W. Kopp

Resonant tunneling oscillations in a GaAs‐AlxGa1−xAs heterostructure at room temperature

This letter reports the first observation of resonant tunneling negative differential resistance (NDR) through a double barrier GaAs‐AlxGa1−xAs ‐GaAs‐AlxGa1−x As‐GaAs structure at room temperature. The NDR yields radio frequency oscillations at 300 K in a coaxial cable circuit that compare closely to those of a standard 1N3716 tunnel diode.

GaAs bipolar transistors grown on (100) Si substrates by molecular beam epitaxy

We have investigated the properties of GaAs homojunction bipolar transistors grown on (100) oriented Si substrates by molecular beam epitaxy. In a structure with a base thickness of 0.2 μm, a small‐signal common emitter current gain β of about 10 at a current density of 10 kA/cm2 has been obtained. Current densities as high as 105 000 A/cm2 were obtained in these devices without degradation demonstrating the excellent stability of this material. Since the minority‐carrier lifetime is quite sensitive to defects in the base region, these measurements demonstrate the high quality of GaAs on Si substrates. From the collector current dependence of current gain, an ideality factor n=1.5 for the emitter junction was obtained, indicating that space‐charge recombination is an important mechanism in these devices. We also demonstrate that the entire GaAs on Si wafer is phase ordered by observing the orientation effect in the mesa etching. These results show that excellent minority‐carrier properties can be obtained...

Incorporation rates of gallium and aluminum on GaAs during molecular beam epitaxy at high substrate temperatures

Gallium arsenide, aluminum arsenide, and aluminum gallium arsenide epitaxial layers were grown by molecular beam epitaxy in the substrate temperature range 590–720 °C. The incorporation rates of Ga and Al in this temperature range were studied by means of thickness measurements. The growth rates of GaAs and AlxGa1−xAs were observed to be dependent on growth temperature above 640 °C while the AlAs growth rate was observed to be independent of growth temperature in the range investigated. The reduction of the GaAs growth rate at a growth temperature above 640 °C was found to be lessened by the presence of minute amounts of Al and excess As. For the fixed Ga flux and a growth temperature of 700 °C the GaAs growth rate and the Ga contribution to the growth rate of Al0.3Ga0.7As were 0.50 and 0.89 times their low temperature values, respectively, while at 680 °C these values were 0.88 and 0.99, respectively.

High transconductance InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistors

Pseudomorphic In0.15Ga0.85As/Al0.15Ga0.85As modulation-doped field effect transistors (MODFETs) exhibiting extremely good dc characteristics have been successfully fabricated, dc transconductance in these strained-layer structures of 270 mS/mm were measured for 1-µm gate, normally-on devices at 300 K. Maximum drain current levels are 290 mA/mm, with excellent pinch-off and saturation characteristics. The transconductance increased to 360 mS/mm at 77 K while no persistent photoconductivity or drain collapse was observed. Preliminary microwave results indicate a 300-K current gain cutoff frequency of about 20 GHz. These results are equivalent to the best GaAs/AlGaAs MODFET results and are due in part to the improved transport properties and carrier confinement in the InGaAs quantum well.

A small‐signal equivalent‐circuit model for GaAs‐AlxGa1−xAs resonant tunneling heterostructures at microwave frequencies

This paper presents a study of the microwave impedance of GaAs‐AlxGa1−xAs resonant tunneling heterostructures. An equivalent‐circuit model is proposed that accounts for the frequency variation of the measured impedance and whose elements correspond to physical phenomena believed to be present in the device. Empirical formulas are obtained which can be used to calculate the values of the equivalent‐circuit elements from the structural parameters and the dc current‐voltage characteristics of the device.

Low threshold laser operation at room temperature in GaAs/(Al,Ga)As structures grown directly on (100)Si

We report the room‐temperature pulsed operation of GaAs/(Al,Ga)As double heterojunction laser structures grown directly on (100)Si. Current thresholds of as low as 170 mA in 10‐μm‐wide stripe lasers have been achieved at 280 K. Power output as high as 44 mW per facet was also obtained. Assuming no current spreading, the corresponding current threshold density is 6.9 kA/cm2. Slope efficiencies and T0 values of 0.18 W/A and 165 K, respectively, have also been obtained. These results are directly attributable to the reduction of dislocations by choosing growth conditions and step density on the surface.

Electron mobility in single and multiple period modulation‐doped (Al,Ga)As/GaAs heterostructures

Single and multiple period modulation‐doped (Al,Ga)As/GaAs heterostructures have been grown by molecular beam epitaxy (MBE) and characterized by Hall measurements. The basic structure consists of alternating layers of undoped GaAs and doped (Al,Ga)As separated by undoped ’’intrinsic’’ layers of (Al,Ga)As. The thickness of each of these layers was varied independently in a one, three, or nine period structure. Mobilities as high as 211 000, 95 500, and 8 360 cm2V−1 s−1 were obtained at 10 K, 78 K, and 300 K, respectively. The total sheet charge concentrations were about 2.25×1012 cm−2 in the highest mobility structures. Single period structures with an AlAs mole fraction of 0.33 typically had higher room‐temperature mobilities while multiple period structures with an AlAs mole fraction of 0.2 gave better results at cryogenic temperatures. The enhanced mobilities are attributed to the separation of electrons and donors by the undoped (Al,Ga)As layers and the high quality interfaces obtainable by MBE. To our...

Photoconductivity effects in extremely high mobility modulation‐doped (Al,Ga)As/GaAs heterostructures

Single period modulation doped Al0.35Ga0.65 As/GaAs heterostructures were grown by molecular beam epitaxy. The mobilities and sheet carrier concentrations were measured as a function of lattice temperature in the dark and in room light. Mobilities as high as 8490, 105 000, and 221 000 cm2/Vs at 300, 78, and 10 K, respectively, were obtained for samples measured in the dark. When measured in light, these mobilities increased to 9090, 136 000, and 286 000 cm2/Vs at the same respective temperatures. In all cases the sheet carrier concentrations were between 4.5×1011 and 9.5×1011 cm−2. These values represent one of the best dark values reported to date and are significant with respect to field effect transistor applications. The 300 K mobility of 9090 is equivalent to the best mobilities obtained in ultrapure GaAs (n⩽1013 cm−3). The change in the mobility and sheet carrier concentration is the result of a persistent photoconductivity effect which is attributed to the ionization of electrons from traps in the ...

Transport in modulation‐doped structures (AlxGa1−xAs/GaAs) and correlations with Monte Carlo calculations (GaAs)

Electron velocity and mobility versus electric field measurements were made in AlxGa1−xAs/GaAs modulation‐doped structures in an electric field range of 0–2 kV/cm and in a temperature range of 77–300 K. The electron velocities obtained at 2 kV/cm were 1.7×107 cm/ s at 300 K and 2.24×107 cm/s at 77 K. The velocity‐field and mobility‐field characteristics obtained were in very good agreement with Monte Carlo results for undoped GaAs, suggesting that existing Monte Carlo calculations are capable of predicting transport in AlxGa1−xAs/GaAs modulation‐doped structures. An ideal modulation‐doped structure can thus be assumed to exhibit transport properties identical to that of undoped GaAs. The advantage of modulation doping is the ability to place 7×1011 cm−2 electrons at the heterointerface for current conduction for devices, while avoiding all the adverse effects of impurity scattering.

Monolithic integration of GaAs/AlGaAs modulation‐doped field‐effect transistors and N‐metal‐oxide‐semiconductor silicon circuits

We have demonstrated for the first time the compatibility of GaAs with Si N‐channel metal‐oxide‐semiconductor (NMOS) transistors by successfully fabricating GaAs/AlGaAs modulation‐doped field‐effect transistors (MODFET’s) on a Si wafer containing NMOS devices. The MODFET’s with 2 μ gate length on 6 μ channels exhibited transconductances of 120 and 180 mS/mm at 300 and 77 K, respectively. The NMOS devices exhibited little if any performance degradation in going through the GaAs growth and fabrication process. These results show that the monolithic integration of GaAs devices with Si devices is possible, which may add a new dimension to the already exploding world of electronics.