Rowland C. Clarke

Growth of large SiC single crystals

We have grown 6H-polytype SiC single crystal boules up to 60 mm in diameter by the physical vapor transport process at 2300 o C. [0001] oriented substrate wafers prepared from these undoped crystals exhibit resistivities of up to 10 5 Ω cm and etch pit defect densities of 10 4 -10 5 cm -2 . Epitaxially-grown microwave MISFIT structures exhibit 5 GHz cutoff frequency; the highest reported to date

Large diameter 6H-SiC for microwave device applications

6H-polytype SiC single crystals with diameters up to 50 mm and lengths up to 75 mm have been grown in the c-and a-axis directions by physical vapor transport (PVT) at growth rates of 0.25 to 1 mm h -1 . Undoped crystals grown from purified source material reveal residual impurity concentrations in the 10 16 cm -3 range and resistivities up to 1000 Ω-cm. N + crystals with resistivities < 0.02 Ω-cm have been produced by controlled nitrogen doping. PVT-grown SiC crystals are characterized by dislocation densities of 10 4 to 10 5 cm -2 and can also exhibit micropipe defects in the 10 2 to 10 3 cm -2 range

SiC microwave power technologies

Two SiC transistors that are investigated for microwave power applications are the 4H-SiC static induction transistor (SIT) and the 4H-SiC metal-semiconductor field-effect transistor (MESFET). Ultrahigh frequency 4H-SiC SITs have demonstrated record-breaking pulsed power per package (900 W) with excellent associated power-added efficiency (PAE) of 78%. S band 4H-SiC MESFETs have shown a record power-density of 5.6 W/mm and 36% PAE, as well as 80 W continuous-wave (CW) power (1.6 W/mm), with an associated PAE of 38%. X-band MESFET power density of 4.3 W/mm was obtained for exploratory CW devices. These performance gains are afforded by the advantageous material properties of silicon carbide. SiC SIT technology offers many military system advantages including lower cost, lower weight, higher power and high temperature of operation and higher efficiency transmitters with minimal cooling requirements. SiC RF MESFETs and circuits are candidates for use in efficient linear transmitters for commercial and military communications.

RF performance of SiC MESFET's on high resistivity substrates

State-of-the art SiC MESFETs showing a record high f/sub max/ of 26 GHz and RF gain of 8.5 dB at 10 GHz are described in this paper. These results were obtained by using high-resistivity SiC substrates for the first time to minimize substrate parasitics. The fabrication and characterization of these devices are discussed.<<ETX>>

High power 4H-SiC static induction transistors

Static induction transistors have been demonstrated in 4H-SiC. SiC specific semiconductor processing technologies such as epitaxy, reactive ion etching, and sidewall Schottky gates were employed. Under pulsed power test conditions, 4H-SiC SITs developed a maximum output power of 225 W at 600 MHz, a power added efficiency of 47%, and a gain of 8.7 dB. Maximum channel current was 1 A/cm, and the maximum blocking voltage was 200 V.

High voltage operation in class B GaAs X-band power MESFETs

Surface trapping effects are shown to affect adversely the RF power performance of high-voltage GaAs MESFETs and a model is presented to explain them. It is shown that the adverse effects of surface trapping can be minimized by: (1) including an undoped layer near the surface, (2) reducing the distance between the gate and n/sup +/ ledge, and (3) making the gate recess narrower than the gate. Devices fabricated with such a structure showed excellent RF power performance at 10 GHz: P/sub 0/=678 mW/mm, G /sub A/=6.8 dB, and eta /sub PA/=51.1% at a drain-bias voltage of 12 V. The design of devices to minimize surface-trapping effects is also expected to lead to self-passivating devices that will be inherently more reliable and show less 1/f noise. The high-voltage, high-efficiency devices described here will be applicable in airborne phased array radar systems where power supply requirements and heat dissipation problems limit system performance.<<ETX>>

30 W VHF 6H-SiC power static induction transistor

6H-SiC Static Induction Transistors (SITs) have been demonstrated, using SiC specific semiconductor processing technologies such as, VPE, reactive ion etching and self aligned sidewall Schottky gates. Under test conditions, 6H-SiC SITs developed 38 W of output power at 175 MHz, a power added efficiency of 60%, and an associated gain of 10 dB. The maximum channel current was 300 mA/cm, and the maximum blocking voltage was 200 V.

Advances in silicon carbide (SiC) device processing and substrate fabrication for high power microwave and high temperature electronics

High-power density, temperature tolerant silicon carbide (SiC) electronics offer an exceptional opportunity to increase the performance and lower the cost of many existing and emerging military and commercial products. Surveillance and tactical radar systems, compact electric tank and aircraft engine controls, high reliability aviation electronics, and radiation resistance satellite components are some examples. Recent technology advances have brought this potential payoff closer to reality. These include the fabrication of a record-setting MESFET device with 12 dB gain at 2 GHz and 2 W/mm of power at 1 GHz and the worlds first 2-inch diameter high-resistivity SiC wafers for planar devices and low resistivity substrates for power devices. Vertical transistor structures have also been fabricated using both Schottky barrier and MOS gates.<<ETX>>

Vapor phase 6H and 4H SiC epitaxy for high-speed devices

Silicon carbide (SiC), a wide bandgap semiconductor, is currently being developed for enhanced high-power and high-temperature microwave devices. Silicon carbide wafers, now available at up to two-inch diameter with resistivities from 0.02 /spl Omega/-cm to 10/sup 7/ /spl Omega/-cm, still exhibit features such as micropipes, surface scratches, and inclusions of other polytypes. Vapor phase epitaxy (VPE) of 6H and 4H SiC, typically performed between 1450 and 1600/spl deg/C using silane and propane reagents, is impacted greatly by the quality of these wafers and the conditions used during in situ etching or the initial stages of growth. By optimizing growth conditions, device-quality homoepitaxial 6H and 4H-SiC has been grown with near specular morphology, background doping levels of less than 1/spl times/10/sup 14/ cm/sup -3/, and controlled n- and p-type doping from less than 5/spl times/10/sup 15/ cm/sup -3/ to greater than 1/spl times/10/sup 19/ cm/sup -3/.

Silicon carbide microwave MESFET's

Summary form only given. The authors describe record-setting 5-GHz SiC MESFET performance and the effects of device design on achieving these results. Bulk growth of 6H-SiC was performed using a physical vapor transport process, and the resultant undoped single-crystal boules were sliced and polished to generate 1-in-diameter wafers. These wafers were then used as substrates for the chemical vapor deposition of doped silicon carbide active layers. Wafers contained 24 chips, each consisting of an array of MESFETs having systematically varied geometry. A sample DC characteristic from a 320- mu m periphery MESFET showed a knee voltage of 8 V, a transconductance of 20 mS/mm, a maximum channel current of 210 mA/mm, and a gate-to-drain breakdown voltage of 100 V. Automated RF probing was used to obtain wafer maps of small signal gain, F/sub T/, and F/sub max/, revealing an excellent transistor yield of 87%. The highest-gain MESFETs in the array developed 12 dB of gain at 2 GHz with a cutoff frequency of 5 GHz. >