There are many different empirical means whereby the optical gap of an amorphous semiconductor may be defined. We analyze some hydrogenated amorphous silicon data with respect to a number of these empirical measures for the optical gap. By plotting these various gap measures as a function of the breadth of the optical absorption tail, we provide a means of relating these disparate measures of the optical gap. The applicability of this calibration to another set of hydrogenated amorphous silicon data is investigated.

The strong spontaneous polarization and piezoelectric effects in the wurtzite IIInitride semiconductors lead to new possibilities for device design. In typical heterojunction field effect transistors these effects are used to create large electron concentrations at the AlGaN/GaN interface. However, we examine several other possible device structures which include heterojunctions of AlGaN, GaN, and InGaN. For example, we find the strong electric fields present in these structures allow us to create quantum wells greater than 1 eV deep. Both Ga-faced and N-faced materials are explored. The two-dimensional electron gas concentrations in these structures are found using a self-consistent 1-D Schrödinger-Poisson solver modified to incorporate the effects of spontaneous and piezoelectric polarization. The boundary conditions at the heterojunction interfaces and at the surface and substrate are discussed in detail. Electron concentrations are compared with those obtained experimentally through capacitance-voltage and Hall effect measurements.

We study electron transport in the alloys of aluminum nitride and gallium nitride and alloys of indium nitride and gallium nitride. In particular, employing Monte Carlo simulations we determine the velocity-field characteristics associated with these alloys for various alloy compositions. We also determine the dependence of the low-field mobility on the alloy composition. We find that while the low-field mobility is a strong function of the alloy composition, the peak and saturation drift velocities exhibit a more mild dependence. Transient electron transport is also considered. We find that the velocity overshoot characteristic is a strong function of the alloy composition. The device implications of these results are discussed.

High quality Ga-face and N-face AlGaN/GaN based heterostructures have been grown by plasma induced molecular beam epitaxy. By using Ga-face material we are able to fabricate conventional heterojunction field effect transistors. Because the N-face material confines electrons at a different heterojunction, the resulting transistors are called inverted. The Ga-face structures use a high temperature AlN nucleation layer to establish the polarity. Structures from these materials, relying only on polarization induced interface charge effects to create the two-dimensional electron gases, are used to confirm the polarity of the material as well as test the electrical properties of the layers. The resulting sheet concentrations of the two dimensional electron gases agree very well with the piezoelectric theory for this materials system. Hall mobilities of the two-dimensional gases for the N-face structures are as high as 1150 cm2/Vs and 3440 cm2/Vs for 300 K and 77 K respectively, while the Ga–face structures yield room temperature mobilities of 1190 cm2/Vs. Both structures were then fabricated into transistors and characterized. The inverted transistors, which were fabricated from the N-face material, yielded a maximum transconductance of 130 mS/mm and a current density of 905 mA/mm. Microwave measurements gave an ft of 7 GHz and an fmax of 12 GHz for a gate length of 1 µm. The normal transistors, fabricated from the Ga-face material, produced a maximum transconductance of 247 mS/mm and a current density of 938 mA/mm. Microwave measurements gave an ft of 50 GHz and an fmax of 97 GHz for a gate length of 0.25 µm. This shows that using plasma induced molecular beam epitaxy N-face and Ga(Al)-face AlGaN/GaN heterostructures can be grown with structural and electrical properties very suitable for high power field effect transistors.

The energy dependent momentum and energy relaxation times, and the effective single valley energy dependent effective mass, are extracted from Monte Carlo simulations of gallium nitride, indium nitride, and aluminum nitride. A simple semi-analytical energy model, which uses these dependencies, is in good agreement with the results of transient Monte Carlo simulations. Both the Monte Carlo and the semi-analytical simulations show that the overshoot effects are most pronounced when the electric field abruptly changes from a value below a critical field to one above. This is attributed to the relatively large difference between the effective energy and momentum relaxation times for such a variation of electric field. Our calculations indicate that gallium nitride and indium nitride should have the most pronounced transient effects. A calculation of the transit times as a function of the gate length shows that an upper bound for the maximum expected cut-off frequencies are 260 GHz and 440 GHz for 0.2 μm gallium nitride and indium nitride field effect transistors, respectively.

High quality Ga-face and N-face AlGaN/GaN based heterostructures have been grown by plasma induced molecular beam epitaxy. By using Ga-face material we are able to fabricate conventional heterojunction field effect transistors. Because the N-face material confines electrons at a different heterojunction, the resulting transistors are called inverted. The Ga-face structures use a high temperature AIN nucleation layer to establish the polarity. Structures from these materials, relying only on polarization induced interface charge effects to create the two-dimensional electron gases, are used to confirm the polarity of the material as well as test the electrical properties of the layers. The resulting sheet concentrations of the two dimensional electron gases agree very well with the piezoelectric theory for this materials system. Hall mobilities of the two-dimensional gases for the N-face structures are as high as 1150 cm2/Vs and 3440 cm2/Vs for 300 K and 77 K respectively, while the Ga-face structures yield room temperature mobilities of 1190 cm2/Vs. Both structures were then fabricated into transistors and characterized. The inverted transistors, which were fabricated from the N-face material, yielded a maximum transconductance of 130 mS/mm and a current density of 905 mA/mm. Microwave measurements gave an ft of 7 GHz and an fmax of 12 GHz for a gate length of 1 μm. The normal transistors, fabricated from the Ga-face material, produced a maximum transconductance of 247 mS/mm and a current density of 938 mA/mm. Microwave measurements gave an ft of 50 GHz and an fmax of 97 GHz for a gate length of 0.25 μm. This shows that using plasma induced molecular beam epitaxy N-face and Ga(A1)-face AlGaN/GaN heterostructures can be grown with structural and electrical properties very suitable for high power field effect transistors.

We develop a simple, one-dimensional, analytical model, which describes electron transport in gallium nitride. We focus on the polar optical phonon scattering mechanism, as this is the dominant energy loss mechanism at room temperature. Equating the power gained from the field with that lost through scattering, we demonstrate that beyond a critical electric field, 114 kV/cm at T = 300 K, the power gained from the field exceeds that lost due to polar optical phonon scattering. This polar optical phonon instability leads to a dramatic increase in the electron energy, this being responsible for the onset of intervalley transitions. The predictions of our analytical model are compared with those of Monte Carlo simulations, and are found to be in satisfactory agreement.

Using an ensemble Monte Carlo approach, ballistic transport and velocity overshoot effects are examined in InN and compared with those in GaN and GaAs. It is found that the peak overshoot velocity is in general greater than both GaN and GaAs. Furthermore, the velocity overshoot in InN occurs over distances in excess of 0.4 μm, which is comparable to GaAs but is significantly longer than the overshoot in GaN. These strong overshoot effects, combined with a high peak drift velocity, large low-field mobility, and large saturation drift velocity, should allow InN based field effect transistors to outperform their GaN and GaAs based counterparts.

We determine the velocity-field characteristic of wurtzite indium nitride using an ensemble Monte Carlo approach. It is found that indium nitride exhibits an extremely high room temperature peak drift velocity, 4.2 × 107 cm/s, at a doping concentration of 1 × 1017 cm−3. This exceeds that of gallium nitride, 2.9 × 107 cm/s, by approximately 40 %. For our nominal parameter selections, the saturation drift velocity of indium nitride is found to be 1.8 × 107 cm/s. The device performance of this material, as characterized by the cut-off frequency, is found to superior to that of gallium nitride, gallium arsenide, and silicon.