Materials for High-Temperature Semiconductor Devices (1995) / Chapter Skim
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Appendix C: High-Temperature Microwave Devices
Appendix C: High-Temperature Microwave Devices
Pages 93-118
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From page 93... ...
Gate lengths achievable for GaAs MESFETs decreased to submicron dimensions, enabling a number of high-performance applications through the millimeter-wave bands. At the same time, GaAs IMPATT diodes set records for power output.
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From page 94... ...
The major implications of the need to limit the temperature of microwave devices are twofold: (1) power density must be limited at the active layers since much of the temperature rise is due to heat-spreading resistance (known as thermal resistance)
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From page 95... ...
The figures of merit suggest that electronically limited devices such as FETs should have higher power density in the wide bandgap materials. For thermally limited devices such as bipolar transistors or IMPATTs, higher 95 power density should also be achievable in the wide bandgap devices.
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From page 96... ...
If p-channel MOSFETs are used in the enhancement mode, channel access must be made by holes that have low mobility or a high parasitic resistance will result. Despite this pessimism for the potential of wide bandgap MOSFETs for microwave application, it should be noted that MOSFETs fabricated from both 6H- and 3C-SiC have shown good DC properties, even at 96 e-type conducting channel formed with positive gate bias voltage Sources ',~,`' Metal ~ ......
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From page 97... ...
These effects severely reduce the frequency response of silicon and GaAs BJTs rendering them essentially useless above 3 GHz. Parasitic effects would have an even more detrimental effect on bipolar transistors constructed of wide bandgap semiconductors such as SiC and GaN.
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From page 98... ...
Low hole mobility ~ at 50 cm2/V s) and an associated high parasitic base resistance is a potential difficulty with GaAlN/GaN heterojunction bipolar transistors.
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From page 99... ...
Compared to the parasitic base resistance of homojunction bipolar transistors, parasitic contact resistance can be reduced since base doping can be much higher. The base layer could be produced by ion implantation of p-type dopant.
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From page 100... ...
Metal-Semiconductor Field Effect Transistors Relatively more attention has been paid to MESFETs than any other microwave device in wide bandgap semiconductors. Goals for MESFETs under development include producing X-band power density three times that of GaAs with power-added efficiency twice that of GaAs.
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From page 101... ...
This indicates that the improvement in fit,,,, might be attributed to the higher mobility of the 4H-polytype and the resulting lower parasitic resistance. Impact Avalanche Transit-Time Diodes GaAs IMPATT diodes have been developed in the last few years, emerging as the solid-state device giving the highest power in the microwave and millimeter-wave frequency bands (Figure C-101.
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From page 102... ...
The parasitic series resistance depicted in Figure C-12 limits the device area and power. Compared to values in GaAs, saturation resistance, Rs, is expected to be much larger in wide bandgap materials because of generally lower mobilities and higher contact resistance.
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From page 103... ...
High parasitic resistance is a particularly troublesome, potential problem for wide bandgap semiconductors. In the past, typical specific contact resistance for SiC n+ material of l0'8 doping range is around l x l0~ Q cm2 and about 5 ~ 104 Q cm2 for p+ at the same doping.
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From page 104... ...
Thus, device area can be doubled with a resulting increase in power at a given power density. Given these basic elements for the operation of IMPATT diodes, and ignoring potentially high parasitic series resistance, the wide bandgap semiconductors provide interesting possibilities.
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From page 105... ...
relative dielectric constant and the high values of avalanche breakdown fields. The high thermal-conductivity of diamond compared with all other wide bandgap materials allows for increased ambient temperature at a given power dissipation at the junction.
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From page 106... ...
Expected transistor-operating temperature rise can then be considered in conjunction with gain predictions. In addition to affecting gain, parasitic resistance can have a profound effect on the current-voltage curves, so this section first considers the intrinsic MESFET, in which behavior depends on the basic velocity versus electric field characteristics.
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From page 107... ...
11) , large built-in voltages typical of wide bandgap semiconductors offset the effect of applied gate voltage by partially depleting the channel at zero applied-gate bias.
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From page 108... ...
When the saturation voltages are high, higher DC bias voltages must be used to obtain efficient RF amplification. Fortunately, such voltages can be achieved in wide bandgap semiconductors.
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From page 109... ...
If contacts are to survive elevated temperatures, refractory metals must be used. Contact resistivities at elevated temperatures are given for refractory metals in Appendix C: High-Temperature Microwave Devices Table C-3 (Shur et al., 19931.
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From page 110... ...
The discrepancy in required load impedances for power and gain could account for Me less-than-optimum results reported for power tests in discrete devices of wide baIldgap material; tuning, operating voltage, and operating current must all be compromised in the test. One way to decrease the power-load resistance at a given bias voltage is to increase ISIS and reduce the -2 c~ E o ,~ ~ -6 o -4 -10 0.5 1 Ro=(ZNV)
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From page 111... ...
Parasitic series resistance was found to be detrimental to drain efficiency, but high power density could still be achieved given the possibility of operating at high bias voltages. To make SiC MESFETs useful, high gain in addition to high power must be achieved.
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From page 112... ...
, Idc 0.231 0.115 0.072 0.098 Dissipated power (W) , P. 4.68 4.72 0.20 0.41 Thermal conductivity (W/cm °C)
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From page 113... ...
Typical transistors show a temperature rise of 50 °C or less, indicating that GaAs MESFETs are electronically limited by available drain current and voltage rather than thermally limited. Reducing the thermal resistance of GaAs MESFETs may have a slight beneficial effect on the performance of the transistors due to higher mobility, higher electron-saturated velocity, and lower leakage currents at lower temperatures, but it is not expected to be a major benefit.
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From page 114... ...
In the foregoing MESFET analysis, the lowest reported values of contact resistance is used. At elevated temperatures, refractory metals would be required.
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From page 115... ...
At 500 °C, the p-type diamond MESFET was predicted to give power density of 0.75 W/mn' at 5 GHz with 33 percent poweradded efficiency and around 8-dB gain. It was concluded that SiC MESFETs were preferred over p-type diamond MESFETs at elevated temperature.
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From page 116... ...
Metal semiconductor field effect transistor based on single crystal GaN. Applied Physics Letters 62:1786-1787.
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From page 117... ...
Pp. 421-430 in Proceedings of the IEEE/Cornell Conference on Advanced Concepts in High Speed Semiconductor Devices and Circuits.
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