T. J. Drummond

Current—Voltage and capacitance—Voltage characteristics of modulation-doped field-effect transistors

A model describingI-VandC-Vcharacteristics of modulation doped FETs is developed and used to predict the performance of AlxGa1-xAs/GaAs FETs in good agreement with our experimental results. It is shown that the change in the Fermi energy with the gate voltage changes the effective separation between the gate and the two-dimensional electron gas by about 80 Å. Current-voltage characteristics were calculated using a two piece as well as a three piece linear approximation for the electron velocity and compared with experimental results. At 300 K, the two piece model overestimates the current predicted by the three piece model only by approximately 10-20 percent. At 77 K, however, the three piece linear approximation for the velocity field characteristic should be used since the electron mobility decreases very abruptly at about 200 V/cm. The effect of the nonlinear source resistance is also discussed along with the gate-to-source and gate-to-drain capacitances, parameters of paramount importance in determining device performance. These capacitances are calculated as functions of gate-to-source and drain-to-source voltages below saturation.

Electron density of the two‐dimensional electron gas in modulation doped layers

The electron density of the two‐dimensional electron gas in modulation doped structures is calculated as a function of the doping density in (Al,Ga)As, the thickness of the undoped (Al,Ga)As layer, the lattice temperature, and other device parameters. The results of the calculation show that the depletion approximation is not accurate enough and that the Fermi–Dirac statistics (rather than the Boltzmann statistics) should be used in the calculation. A simple analytical model which takes these factors into account is shown to be in good agreement with our computer calculations and experimental data. The obtained results may be used to evaluate the maximum intrinsic transconductance and the maximum gate voltage swing for the modulation doped field effect transistors.

Low field mobility of 2‐d electron gas in modulation doped AlxGa1−xAs/GaAs layers

We derive a simple analytical formula for the low field electron mobility which uses the 2‐d degenerate statistics of the 2‐d electron gas. This takes into account the finite width of the depletion layer in (Al,Ga)As for the scattering by remote donors, scattering by the interface charge, and the polar‐optical and acoustic deformation potential and piezoelectric scattering. The largest measured value of mobility is determined by scattering due to interface charge in some cases. The ultimate value of the mobility which may be achieved is limited by the acoustic deformation potential and piezoelectric scattering at about 6.5×106 cm2/V s for an interface carrier density of the 2‐d electron gas ns0 ≂4×1011 cm−2. Our results agree very well with experimental data obtained in our laboratory as well as other laboratories.

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.

On the collapse of drain I-V characteristics in modulation-doped FET's at cryogenic temperatures

The collapse of the drain current-voltage characteristics of modulation-doped field-effect transistors (MODFETs) at cryogenic temperatures, previously thought to be unavoidable, has been investigated. The results indicate that the mechanism responsible for the collapse is dependent on both the device fabrication steps and the parameters of crystal growth. Bulk AlxGa1 - xAsFETs fabricated in our laboratory exhibited little or no collapse in the I-V characteristics at 77 K in the dark, demonstrating that the mechanism responsible for this pheonomenon is not related to problems associated with contacting AlxGa1 - xAs. MODFETs with proper fabrication and growth procedures showed no collapse. In those devices exhibiting no collapse, the source resistance exhibited a substantial decrease upon cooling. At 300 K source resistances slightly over 1.0 Ω . mm with a transconductance of 170 mS/mm were obtained. Upon cooling, the source resistance decreased to 0.5 Ω . mm with a transconductance of 280 mS/mm. These results demonstrate that MODFETs will exhibit enhanced performance at 77 K without exposure to light. Specific contact resistivities measured at room temperature ranged from 2 × 10-7to 2 × 10-6Ω cm2depending on the structural parameters.

Use of a superlattice to enhance the interface properties between two bulk heterolayers

Single interface modulation‐doped Alx Ga1−xAs /GaAs heterostructures with the binary on top of ternary were grown by molecular beam epitaxy. By incorporating a 150‐A‐thick Alx Ga1−xAs /GaAs three‐period superlattice in place of an undoped Alx Ga1−xAs spacer layer, 10‐K mobilities of up to 256 000 cm2/Vs were obtained. This value is about 6.5 times that of the previous best value. This dramatic improvement is tentatively attributed to the relief of strain caused by the small but significant lattice mismatch although impurity trapping by the superlattice may also play a role. Normal modulation‐doped structures where the ternary is grown on top of binary also showed mobility improvement (about 30%) when the undoped AlGaAs spacer layer is replaced with a three‐period superlattice of the same thickness. This concept should have a significant role in heterojunction bipolar transistors, field‐effect transistors, lasers, and other heterojunction devices.

Parasitic MESFET in (Al, Ga) As/GaAs modulation doped FET's and MODFET characterization

A charge-control model for n-channel modulation doped FETs (MODFETs) is extended to include the drain-to-source current through the doped (Al, Ga)As layer which becomes important for large positive gate voltages. This parasitic conduction leads to decreased device transconductances at high gate voltages. A unified and complete characterization technique for deducing the parameters of our model is introduced and used for the device characterization. Parameters, e.g., the saturation velocity, two-dimensional gas concentration at equilibrium, thickness of the doped (Al, Ga)As layer, etc., deduced using the model, are in good agreement with the independent calculations and measurements. However, the deduced values of the room-temperature low field mobility of the two-dimensional electron gas are considerably smaller than those measured by Hall effect and in long-gate MODFETs. This model is in good agreement with the characteristics of high-current normally on MODFETs. The maximum measured current swing of 300 mA/mm gate is reported.

Bias dependence and light sensitivity of (Al, Ga)As/GaAs MODFET's at 77 K

Modulation doped field-effect transistors typically show a threshold-voltage shift of about 0.2 V at 77 K with respect to room temperature. An investigation of the characteristics of Al0.33Ga0.67As/ GaAs and Al0.24Ga0.76As/GaAs MODFETs confirms that the low temperature performance of these devices is affected by the presence of persistent photoconductive traps in the bulk (Al, Ga) As and the properties of the surface, both of which depend strongly on the Al mole fraction and the growth conditions. Al0.33Ga0.67As/GaAs MODFETs grown at 610°C show a threshold voltage shift of less than 0.05 V at 77 K with respect to room temperature and little sensitivity of the current-voltage characteristics on illumination and on bias condition, indicating that by proper control of the growth parameters it is possible to obtain high quality (Al, Ga)As/GaAs MODFETs suitable for operation 77K.

Interfacial properties of (Al,Ga)As/GaAs structures: Effect of substrate temperature during growth by molecular beam epitaxy

Selectively doped (Al,Ga)As/GaAs heterostructures exhibit extremely high electron mobilities, particularly at low temperatures, as a result of electron transfer from doped (Al,Ga)As layer(s) into adjoining GaAs layers. These transferred electrons are confined to within about 100 A of the interface and the electron mobility is extremely dependent on the structural quality of the heterointerface. By studying low‐field mobility of single period (Al,Ga)As/GaAs and inverted GaAs/(Al,Ga)As structures, the quality of each interface can be ascertained. Such single period structures have been prepared by molecular beam epitaxy in a substrate growth temperature range of 600–770 °C to investigate heterointerfaces with GaAs on top of (Al,Ga)As and (Al,Ga)As on top of GaAs. While very good interfaces with (Al,Ga)As on top have been obtained in the growth temperature range of 600–750 °C, the interface quality of the inverted structure has shown a very strong dependence on the substrate temperature. The best interface h...

Electron mobilities in modulation doped Ga0.47In0.53As/Al0.48In0.52 As heterojunctions grown by molecular beam epitaxy

Modulation doped Ga0.47In0.53As‐Al0.48In0.52 As single‐period heterostructures have been prepared by molecular beam epitaxy (MBE). Electron mobilities as high as 8915 cm2/V s at 300 K, 60 120 cm2/V s at 77 K, and 90 420 cm2/V s at 10 K were obtained with an average electron concentration of ∼1017 cm−3. These results represent a mobility enhancement over uniformly doped Ga0.47In0.53 As with the same carrier concentration by about a factor of 6 at 77 K and by a factor of 2 at 300 K. Single‐period modulation doped heterostructures have shown enhanced mobility in spite of the relative positions of the Si‐doped Al0.48In0.52 As layer and the undoped Ga0.47In0.53 As layer in contrast to the case of GaAs/AlxGa1−x As system where mobility enhancement has only been observed for MBE grown AlxGa1−x As on top of the undoped GaAs layer.