity of the electrical
properties of the network channel CNTFETs were very good.
Finally, it has been shown that the surface potential profile measurement based on the electrostatic force detection in the
scanning probe microscopy was effective in studying the behavior of the CNTFETs such as the transient behavior and
the effect of the defects.
We have studied transient behavior of AlGaN/GaN high electron mobility transistors (HEMTs) using various kinds of
measurement techniques such as frequency dispersion of the drain conductance, low-frequency noise, drain current
DLTS, and Kelvin probe force microscopy. It has been shown that the frequency dispersion has a correlation with the
low-frequency noise. In order to study the transient behavior of the device in more detail, the drain current DLTS was
applied to various types of HEMTs. Even though electron-trap-related negative peaks were observed for all types of
devices, surface-states-related positive peaks were observed only for devices without Si3N4 passivation film. It has been
shown by KFM measurement that the surface potential of the device which was subjected to the gate bias stress
increased with time due to the emission of electrons which were captured at the surface states. These phenomena are
consistent with the current collapse model where the collapse is caused by the electrons which are injected from the gate
electrode and captured at the surface states.
We have fabricated carbon nanotube (CNT) FETs using position-controlled growth technique. The CNT-FETs showed p-type conduction. The chirality of the CNT FET channel was determined by using microphotocurrent spectroscopy. Novel peapods FETs with fullerenes inserted in the CNTs were also fabricated. They showed ambipolar I-V characteristics with both n- and p-type conductions depending on the gate voltage. The ambipolar behavior was explained based on the Schottky-barrier-controlled transistor model, where the transistor action occurs primarily by changing the Schottky contact resistance by the gate voltage. It has been shown that the bandgap of the peapod FETs was dependent on what kind of fullerene was inserted in the CNT channel. We have also demonstrated that it was possible to control the conduction type of the FET channel by choosing contact metal with different work function without any doping in the CNT channel.