Global Consortium Pioneers High-frequency SiC PIN Diodes
PIN diodes are widely used as phase-shifters, switches, attenuators, and limiters in RF, ultrahigh frequency and microwave systems. These devices – which serve in applications ranging from civilian and military radar to mobile-phone base-station transmitters and satellite communication units – are preferred to their electromechanical equivalents because they are cheaper, more reliable, last longer, and can operate at higher speeds. They are also more efficient, as they can operate at very high microwave power levels using a low direct-current (DC) bias. The frequency response of these PINs is also good, due to their relatively low off-state capacitance for a given on-resistance, while their higher blocking voltages give them the edge over MESFETs for applications requiring either high powers or high frequencies.
High-frequency versions of the device are currently made from GaAs and silicon. However, improvements in performance are promised by switching to SiC, which has superior intrinsic properties. This material could enable higher switching speeds and similar microwave-power-control capabilities, or provide greater power handling at equivalent switching speeds.
Switching to SiC PINs could also cut the number of devices required in a circuit. When silicon PINs are used, several diodes have to be combined to handle powers of more than a few kilowatts in pulsed mode, resulting in complex, narrow-band circuits that have the added disadvantage of needing a dedicated cooling system. With SiC, however, only one or two diodes would be needed to handle a few kilowatts of RF power, leading to a simpler, broadband circuit that is easier to cool.
Changes in radar technology are also increasing the attractiveness of SiC PINs. Frequency bands that were once set aside for radar are now used for wireless communication, which has forced radar systems to turn to higher frequencies. This means that diodes need to have smaller capacitances and smaller physical sizes to significantly increase their thermal resistance.
The power-handling requirements for today s diodes are also more stringent, because higher powers are needed to pinpoint increasingly sophisticated planes operating in stealth mode, which reflect as little energy as possible.
When used in this application, silicon s low breakdown voltage limits the output from the diode and its low thermal conductivity hampers heat dissipation. SiC PINs, in comparison, excel in both of these areas. They can also operate at higher temperatures than both silicon and GaAs devices, which are typically limited to environments below 175 °C. This enables SiC PINs to be placed in other harsh environments, such as those found next to engines and airplane turbines.
GaN, like SiC, has several characteristics that make it a potential candidate for high-power microwave PIN diodes, but it is actually inappropriate for several reasons. Unlike SiC, it is not suited to the fabrication of vertical transport devices that offer excellent stability at high power levels. Heavy p-doping is also difficult, which makes contact resistance much higher, and it is impossible to grow thermal oxides that lead to high-quality passivation at the device s edge. Lastly, the restricted availability of GaN substrates means that fabrication has to be carried out on a foreign platform, such as sapphire, which results in poor heat dissipation and higher device resistance.
PINs versus MESFETs
For these reasons research and commercial efforts using wide bandgap materials are focused on SiC. In the main, PIN diodes are being developed for low-frequency switching requirements and SiC MESFETs for high-frequency microwave applications. This is not surprising because the market for low-frequency SiC PIN diodes is orders of magnitude larger than that for its higher-frequency counterparts. In addition, the current tendency in all forms of compound semiconductor technology is to replace microwave diodes with MMICs, which explains the popularity of SiC-related microwave component research programs dedicated to MESFETs.
However, SiC microwave PIN diodes do have the potential to deliver high performance as they do not suffer from the traditional problems of a high number of defects and forward-bias degradation that plague SiC devices used for power electronics. Having fewer defects is a direct consequence of the smaller device dimensions, which are needed to reduce capacitance and boost performance at high frequencies. These chips are smaller in all dimensions, and thicknesses of the active layer can be reduced to below 6 μm for applications requiring power handling of 5 kW or more.
To speed up SiC PIN development, the Foundation for Research and Technology–Hellas (FORTH) formed a consortium five years ago with Svetlana Electronpribor and Ioffe Institute from Russia and ORION from Ukraine. This team has been supervised by George Haddad from the University of Michigan, who has been a leading developer of silicon and GaAs-based diodes for the last 40 years.
Our consortium has built SiC PIN diodes on 4H substrates that can operate in the X-band (8–12 GHz) and tested them in broadband switches. These devices were fabricated after the epitaxial structure and device geometry had been optimized, and they make the best use of available material and existing process technology.
The 4H-SiC diodes were fabricated from CVD-grown, commercially available epiwafers and material grown by the sublimation method at the Ioffe Institute (see"Fabricating 4H-SiC PINs", and figure 1 for a description of the diode s packaging). The CVD-grown diodes produced excellent DC characteristics and fast switching. The drift layer resistance at a 100 mA forward current is 1.6 × 10−4 Ω cm2, indicating that the base layer is effectively conductivity modulated and a switching speed of less than 10 ns is possible. Capacitance at a punch-through voltage of –100 V was well below 0.5 pF for mesa structures with a diameter of up to 150 μm, which shows that this structure has the potential to operate at high frequencies.
We have evaluated the performance of our packaged 80–150 μm diameter devices in special tunable waveguide single-pole-single-throw (SPST) switches suitable for power X-band applications (see figure 2). The key attributes for this type of switch are high speed, good power handling, and high and low transmission of the input signal in the "on" and "off" states, respectively. In the "on" state a perfect switch would produce an output signal that is identical to the input, giving it an insertion loss of 0 dB, and in its "off" state no signal would be transmitted, giving it an infinite value for isolation.
Over a narrow band between 8.5 and 10.5 GHz, our SPST switches produced an isolation of 19–25 dB and insertion loss of less than 2 dB. This loss is similar to that for commercial silicon RF switches operating at several gigahertz, which have an isolation figure that usually exceeds 30 dB. The stability of our switches electrical characteristics is excellent, according to extensive high-thermal-stress tests that were conducted over long periods.
We have also carried out high-power tests at 9.5 GHz on 100 μm diameter diodes to determine the power-handling capabilities. These tests involved using 1 μs pulses, an on-off time ratio of 1000, and drive currents of 100 mA for the "on" state and voltages of 100 V for the "off" state. In the "off" configuration the switches produced a very stable isolation of 22.5 dB at microwave powers of 2 kW, and in the "on" state the devices delivered an insertion loss below 1 dB for powers up to 1.8 kW. The relatively low isolation is due to a differential resistance of more than 1 Ω for the 100 mA drive current used.
We have improved the isolation and broadband operation of our switches by building two types of modulator featuring multiple diodes. Our three-diode modulator is a compact and convenient design for portable communication systems, while our version containing two diodes is suitable for high-temperature operation. The three-diode package produces 30 dB isolation, an insertion loss below 2 dB and switching in less than 30 ns in the 1–6 GHz band. Its two-diode counterpart produced a transmission loss of just 1–2.5 dB and isolation of 33–45 dB between 2–7 GHz at temperatures up to 300 °C (see figure 3). The peak value for the isolation actually increases with temperature, proving for the first time the suitability of SiC PINs operating at high temperature for high-frequency applications. We are continuing to investigate the behavior of these PIN circuits under high-power CW and pulsed-signal operation.
The results of our SiC PIN development program demonstrate that these devices offer higher operating temperatures and comparable power-handling characteristics to their commercial silicon and GaAs equivalents. However, this work is still in its infancy, and we believe that the great potential of SiC indicates that there is still plenty of room for improving devices.
One target is a reduction of resistance in the "on" state to improve the isolation, which can be achieved by additional thinning of the substrate. However, we will also be trying to develop a full 4H-SiC modulator that will feature a switching diode and a driving circuit made from the same material. We will begin by developing separate circuits for the switch and its driver, before we unite them with a MMIC approach that will produce an integrated module suitable for harsh environments.