M. Kubovic

Diamond field effect transistors—concepts and challenges

Abstract Field effect transistors (FETs) in diamond should outperform FET structures on other wide bandgap materials like SiC and GaN in high power/high temperature applications due to the ideal diamond materials properties. However, the technology of these structures proved difficult leaving two device concepts to investigate: (1) the boron δ-doped p-channel FET and (2) the hydrogen induced p-type surface-channel-FET. The δ-channel-FET approach follows a traditional design path of power FET structures. Here, simulation results have enabled the extrapolation of a maximum RF output power to 27 W/mm, a value which is indeed higher than for any FET based on III-Nitrides or SiC. However, due to the narrow technological parameter window, fabricated δ-channel-FETs are still well behind expectations. In contrast, concerning the surface-channel-FET the physical/chemical nature of its channel remains still under discussion. Nevertheless, results obtained with this FET concept yielded a VDmax>200 V (LG=1 μm) and a IDmax>360 mA/mm a fT=11.5 GHz and fmaxU>40 GHz (LG=0.2 μm) and a recently obtained RF power measurement at 1 GHz. Furthermore, the 1 GHz power measurement result has been obtained on a diamond quasi-substrate grown on a Ir/SrTiO3 substrate. This result may therefore open up the perspective for wafer scale diamond electronics.

Field effect transistor fabricated on hydrogen-terminated diamond grown on SrTiO3 substrate and iridium buffer layer

Up to now high performance devices have mainly been realized on HTHP single crystals with limited size. However, diamond of single crystal quality can also be grown on SrTiO 3 substrate using an iridium buffer layer. For the first time p-type surface channel FETs with sub-micron gatelength have been fabricated on such a substrate. Small signal, large signal and power measurements could be performed up to gigahertz frequencies. This has resulted in cut-off frequencies f T =9.6 GHz, f max(MAG) = 16.3 GHz and f max(U) =17.3 GHz for L G =0.24 μm. For L G =0.9 μm a saturated RF power at 1 GHz of 0.2 W/mm could be measured. These results indicate the high quality of this quasi-substrate of approximately 0.6 cm 2 in size.

Enhancement and Stabilization of Hole Concentration of Hydrogen-Terminated Diamond Surface Using Ozone Adsorbates

The p-type conductivity of H-terminated diamond surface can be linked to adsorption of a specific gas species on the surface. O3, NO2, NO, and SO2 were identified as adsorbates, which induce holes on the H-terminated diamond surface. Among them, exposure to O3 increases hole concentration the most. The O3-increased concentration remains high even after exposure to the gas has stopped, indicating that ozone is the most stable adsorbent. X-ray photospectroscopy spectra of O3-adsorbed H-terminated diamond surface show partial oxidation of the surface and upward band bending and are very similar to those of NO2 exposed diamond surfaces.

Improvement of Hydrogen-Terminated Diamond Field Effect Transistors in Nitrogen Dioxide Atmosphere

The electrical properties of field effect transistors (FETs) fabricated on hydrogen-terminated diamond have been greatly improved by exposing the diamond surface to nitrogen dioxide. Exposure to NO2 gas significantly increased the hole sheet charge density up to 1.3×1014 cm-2, which is several times higher than previously reported values. FETs exposed to NO2 gas exhibited lower source and drain resistances, which facilitated a 1.8 fold increase in maximum drain current, transconductance increased 1.5 times and power gain cut-off frequency increased 1.6 times.

Sorption properties of NO2 gas and its strong influence on hole concentration of H-terminated diamond surfaces

The hole concentration of hydrogen-terminated diamond surfaces was studied during exposure to different concentrations of NO2 gas. The hole concentration increased during adsorption of NO2 molecules on the diamond surface, and decreased when the exposure stopped and NO2 molecules desorbed from the surface. The increase in hole concentration can be directly linked to the NO2 concentration. The low NO2 concentration in air (∼20 ppb) is responsible the hole concentration normally measured in air, and with increasing NO2 concentration the maximum hole concentration increases even more. The time evolution of hole concentration was analyzed using the Elovich sorption model. Further analysis based on the Ritchie model indicated that an adsorbed NO2 molecule occupies two different surface sites. Temperature-dependent measurements indicate low activation energy between 0.1 and 0.2 eV.

A new diamond based heterostructure diode

A diamond based heterostructure diode containing a p-type doped diamond active layer and an n-type doped ultra-nano-crystalline top layer has been investigated. Analysis suggests that the configuration is that of a merged diode, containing two areas of different interfacial barrier potentials in parallel related to the ultra-nano-crystalline grains and the grain boundaries, respectively. Thus this heterostructure may be ideally suited to combine low forward losses with high blocking voltages in diamond high power rectifiers.

Electronic properties and applications of ultrananocrystalline diamond

Ultrananocrystalline diamond (UNCD) is a 3–5 nm grain size material with many of the properties of diamond. Whilst intrinsic UNCD films display a mild p-type characteristic with high resistivity, the addition of nitrogen to the gas phase during deposition renders the material n-type with low resistivity and activation energy. Hall effect measurements as a function of temperature show that this conductivity mechanism is semi - metallic, with the carrier concentration decreasing very gradually with decreasing temperature. Increasing the nitrogen content in the gas phase during deposition results in higher carrier concentrations in the deposited films and lower activation energies. The carrier mobilities of the films are limited by the grain size of the films. A prototype heterostructure diode is demonstrated, combining single crystal and ultrananocrystalline diamond.

Properties of (111) Diamond Homoepitaxial Layer and Its Application to Field-Effect Transistor

The (111)-oriented chemical-vapor-deposited diamond homoepitaxial layers with low defect density exhibited well-resolved free-exciton transitions in cathodoluminescence at 13 K and a sharp peak at 1332 cm-1 (linewidth: 1.9 cm-1) in Raman scattering. Furthermore, using these (111) layers, we fabricated metal-semiconductor field-effect transistors (FETs). FETs with an 11-µm-long gate exhibited a maximum drain current of 24 mA/mm and maximum transconductance of 14 mS/mm. These values are of the same order as those for the (001) orientation.

First diamond FET RF power measurement on diamond quasi-substrate

Diamond is an exceptional widegap material predestined for high power high frequency electronics. However, up to now devices could only be fabricated on chips cut from synthetic single crystal stones of small size, and device performance had been restricted to small signal measurements due to severe large signal instabilities However, diamond 100-oriented quasi-substrates of single crystal quality can be grown on ceramic substrates such as SrTiO/sub 3/ using a single crystal iridium buffer layer. In this experiment we have succeeded in fabricating the first surface channel FETs (using a hydrogen induced p-type channel) on such a diamond quasi-substrate. The FETs show high electrical stability, enabling large signal and power measurements for the first time, and thus demonstrating the feasibility of diamond microwave high power electronics.

Diamond-MESFETs - synthesis and integration

We report the utilization of synthetic diamond grown by chemical-vapour-deposition (CVD) for use as metal-semiconductor field-effect-transistors (MESFETs). The lack of a shallow n-type donor means that diamond-based electronic devices are unipolar (p-type). The devices presented in this paper are based on delta-doping. Delta-doping stands for the use of very thin (<5 mm) highly doped (NA 1020 cm-3 ) buried layers. This approach poses a huge challenge in terms of synthesis as well as processing. First successful attempts of fully integrating working delta-doped diamond MESFETs are presented