Development of field effect transistors (MESFET) and optically activated transistors (OPFET) based on single crystal and polycrystalline diamond films.

Project leaders

Gennaro Conte, Viktor Ralchenko

Agreement

RUSSIA - RAS old - Russian Academy of Sciences old

Call

CNR/RAS 2011-2013

Department

Materials and Devices

Thematic area

Physical sciences and technologies of matter

Status of the project

New

Research proposal

The present proposal focuses on the realization of devices that receive signals of desired spectral band and sensitivity suitable as switches operating at GHz frequencies. High quality, low-surface roughness, free-standing polycrystalline diamond films will be used for the fabrication of devices with contacts suitable for RF characterization up to 40 GHz. Further, new devices will be designed and fabricated to study the dynamic response to UV light excitation starting from modulated light up to nanosecond excimer lasers pulses, by using opaque-gate MESFET structures. On-off ratio, output resistance as well as switching noise will be analyzed through static and dynamic characterization. Such optical receivers will be based on surface channels induced by hydrogenation of diamond specimens with charge diffusion below the opaque gate and drift in the illuminated gate-drain region. The generation-transport mechanism in the low-intensity regime will be also inferred to evaluate and model the light sensitivity-speed of response relationship. This activity is aimed to fabricate MESFET devices suitable to be used as low-interference power switches in next generation safety information transport.
State of the art
In an environment where external radio-frequency (RF) signals can interact with power electronics, for example fly-by-light architectures for next generation avionics, electromagnetic interference (EMI) is a critical issue. Recent research by US Air Force has shown that tangible reduction in weight, volume and cost are possible through the application of emerging photonic technologies for vehicular power-management systems by elimination of EM shielding around copper wiring, which is replaced by lightweight optical fibres. In this respect, next-generation photonic power-electronic systems based on optically triggered devices (OTDs) provide key advantages over conventional electrically triggered device based switching power electronics. Prospective applications for such OTDs include power management systems in military and commercial aircrafts, electric warships, naval planes and helicopters. Such devices are triggered directly by light and therefore does not need a voltage differential between source and gate like most widely used power devices, i.e. MOSFET, MESFET or IGBT. Direct optically controlled power devices is the first major step toward photonic power electronic systems. Main desirable properties of such a device would be: optical and electrical triggering gate; on-off controllability with a single optical monochromatic source; optical fibre coupling; high breakdown voltage; fast switching time and low on-state resistance. Such requirements can be matched by UV operated Schottky junctions FET based on polycrystalline diamond. The finalization of this research to the exploitation of the poly-diamond technology as emerging and able to satisfy future needs of the industry is based on the unpaired chemical and physical characteristics of this material. Polycrystalline chemical vapour deposited (CVD) diamond films have many excellent physical and chemical properties, which have approached or reached the level of natural diamond. Being a kind of continuous film, CVD diamond is much better than natural diamond and high-temperature high-pressure (HPHT) synthetic diamond, which are in the shape of grains, to be used as the coatings for heat dissipation or wear resistance. It can be expected that CVD diamond films will be widely used in lots of fields such as acoustics, optics and power electronics due to the best Johnson’s figure of merit, higher thermal conductivity and cohesive energy.surface conductivity make H-terminated diamond surface very appealing for the fabrication of electronic devices. Surface p-type channel devices show extremely low leakage current and high breakdown voltages. Single-crystal diamond layers show high sheet charge densities in the range of 1013 cm-2 and carriers mobility above 100 cm2/V/s.
M. Kasu et al, have demonstrated high RF output power equal to1.26 W @ 1 GHz, for a device with a gate width of 1 mm and length of 0.4 um. The maximum power gain of this surface channel MESFET realized on homo-epitaxial diamond was 23.2 dB, with a power added efficiency of 56.3%. Exploitation of polycrystalline diamond produced by research groups active in this field demonstrated the foreseen performance with MESFETs operating in the frequency range 0.3-30 GHz. The best result obtained on a sub-micrometer polycrystalline diamond MESFET has been recently presented by a Japanese group, and it has been well over those obtained by similar devices realised on natural diamond: transition frequency (fT) equal to 45 GHz and fmax equal to 120 GHz, with drain current density of 550 mA/mm at VGS = –3.5 V, and maximum trans-conductance gm = 143 mS/mm at VDS = –8 V.  No indication of power output was reported. This group is leading the research in this field. The proposing group currently fabricates surface channel MESFETs with fT=10 GHz and fmax= 35 GHz, transconductance equal to 50 mS/mm and density of current of 140 mA/mm. Because of its wide bandgap (5.47 eV) diamond is expected to be an ideal candidate for ultraviolet activated devices blind to the visible spectra. Moreover, due to the high thermal conductivity, the fabrication of power switches optically activated could be an important application for this high performing electronic material. Selectivity, sensitivity and high speed operation are stringent requirements for such application, as well as optical gain. The first optically activated diamond FET was demonstrated by Lansley et al. by using a semitransparent-gate structure. The possibility to use opaque-gate FET structures to monitor chopped UV light has been recently proposed for polycrystalline diamond MESFETs on hydrogen terminated polycrystalline diamond. Observed trends were interpreted on the basis of the theory proposed for ion-implanted GaAs MESFET with opaque-gate.

Research goals

The demonstration of high frequency and high power switching performances is here intended as a strategy which imposes to meet a stringent set of requirements on the specific electronic physical characteristics to be obtained. This will be used as a testing field for the specific application on hand, but also to produce that know-how needed to achieve the control of the electronic quality of polycrystalline diamond, to such extent to allow the complementation and the substitution of silicon, and III-V semiconductors, in many passive and active electronic applications and to open the market to this material in electronics.
The main scientific objective of the collaboration is to improve the understanding of the factors which control the performance of UV activated opaque-gate transistors through the analysis of the response in the dark and under illumination. The control of surface conductivity, and then device trans-conductance, will be considered together with the collection of data on 2DHG for conductivity transport modelling development of diamond MESFETs suitable as the main detector component of guided UV light. The main technical objective is to transform this know-how into realisation of improved optically activated devices.
During the project will be fabricated and characterized:
-  surface channel MESFETs suitable for high-frequency and high-power output operation
-  opaque-gate UV activated FETs
-  light sensitive switches

Last update: 08/06/2025