A. Aleksov

Diamond surface-channel FET structure with 200 V breakdown voltage

An enhancement mode diamond FET using a hydrogen-terminated surface as hole conductive channel has been fabricated with 200 V gate to drain breakdown voltage. At the 8.5-/spl mu/m gate length the maximum drain current was 22 mA/mm. 90 mA/mm maximum drain current was obtained at a gate length of 3.0 /spl mu/m. Scaling to below 1 /spl mu/m gate length assuming undegraded breakdown conditions will result in a projected RF power handling capability above 6 W/mm.

RF performance of surface channel diamond FETs with sub-micron gate length

Abstract Diamond hydrogen-induced surface channel field effect transistors (FETs) were fabricated with gate lengths down to 0.2 μm in part using electron beam lithography. Down-scaling of the gate-length resulted in both improved DC- and RF characteristics, especially for a 0.2-μm gate length in a maximum output current of I D max =360 mA/mm with a peak transconductance of 148 mS/mm. The optimum cut-off frequencies were f T =11.5 GHz, f max(MAG) =31.7 GHz and f max(U) =40.2 GHz. A maximum drain voltage of 68 V was obtained before pattern-related destructive breakdown occurred. This allows estimation of the RF power handling capability to above 3.0 W/mm. The data are the highest reported for diamond FETs. Scaling of the device parameters with gate length allows to estimate a velocity-limited maximum output current to 750 mA/mm and an f T above 20 GHz at 0.1-μm gate length. At high drive, current drift and current compression is observed in the quasi-DC output characteristics as well as in first microwave large signal measurements. These instabilities seem at present to be the main hurdle of hydrogen-induced surface channel FETs in high power microwave applications.

Prospects of diamond devices

The prospects of two classes of diamond devices are reviewed, namely electronic devices on single-crystal substrates and microsystems devices on Si substrate. The transistor structures on single-crystal diamond represent still proof-of-concept experiments; however, they already allow us to extract their potential. The microsystems actuator and sensor devices already reflect the materials properties in their characteristics. Two of the most complex structures and future trends are discussed.

pH sensing by surface-doped diamond and effect of the diamond surface termination

Abstract For the first time, pH sensing of liquid solutions by a free diamond surface is demonstrated using ungated FET structures. The surface channel is in direct contact with the liquid, forming a liquid gate. The sensors tested were acidic and basic water solutions with pH varying from 1 to 13. Three different types of surface channel were tested: (a) p-type boron-doped channel /hydrogen-terminated surface; (b) p-type hydrogen-induced surface channel; and (c) boron-doped channel/oxygen-terminated surface. The surface termination determined the pH response. For the H-terminated surface (a, b), the surface channel was gradually depleted with increasing pH. No charge transfer across the diamond surface to reach equilibrium between the semiconductor and liquid solution was observed. It is proposed that CH surface bonds pin the Fermi level at the valence band edge due to a high density of states. Shifting the surface state level into the bandgap results in channel depletion. The effect is believed to be related to interaction between the CH surface dipoles and negatively charged radicals in solution. The oxygen termination (c) resulted in a high density of surface states at approximately 1.7 eV above the valence band. No pH sensitivity was observed, thus the energy level of the state is not moved.

Microwave performance of diamond surface-channel FETs

S-parameters measurements were carried out on diamond-based FET devices with a p-type channel induced by hydrogen surface termination extracting f/sub T/ and f/sub max/ for devices with gate lengths ranging from 5 /spl mu/m to 0.2 /spl mu/m. For the 0.2 /spl mu/m gate length FET f/sub T/=11.55 GHz and f/sub max/ (MAG)=33.3 GHz values were obtained. High f/sub max/ (MAG)/f/sub T/ ratios of above 2.5 were obtained for all devices. Further downscaling may result in an f/sub T/ above 20 GHz and in addition, an f/sub max/ (MAG) above 50 GHz.

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.

Diamond power FET concept

In this paper a novel concept for diamond power FETs is presented. This concept is based on a /spl delta/-doped active channel using homoepitaxial CVD-layers on 100-oriented single crystals. The channel is controlled by a recessed sub-/spl mu/m pn-junction gate. Based on technological building blocks developed previously, the structure has been simulated and 13 W/mm RF power are predicted. First fabricated FETs show that the concept is feasible.

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.

Microwave performance of diamond surface-channel FET

In this paper, the microwave characteristics of diamond FETs with 0.2/spl mu/m gate length (L/sub G/) are discussed.

Advances in diamond surface channel FET technology with focus on large signal properties

Field effect transistors based on a hydrogen induced p-type surface channel (surface channel FETs) have shown steady progress in the past. Devices with sub-/spl mu/m gatelength have been fabricated and cut-off frequencies up to the mm-wave range could be extracted. However, large signal and power performance could only be reported recently. This is due to severe stability and degradation problems. These phenomena are largely related to the highly polar H-terminated diamond surface, although details are still in discussion. This contribution describes these instabilities and the recent progress obtained. To some extent this may also shine some light onto the nature of instabilities observed in GaN based devices.