Maurice Weiner

4H-SiC UV photo detectors with large area and very high specific detectivity

Pt/4H-SiC Schottky photodiodes have been fabricated with the device areas up to 1 cm/sup 2/. The I-V characteristics and photoresponse spectra have been measured and analyzed. For a 5 mm/spl times/5 mm area device leakage current lower than 10/sup -15/ A at zero bias and 1.2/spl times/10/sup -14/ A at -1 V have been established. The quantum efficiency is over 30% from 240 to 320 nm. The specific detectivity, D/sup */, has been calculated from the directly measured leakage current and quantum efficiency are shown to be higher than 10/sup 15/ cmHz/sup 1/2//W from 210 to 350 nm with a peak D/sup */ of 3.6/spl times/10/sup 15/ cmHz/sup 1/2//W at 300 nm.

4H-SiC normally-off vertical junction field-effect transistor with high current density

4H-silicon carbide (SiC) normally-off vertical junction field-effect transistor (JFET) is developed in a purely vertical configuration without internal lateral JFET gates. The 2.1-/spl mu/m vertical p/sup +/n junction gates are created on the side walls of deep trenches by tilted aluminum (Al) implantation. Normally-off operation with blocking voltage V/sub bl/ of 1 726 V is demonstrated with an on-state current density of 300 A/cm/sup 2/ at a drain voltage of 3 V. The low specific on-resistance R/sub on-sp/ of 3.6 m/spl Omega/cm/sup 2/ gives the V/sub bl//sup 2//R/sub on-sp/ value of 830 MW/cm/sup 2/, surpassing the past records of both unipolar and bipolar 4H-SiC power switches.

Nonlocal effects in thin 4H-SiC UV avalanche photodiodes

The avalanche multiplication and excess noise characteristics of 4H-SiC avalanche photodiodes with i-region widths of 0.105 and 0.285 /spl mu/m have been investigated using 230-365-nm light, while the responsivities of the photodiodes at unity gain were examined for wavelengths up to 375 nm. Peak unity gain responsivities of more than 130 mA/W at 265 nm, equivalent to quantum efficiencies of more than 60%, were obtained for both structures. The measured avalanche characteristics show that /spl beta/>/spl alpha/ and that the /spl beta///spl alpha/ ratio remains large even in thin 4H-SiC avalanche regions. Very low excess noise, corresponding to k/sub eff/<0.15 in the local noise model, where k/sub eff/=/spl alpha///spl beta/(/spl beta///spl alpha/) for hole (electron) injection, was measured with 365-nm light in both structures. Modeling the experimental results using a simple quantum efficiency model and a nonlocal description yields effective ionization threshold energies of 12 and 8 eV for electrons and holes, respectively, and suggests that the dead space in 4H-SiC is soft. Although dead space is important, pure hole injection is still required to ensure low excess noise in thin 4H-SiC APDs owing to /spl beta///spl alpha/ ratios that remain large, even at very high fields.

On the temperature coefficient of 4H-SiC BJT current gain

Abstract In this paper, the temperature coefficient of 4H-SiC NPN BJT current gain is studied by way of numerical simulations. In general, 4H-SiC NPN BJT would have a positive temperature coefficient (PTC) for the common emitter current gain if the acceptor ionization energy is smaller than 170 meV. Both PTC and negative temperature coefficient (NTC) can occur in 4H-SiC NPN BJT with an aluminum-doped base. High base doping concentration is required to obtain a wide range of current density with a NTC for current gain, especially when the electron lifetime in base is low. For a base doping concentration of 2.5×10 17 cm −3 , the NTC for current gain is obtained for current density up to 300 A/cm 2 , even when the electron lifetime is as low as 48 ns. The experimental results are also reported.

Monte Carlo simulation of 4H-SiC IMPATT diodes

A Monte Carlo particle (MCP) bipolar model for 4H-SiC consisting of three electron and two hole bands is developed to simulate the millimetre wave power generation by 4H-SiC IMPATT diodes. Validation of the model is provided by comparing (i) carrier transport properties with full band simulation results and (ii) hole impact ionization coefficients with the most recent experimental results. MCP simulation results are reported for a low-voltage 4H-SiC IMPATT diode connected directly in a parallel resonant circuit with a standard 50 O load resistor. The detailed evolution of carrier generation, accumulation and drift are presented to confirm the design of an efficient hi-lo IMPATT diode structure. Critical performance parameters investigated include bias and frequency dependences of millimetre wave output power, generation efficiency, conduction current and frequency stability at an operating frequency around 200 GHz. It is predicted that very high-power millimetre waves at around 200 GHz can be generated at pulse mode.

1710-V 2.77-m/spl Omega/cm sub 2 4H-SiC trenched and implanted vertical junction field-effect transistors

This letter reports the demonstration of a 4H-SiC trenched and implanted vertical-junction field-effect transistor (TI-VJFET). The p/sup +/n junction gates are created on the sidewalls of deep trenches by angled Al implantation, which eliminates the need for epitaxial regrowth during the JFET fabrication. Blocking voltages up to 1710 V has been achieved with a voltage supporting drift layer of only 9.5 /spl mu/m by using a two-step junction termination extension. The TI-VJFET shows a low specific on-resistance R/sub ON-sp/ of 2.77m/spl Omega/cm/sup 2/, corresponding to a record high value of V/sub B//R/sub ON-sp/ equal to 1056 MW/cm/sup 2/.

Development of 4H-SiC LJFET-Based Power IC

A novel lateral junction field-effect transistor (JFET)-based power IC technology in 4H-SiC is presented in detail covering device and circuit design, fabrication, and characterization. The optimal reduced surface field design for the lateral power JFET has been carried out and implemented in the IC fabrication. Since this technology has great promise at high temperatures, the temperature dependences (from room temperature to 300degC) of the threshold voltage, transconductance, resistance, and electron mobility have been fully characterized. Advantages of the SiC vertical-channel lateral JFET (VC-LJFET) technology, such as lower output capacitance (Coss) for lateral power JFETs and adjustable threshold voltages at mask design level, are also discussed. Finally, a monolithic power IC chip integrating a power lateral JFET with its low-voltage buffers is presented, which demonstrated megahertz switching at a power level of 270 W. The successful development of the VC-LJFET technology should hasten the introduction of SiC smart power ICs and eventually the system-on-a-chip applications in harsh environments.

Demonstration of high voltage (600–1300 V), high current (10–140 A), fast recovery 4H-SiC p-i-n/Schottky(MPS) barrier diodes

Abstract In this work, the design, fabrication, and testing of improved, high power 4H-SiC merged p-i-n/Schottky(MPS) diodes, in the 600–1300 V voltage range, are described. Both DC and transient test results are presented. The diodes were designed with a multi-step junction termination extension (MJTE) to improve the blocking voltage. The MJTE design allows full utilization of the superior breakdown properties of SiC. At 600 V, the highest forward current capability of a packaged MPS diode showed a current of 50 A at 2 V and 140 A at 4 V. At 1300 V, the current was 4 A at 2 V and 10 A at 4 V. In addition, the SiC MPS diode reverse voltage showed excellent suppression of the Schottky leakage current at temperatures up to 250 °C. The transient properties were measured with an inductively loaded half-bridge inverter circuit at high current and high temperatures (high- T ). Results showed that the replacement of Si freewheeling diodes by SiC diodes results in far less storage charge in the diodes and substantial reduction in diode turn-off energy loss, especially at high- T . Switching measurements at currents up to 230 A compared a SiC MPS and a state-of-the-art Si diode; the MPS showed a 47% energy loss reduction at room temperature and 84% at 200 °C. The IGBT energy loss reduction, when using an MPS diode, was 15% at room temperature and 45% at 150 °C. The use of MPS diodes should result in improved efficiency when used in power electronics applications.

Demonstration of 4H-SiC avalanche photodiodes linear array

Abstract 4H-SiC visible blind avalanche photodiode (APD) linear arrays have been fabricated and successfully tested. A 40 pixel linear array with only one bad pixel has been demonstrated. The linear arrays show uniform breakdown voltage and low leakage current. The photoresponse and the excess noise factor of 4H-SiC APD pixels have been studied. A very high multiplication gain with very low excess noise factors is reported.

Sensitive optical gating of reverse-biased AlGaAs/GaAs optothyristors for pulsed power switching applications

A heterojunction-based optothyristor has been fabricated and tested with biasing field intensity up to 34 kV/cm for pulsed power applications. The reverse-biased optothyristor can even be triggered by a light-emitting diode (LED) of a few microwatts power, and more than 500 times reduction in the required LED power for triggering has been observed when compared to bulk photoconductive switches. The optothyristor, however, does not turn on under similar triggering conditions if bias polarity is changed. The sensitive optical gating of the reverse-biased optothyristor is explained. The turn-on delay time under reverse bias has been found to be inversely proportional to the square root of the LED power. The possibility of improving the switching efficiency by superimposing the laser pulse on a constant lower level background illumination has been demonstrated. >