Materials such as silicon (Si) and gallium arsenide (GaAs) have found wide application in semiconductor devices. These, more familiar, semiconductor materials may not be well suited for higher power and/or high frequency applications, however, because of their relatively small bandgaps (e.g., 1.12 eV for Si and 1.42 for GaAs at room temperature) and/or relatively small breakdown voltages.
In light of the difficulties presented by Si and GaAs, interest in high power, high temperature and/or high frequency applications and devices has turned to wide bandgap semiconductor materials such as silicon carbide (2.996 eV for alpha SiC at room temperature) and the Group III nitrides (e.g., 3.36 eV for GaN at room temperature). These materials, typically, have higher electric field breakdown strengths and higher electron saturation velocities as compared to gallium arsenide and silicon.
A device of particular interest for high power and/or high frequency applications is the High Electron Mobility Transistor (HEMT), which, in certain cases, is also known as a modulation doped field effect transistor (MODFET). These devices may offer operational advantages under a number of circumstances because a two-dimensional electron gas (2DEG) is formed at the heterojunction of two semiconductor materials with different bandgap energies, and where the smaller bandgap material has a higher electron affinity. The 2DEG is an accumulation layer in the undoped (“unintentionally doped”), smaller bandgap material and can contain a very high sheet electron concentration in excess of, for example, 1013 carriers/cm2. Additionally, electrons that originate in the wider-bandgap semiconductor transfer to the 2DEG, allowing a high electron mobility due to reduced ionized impurity scattering.
This combination of high carrier concentration and high carrier mobility can give the HEMT a very large transconductance and may provide a strong performance advantage over metal-semiconductor field effect transistors (MESFETs) for high-frequency applications.
High electron mobility transistors fabricated in the gallium nitride/aluminum gallium nitride (GaN/AlGaN) material system have the potential to generate large amounts of RF power because of the combination of material characteristics that includes the aforementioned high breakdown fields, their wide bandgaps, large conduction band offset, and/or high saturated electron drift velocity. A major portion of the electrons in the 2DEG is attributed to polarization in the AlGaN. HEMTs in the GaN/AlGaN system have already been demonstrated. U.S. Pat. Nos. 5,192,987 and 5,296,395 describe AlGaN/GaN HEMT structures and methods of manufacture. U.S. Pat. No. 6,316,793, to Sheppard et al., which is commonly assigned and is incorporated herein by reference, describes a HEMT device having a semi-insulating silicon carbide substrate, an aluminum nitride buffer layer on the substrate, an insulating gallium nitride layer on the buffer layer, an aluminum gallium nitride barrier layer on the gallium nitride layer, and a passivation layer on the aluminum gallium nitride active structure.
Wide bandgap GaN-based high-electron-mobility-transistors (HEMTs) have come a long way as microwave devices since their description in 1993 in Khan et al., Appl. Phys. Lett., vol. 63, p. 1214, 1993, and a demonstration of their power capability in 1996 in Wu et al., IEEE Electron Device Lett., vol. 17, pp. 455–457, September, 1996. Many research groups have presented devices with power densities exceeding 10 W/mm, a ten-fold improvement over conventional III–V devices. See Tilak et al., IEEE Electron Device Lett., vol. 22, pp. 504–506, November, 2001; Wu et al., IEDM Tech Dig., Dec. 2–5, 2001, pp. 378–380; and Ando et al., IEEE Electron Device Lett., vol. 24, pp. 289–291, May, 2003. Much of the previous work covered material quality, choice of substrate, epi-layer structures and processing techniques. Less effort has been put on advanced device designs, leaving room for further improvement. An overlapping gate structure, or field plate, was used by Zhang et al. with GaN HEMTs for high-voltage switching applications. Zhang et al., IEEE Electron Device Lett., vol. 21, pp. 421–423, September, 2000. Following this, Karmalkar et al. performed simulations for the field plate structure, predicting up to five times enhancement in breakdown voltages. Karmalkar et al., IEEE Trans. Electron Devices, vol. 48, pp. 1515–1521, August, 2001. However, fabricated devices at that time had low cutoff frequencies, not suitable for microwave operation. Ando et al. recently used a similar structure with smaller gate dimensions and demonstrated performance of 10.3 W output power at 2 GHz using a 1-mm-wide device on a SiC substrate. Ando et al., IEEE Electron Device Lett., vol. 24, pp. 289–291, May, 2003. Chini et al. implemented a new variation of the field-plate design with further reduced gate dimensions and obtained 12 W/mm at 4 GHz from a 150-μm-wide device on a sapphire substrate. Chini et al., IEEE Electron Device Lett., vol. 25, No. 5, pp. 229–231, May, 2004.
Modern communication applications also may require high linearity for power devices. Chini et. al. reported two-tone linear power of 2.4 W/mm with PAE of 53% at 4 GHz from FP devices at a 3rd-order-intermodulation level (IM3) of −30 dBc. Chini et al., IEEE Electron Device Lett., vol. 25, No. 5, pp. 229–231, May, 2004.
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