Solar-blind ultraviolet photodiodes with a band-edge wavelength of 285 nm were fabricated on laterally epitaxially overgrown GaN grown by metalorganic chemical vapor deposition. Current–voltage measurements of the diodes exhibited dark current densities as low as 10 nA/cm2 at −5 V. Spectral response measurements revealed peak responsivities of up to 0.05 A/W. Response times for these diodes were measured to be as low as 4.5 ns for 90%-to-10% fall time. For comparison, diodes were fabricated using the same p–i–n structure deposited on dislocated GaN. These diodes had dark current densities many orders of magnitude higher, as well as a less sharp cutoff, and a significant slow tail under impulse excitation.

In this article, we discuss parameters influencing (a) the properties of thin AlxGa1−xN layers grown by metalorganic chemical vapor deposition and (b) the electrical properties of the two-dimensional electron gas (2DEG) forming at the AlxGa1−xN/GaN heterojunction. For xAl>0.3, the AlxGa1−xN layers showed a strong tendency towards defect formation and transition into an island growth mode. Atomically smooth, coherently strained AlxGa1−xN layers were obtained under conditions that ensured a high surface mobility of adsorbed metal species during growth. The electron mobility of the 2DEG formed at the AlxGa1−xN/GaN interface strongly decreased with increasing aluminum mole fraction in the AlxGa1−xN layer and increasing interface roughness, as evaluated by atomic force microscopy of the surfaces prior to AlxGa1−xN deposition. In the case of modulation doped structures (GaN/AlxGa1−xN/AlxGa1−xN:Si/AlxGa1−xN), the electron mobility decreased with decreasing thickness of the undoped spacer layer and increasing silicon doping. The electron mobility was only moderately affected by the dislocation density in the films and independent of the growth temperature of the AlxGa1−xN layers at xAl=0.3. For Al0.3Ga0.7N/GaN heterojunctions, electron mobility values up to 1650 and 4400 cm2/V s were measured at 300 and 15 K, respectively.

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An overview is presented of progress in GaN electronic devices along with recent results from work at UCSB. From 1995 to 2001, the power performance of AlGaN/GaN high electron mobility transistors (HEMT) improved from 1.1 to 11 W mm-1, respectively. The disadvantage of the low thermal conductivity of the sapphire substrate was mitigated by flip-chip bonding onto AlN substrates, yielding large periphery devices with an output power of 7.6 W. A variety of HEMT amplifier circuits have been demonstrated. The first AlGaN/GaN heterojunction bipolar transistor (HBT) was demonstrated in 1998, with a current gain of about 3. By developing the technique of emitter regrowth, a current gain of 10 was achieved in both GaN BJTs and AlGaN/GaN HBTs. A common emitter current gain cutoff frequency of 2 GHz was measured. Critical issues involved in the growth of high quality AlGaN/(AlN)/GaN heterostructures and GaN:Mg by metal-organic chemical vapour deposition (MOCVD) and molecular beam epitaxy (MBE) and the device fabrication are discussed.

In order to characterize the electron transport properties of the two-dimensional electron gas (2DEG) in AlGaN/GaN modulation-doped field-effect transistors, channel magnetoresistance has been measured in the magnetic field range of 0–12 T, the temperature range of 25–300 K, and gate bias range of +0.5 to −2.0 V. By assuming that the 2DEG provides the dominant contribution to the total conductivity, a one-carrier fitting procedure has been applied to extract the electron mobility and carrier sheet density at each particular value of temperature and gate bias. Consequently, the electron mobility versus 2DEG sheet density has been obtained for each measurement temperature. Theoretical analysis of these results suggests that for 2DEG densities below 7×1012 cm−2, the electron mobility in these devices is limited by interface charge, whereas for densities above this level, electron mobility is dominated by scattering associated with the AlGaN/GaN interface roughness.