Seite - 24 - in Adaptive and Intelligent Temperature Control of Microwave Heating Systems with Multiple Sources

2. IntroductionofHEPHAISTOS TREW: HIGH-FREQUENCY SOLID-STATE ELECTRONIC DEVICES 641 Fig. 4. Average RF output power versus frequency for various electronic devices (courtesy of the Naval Research Laboratory). function of bandgap energy and wide bandgap semiconductors are desirable for power applications. Semiconductors, such as SiC andGaN, showsignificantpotential for these applications. Electronic devices designed for microwave and RF applica- tions operate in a transit-time mode and are scaled in size by frequency considerations. Under normal operation the electric fieldswithinthedevicesvaryfromlowmagnitudeneartheelec- tron injection location to magnitude sufficient to produce elec- tronvelocitysaturationinthechargecontrol/modulationregion. Therefore, large current capability requires semiconductor ma- terials that have high electron velocity. In general, both high mobility and high saturation velocity are desirable for high RF current. Traditional semiconductors such as Si and GaAs have electron saturation velocities that are limited to about cm/s,andthislimitsboththepowerthatcanbegenerated and the frequency response of the device. Wide bandgap semi- conductors have electron saturation velocities that are a factor of twohigher.Thecombinationofhighcurrentandhighvoltage capability make wide bandgap semiconductors very attractive candidatematerials for fabricationofhigh-powerandhigh-per- formance electronic devices [4]. IV. SOLID-STATE ELECTRONIC DEVICES A large variety of semiconductor devices for high-frequency applications have been proposed, fabricated, demonstrated, and used in practical applications. A review of these devices has been presented [5]. The present state-of-the-art RF power performance of microwave solid-state devices is compared to that for microwave tubes in Fig. 4. Solid-state devices produce RF power of about 100 W in S-band and 1 W at 100 GHz. As indicated, the RF performance of solid-state devices is significantly lower than obtained from electronic vacuum tubes. The reduced RF power capability of solid-state devices is due to 1) lower bias voltage that can be applied; 2) reduced electronvelocity in thesemiconductorwhichproduces reduced current and; 3) a thermal limitation caused by the semicon- ductor thermal impedance. The relatively low bias voltage at which solid-state devices operate permit high reliability due to reduced electric field stress, and the ability to use lithography technologypermits lowfabricationcosts.Solid-statedeviceRF outputpowercanbe increasedbyusingpowercombining tech- nology, although system RF output power is generally limited to the 10’s to 100’s of kilowatt level in the microwave region. For megawatt systems it is difficult to efficiently combine the large number of devices necessary for practical systems. 1) Transistors: Three-terminal transistor structures can be fabricatedwithavarietyofgeometriesandoperatingcharacter- istics. Fundamentally, they all function by controlling the con- ductivity of a conducting channel by establishment and mod- ulation of an electrostatic barrier. However, the various tran- sistor structures differ regarding the details of how the electro- static barrier is formed, and how it modulates the channel con- ductivity. Field-effect transistors are majority carrier devices, where the modulation region controls majority carrier current, and bipolar transistors are minority carrier devices where the modulationregioncontrolsminoritycarriercurrent.Theopera- tion and performance of these devices are reviewed. The first practical work on the development of FETs was re- ported in patents by Lilienfeld in 1930 [6] and 1933 [7], and reports by Stuetzer [8], [9] in 1950 and Shockley in 1952 [10]. Theearlydevicesdemonstrated limitedperformanceduetorel- atively poor semiconductor material quality and an inability to fabricate a gate electrode with fine line geometry. The realiza- tion of high performance devices needed to wait for the ad- vancement of epitaxial semiconductor growth technology and the development of optical lithography to produce gate lengths Figure2.8. Power versus frequency performance of different solid-state de- vicesandmicrowavet be [Tre05]. In HEPHAISTOS, magnetron [Poz09] is selected as the microwave s urce, such as shown in figure 2.9. Magnetron is ne of the most popular microwave tubes used in industrial applications. Its working principles are well explained in [Jon98] and [Gil11]. The reason why magnetron is used in HEPHAISTOS is due to considerations of size, cost, efficiencyandmost important, theability toproducehighpower microwave(700Wto2kW). Thesemagnetronsa edirectlyconnectedwithanumber fpowersup- plies. In the old HEPHAISTOS cavity 3, power supplies with only puls DC output were pplied. All magnetrons h d to work in e pulse mode (only ON and OFF), and different microwave power lev- els were only realized by adjusting the ON/OFF time ratio via the pulsewidthmodulation(PWM)method[Bar01]. After theupgradeof the whole system, the new HEPHAISTOS cavity 3 are equipped with two different types of power supplies, such as shown in figure 2.10. Both of them are able to operate the magnetron in the continuous- wave(CW)mode,whichmeansthemicrowavepowercanbeadjusted con inuouslywi houtPWM. 24