GaN/AIGaN Heterostructure Devices: Photodetectors and Field-Effect Transistors
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tectors and for deep submicron GaNbased FETs. GaN-Based Photodetectors GaN Photodetector Types The recent discovery and rapid progress in blue and violet semiconductor lasers based on GaN has brought a lot of attention to GaN-based light emitters.
The second most important device for optoelectronics applications is a photodetector. One of the best photodetectors with the sensitivity range from 0.4 to 0.76 jam is a human eye, which is quite sensitive to solar radiation. The solar-radiation spectrum is somewhat similar to the radiation spectrum of a black body with a temperature of 5800 K. It has a maximum corresponding to yellow light (with the frequency not too different from the frequency corresponding to the peak sensitivity of a human eye at 0.555 fim, which is green). Visible-blind detectors have to respond to the radiation with a wavelength shorter than 400 nm. Semiconductor photodetectors span a very wide spectral range. GaN-based and other wide-bandgap semiconductors primarily cover shorter wavelengths (smaller than 400 nm). These ultraviolet (uv, visible-blind) photodetectors may find numerous applications in chemical analysis in industrial and environmental controls, and in avionics and defense. Both photoconductive and photovoltaic detectors have been demonstrated in the GaN-based materials system. The energy gap of GaN-based materials changes from 1.89 eV for InN to 3.5 eV for GaN and 6.2 eV for A1N. A wide range of GaN-AIN solid-state solutions has been demonstrated. Hence in principle, intrinsic photodetectors based on these materials systems may span the wavelength range from 656 to 200 nm. This includes practically the full visible range (390770 nm), as well as much of the uv range.
Table 1: Materials Parameters of GaN, AIN, and InN. Property
GaN
AIN
InN
Energy gap (eV) (direct) Lattice constant, a (A) Lattice constant, c(A) Density (g/cm3) Static dielectric constant Dynamic dielectric constant Electron mobility (cm2/Vs) Hole mobility (cm2/Vs) Breakdown field (V/cm) Saturation velocity (m/s) Electron effective mass Light hole mass Sound velocity (m/s) Optical polar phonon energy (eV) Deformation potential (eV) Piezoelectric constant Kth. (W/cm°C) Melting temperature (°C)
3.4 3.189 5.185 6.1 9.5 (8.9) 5.3 1,000 30 >5 x 106 2.5 x 105 0.2 0.259 5 X 103 0.092 8 0.65 C/m2 1.5 >1700
6.2 3.11 4.98 3.26 8.5 4.84
1.89 3.54 5.70 6.88 19.6 9.3
14
0.314 0.471 104 0.11 19 4.10"m/Vdi5 2 3000
0.11
=1100
MRS BULLETIN/FEBRUARY 1997
GaN/AIGaN Heterostructure Devices: Photodetectors and Field-Effect Transistors
Extrinsic GaN photodetectors (using donor and acceptor levels) are also possible. However in this article, we will only consider intrinsic photodetectors. Figure 2 schematically shows different kinds of the photodetectors realized using the GaN materials system. Conspicuously absent in Figure 2 is an avalanchephotodiode (APD) photodetector. Avalanche photodiodes realized in other material systems demonstrated superior performance. However a very high bulk breakdown field in GaN makes the realizati
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