1. Field of the Invention
This invention relates to aluminum gallium nitride and gallium nitride based high electron mobility transistors.
2. Description of the Related Art
Microwave systems commonly use solid state transistors as amplifiers and oscillators which has resulted in significantly reduced system size and increased reliability. To accommodate the expanding number of microwave systems, there is an interest in increasing their operating frequency and power. Higher frequency signals can carry more information (bandwidth), allow for smaller antennas with very high gain, and provide radar with improved resolution.
Field effect transistors (FETs) and high electron mobility transistors (HEMTs) are common types of solid state transistors that are fabricated from semiconductor materials such as Silicon (Si) or Gallium Arsenide (GaAs). One disadvantage of Si is that it has low electron mobility (approximately 1450 cm2/V−s), which produces a high source resistance. This resistance seriously degrades the high performance gain otherwise possible from Si based HEMTs. [CRC Press, The Electrical Engineering Handbook, Second Edition, Dorf, p.994, (1997)]
GaAs is also a common material for use in HEMTs and has become the standard for signal amplification in civil and military radar, handset cellular, and satellite communications. GaAs has a higher electron mobility (approximately 6000 cm2/V−s) and a lower source resistance than Si, which allows GaAs based devices to function at higher frequencies. However, GaAs has a relatively small bandgap (1.42 eV at room temperature) and relatively small breakdown voltage, which prevents GaAs based HEMTs from providing high power at high frequencies.
Improvements in the manufacturing of gallium nitride (GaN) and aluminum gallium nitride (AlGaN) semiconductor materials have focused interest on the development of AlGaN/GaN based HEMTs. These devices can generate large amounts of power because of their unique combination of material characteristics including high breakdown fields, wide bandgaps (3.36 eV for GaN at room temperature), large conduction band offset, and high saturated electron drift velocity. The same size AlGaN/GaN amplifier can produce up to ten times the power of a GaAs amplifier operating at the same frequency.
U.S. Pat. No. 5,192,987 to Khan et al discloses AlGaN/GaN based HEMTs grown on a buffer and a substrate, and a method for producing them. Other HEMTs have been described by Gaska et al., “High-Temperature Performance of AlGaN/GaN HFET's on SiC Substrates,” IEEE Electron Device Letters, Vol. 18, No 10, October 1997, Page 492; and Wu et al. “High Al-content AlGaN/GaN HEMTs With Very High Performance”, IEDM-1999 Digest pp. 925–927, Washington D.C., December 1999. Some of these devices have shown a gain-bandwidth product (fT) as high as 100 gigahertz (Lu et al. “AlGaN/GaN HEMTs on SiC With Over 100 GHz ft and Low Microwave Noise”, IEEE Transactions on Electron Devices, Vol. 48, No. 3, March 2001, pp. 581–585) and high power densities up to 10 W/mm at X-band (Wu et al., “Bias-dependent Performance of High-Power AlGaN/GaN HEMTs”, IEDM-2001, Washington D.C., Dec. 2–6, 2001).
Despite these advances, AlGaN/GaN based FETs and HEMTs have been unable to produce significant amounts of total microwave power with high efficiency and high gain. They produce significant power gain with DC gate drives, but with frequency step-ups as low as a millihertz to a few kilohertz, their amplification drops off significantly.
It is believed that the difference between AC and DC amplification is primarily caused by surface traps in the device's channel. Although the nomenclature varies somewhat, it is common to refer to an impurity or defect center as a trapping center (or simply trap) if, after capture of one type of carrier, the most probable next event is re-excitation.
At equilibrium, the traps donate electrons to the 2-dimensional electron gas (2-DEG) in HEMTs. Trapping levels located deep in a band gap are also slower in releasing trapped carriers than other levels located near the conduction of valence bands. This is due to the increased energy that is required to re-excite a trapped electron from a center near the middle of the band gap to the conduction band, compared to the energy required to re-excite the electron from a level closer to the conduction band.
AlxGa1−xN (X=0˜1) has a surface trap density comparable to the channel charge of the transistor with the traps in deep donor states with activation energy ranging from 0.7 to 1.8 eV (depending on X). During HEMT operation, the traps capture channel electrons. The slow trapping and de-trapping process degrades transistor speed, which largely degrades the power performance at microwave frequencies.
It is believed that the trap density of a AlGaN/GaN based HEMTs is dependent upon the surface and volume of the AlGaN barrier layer. Reducing the thickness of the AlGaN layer reduces the total trapping volume, thereby reducing the trapping effect during high frequency operation. However, reducing the thickness of the AlGaN layer can have the undesirable effect of increasing the gate leakage. During normal operation a bias is applied across the source and drain contacts and current flows between the contacts, primarily through the 2DEG. However, in HEMTs having thinner AlGaN layers, current can instead leak into the gate creating an undesirable current flow from the source to the gate. Also, the thinner AlGaN layer can result in a reduction in the HEMT's available maximum drive current.