AlAs/GaAs Schottky-collector resonant-tunnel-diodes

The Schottky-collector resonant-tunnel-diode (SRTD) is an resonant-tunnel-diode with the normal N⁺ collector layer and ohmic contact replaced by direct Schottky contact to the space-charge layer, thereby eliminating the associated parasitic series resistance Rins. By scaling the Schottky collector contact to submicron dimensions, the device periphery-to-area ratio is increased, decreasing the periphery-dependent components of the parasitic resistance, and substantially increasing the device's maximum frequency of oscillation. We report measured d.c. and microwave parameters of planar SRTDs fabricated with 1 μm-geometries in AlAs/GaAs.     

Process and device characterization of high voltage gallium arsenide P-i-N layers grown by an improved liquid phase epitaxy method

GaAs P-i-N layers with an i-region net doping of less than 10¹² cm⁻³ were grown on P⁺ and N⁺ substrates by a modified liquid phase epitaxy (LPE) method. Doping profiles and structural data obtained by varius characterization techniques are presented and discussed. A P⁺-P-i-N-N⁺ diode with a 25 μm-wide i-region exhibits a breakdown voltage of 1000 V, a t_rr of 50 ns, and reverse current densities (at V_R = 800 V) of − 3 × 10⁻⁶ A/cm² at 25°C and 10⁻² A/cm² at 260° C.     

Monte Carlo study of electron relaxation in quantum-wire laserstructures

Very-low threshold currents are expected to be achieved in quantum-wire lasers owing to the singularity in the density of states occurring at the bandedge. On the other hand, the high-speed modulation of quantum-wire lasers may be limited by carrier relaxation processes that are greatly affected by the reduction in the momentum space. In this paper, we calculate the electron relaxation times for GaAs/AlGaAs wires of various cross sections assuming that electrons are injected in a thermal distribution at the edge of the potential well formed by the barrier. The relaxation times are extracted from the time evolution of the carrier distribution as the electrons come to thermal equilibrium with the lattice. The Monte Carlo method is used to simulate the details of the relaxation process with the inclusion of electron-bulklike phonon, electron-electron and electron-hole interactions. We find that the electron relaxation times range from 120 ps for the 100×100 Å wire to 30 ps for the 200×200 Å wire for a carrier density of 10¹⁸ cm^-3. When the electron-hole interaction is included into the calculations, the equilibration time for the 100×100 Å wire is reduced to ≈50 ps. Screening effects are incorporated using the Thomas-Fermi formalism. At a carrier concentration of 10¹⁶ cm^-1, the equilibration times for the corresponding wire sizes are 20 and 5 ps. Thus, the relaxation time calculated within the limits of our model decreases with an increased wire cross section. This trend indicates the presence of a trade-off between speed and efficiency in quantum-wire lasers considering that the threshold current is decreased by reducing the wire cross section     

Physics-based modeling of submicron GaN permeable base transistors

We present the first physics-based nonstationary modeling of a submicron GaN permeable base transistor. Three different transport models are compared: drift-diffusion, energy balance, and ensemble Monte Carlo. Transport parameters and relaxation times used by the carrier transport equations are consistently derived from particle simulation. The current-voltage (I-V) characteristics predicted with the energy balance model are in good agreement with those obtained from direct Monte Carlo device simulation. On the other hand, the drift-diffusion approach appears to be inadequate for the device under study, even if improved high-field mobility models are adopted     

Cyclotron oscillations in a tilted magnetic field: A tool to measure the mean free path of ballistic electrons in high purity GaAs

The mean free path (mfp) of ballistic electrons in high purity GaAs was measured using a novel effect due to the cyclotron motion of ballistic electrons in tilted magnetic fields. Whenever the cyclotron orbit is commensurate with the device length an increase in the collected current is observed, leading to pronounced oscillations. We utilize the fact that the total path length varies as we change the angle of the magnetic field to extract the mfp using a single device. For high purity GaAs with an impurity concentration N_a+N_d≈6 × 10¹⁴ cm⁻³ we measure an mfp of ≈4 μm for electrons below the optical phonon energy. We suggest that the observed mfp is primarily due to impact ionization of neutral donors by the ballistic electrons. The large contrast of the oscillations proves that both injection and collection give preference to the direction normal to the layers and thus, that the momentum component parallel to the layers is conserved during transmission over the barrier.     

Assessment of approximate balance equation transport models used for submicron silicon device modelling

The common approximations found in literature for the balance equation method used for non-stationary submicron device simulation are presented and discussed. To assess different approximations, the full balance equation model is solved numerically solved for the n⁺-i-n⁺ submicron silicon structure where nonstationary transport effects eventually take place. The results obtained from the derived approximate models applied to the same silicon structure are compared with those obtained from the full model. In all cases, empirical formulae for momentum and energy relaxation times are used to clarify only the effect of approximations on the simulation results.     

Hot electron energy relaxation via acoustic phonon emission in GaAs/Al0.24Ga0.76As single and multiple quantum wells

The energy relaxation associated with acoustic phonons has been investigated in a series of modulation doped GaAs/AlGaAs single and multiple quantum wells grown by molecular beam epitaxy, using the hot electron Shubnikov — de Haas effect. The power loss is shown to be proportional to (T_e² − T_L²) for electron temperatures 2.2K < T_e < 8K and proportional to (T_e³ − T_L³) for 8K < T_e < 20K. The energy loss rates due to acoustic phonon scattering via both deformation potential coupling and piezoelectric coupling have been calculated. The total energy loss rate as a function of electron temperature is compared with the experimental results. Good agreement is obtained for 2.2K < T_e < 8K. Above 8K the energy loss rate is seen to rise above the predicted values, indicating the onset of an extra energy relaxation mechanism. The application of a high electric field (E = 3kV/cm) at low lattice temperatures is shown to induce persistent parallel conduction and a subsequent reduction of the low field well mobility.     

The use of generalised models to explain the behaviour of ohmic contacts to n-type GaAs

The use of two generalised carrier transport models to account for the N_D⁻¹ dependence of the specific contact resistance (ρ_c) of metal-semiconductor Ohmic contacts to n-type GaAs is proposed. Both models include the effects of thermionic emission and diffusion across the high-low barrier junction a priori. Calculations of ρ_c, and comparison with experimental data, show conclusively that thermionic emission is the dominant transport mechanism across the barrier. It is stressed that these models do not rely on prior choices of either of the transport processes. These conclusions are arrived at a posteriori.     

Scaled performance for submicron GaAs MESFET's

Scaling schemes for GaAs MESFET's below the submicron gate length are proposed. The corresponding switching times are calculated accurately down to 0.25µm gate length devices using an ensemble Monte Carlo simulation program. It is demonstrated that the proposed scaled devices offer ultrashort switching time due to the nonstationary carrier transport effects.     

Energy relaxation of light holes in InAs.15Sb.85/InSb multiple quantum wells

The energy relaxation rate of light in-plane holes in InAs_.15Sb_.85/InSb quantum wells has been measured using a Shubnikov-de Haas technique. In this Type II system, the holes reside in the InSb layers; strain reverses the heavy-light hole ordering and thus light holes are the charge carriers. The samples consist of 20 to 100 InAs_.15Sb_.85/InSb periods 100Å/200Å thick. The InAsSb barriers are doped with Be. The total carrier concentration p ≈ 1×10¹¹ cm⁻² is obtained from Hall data. Shubnikov-de Haas oscillation amplitudes are measured and used to determine the light hole temperature for a given applied power. The steady state power per carrier is equated to the energy relaxation rate to determine the carrier temperature T_H. The measured rate for low electric fields is proportional to T_Hⁿ-T_Lⁿ with n ≈ 3.2 and 2.5 for two different samples. These data are compared with theory and experiment for light holes in InGaAs/GaAs quantum wells.     

n-Channel AlSb/GaSb modulation-doped field-effect transistors

We report the first fabrication of a GaSb n-channel modulation-doped field-effect transistor (MODFET) grown by molecular beam epitaxy. The modulation-doped structure exhibits a room temperature Hall mobility of 3140 cm² V⁻¹ s⁻¹ and 77 K value of 16000 cm² V⁻¹ s⁻¹, with corresponding sheet carrier densities of 1.3 × 10¹² cm⁻² and 1.2 × 10¹² cm⁻². Devices with 1 μm gate length yield transconductances of 180 mS mm⁻¹ and output of 5 mS mm⁻¹ at 85 K. The device characteristics indicate that electron transport in the channel occurs primarily via the L-valley of GaSb above 85 K. The effective electron saturation velocity is estimated to be 0.9 × 10⁷ cm s⁻¹. Calculations show that a complementary circuit consisting of GaSb n- and p-channel MODFETs can provide at least two times improvement in performance over AlGaAs/GaAs complementary circuits.     

Quantized conductance in a Si/Si0.7Ge0.3 split-gate device and impurity-related magnetotransport phenomena

We have defined a quantum point-contact by the split-gate technique in a Si/SiGe heterostructure containing a two-dimensional electron gas with an elastic mean free path of about 1.3 μm. The conductance of this device shows typical steps very close to multiples of 4e² h⁻¹. Upon application of a perpendicular magnetic field the spin and valley degeneracies are lifted and magnetic depopulation of the one-dimensional subbands can be observed. The appearance of Aharonov-Bohm oscillations for B ≥ 2T and of resonant tunneling peaks close to “pinch-off” indicates the presence of impurities close to the constriction.     

Determination of the LO-phonon and Γ→L intervalley scattering time in GaAs from hot electron luminescence spectroscopy

Both the LO-phonon scattering time and the Γ→L intervalley scattering time for electrons in the conduction band of GaAs are of fundamental importance, and they are needed for the modelling of devices. We measure the steady state distribution of hot electrons in lightly p-doped bulk GaAs under carrier densities of 10¹³–10¹⁴cm⁻³, which is orders of magnitude lower than in pulsed laser experiments. Using a 16×16 k.p Hamiltonian and taking into account the transition matrix elements in a dipole model, we determine the hot electron lifetime from comparison with the experimentally found lifetime broadening. For electrons with a kinetic energy of 100meV to 300meV we obtain τ_LO=(132±10)fs. We also determine the Γ→L intervalley separation as E_ΓL=(300±10)meV. We find Γ→L scattering times around 150fs to 200fs, corresponding to a value for the associated deformation potential of D_ΓL=(9.4±1.5)×10⁸eV/cm.     

GaN based transistors for high power applications

Unique properties of GaN and related semiconductors make them superior for high-power applications. The maximum density of the two-dimensional electron gas at the GaN/AlGaN heterointerface or in GaN/AlGaN quantum well structures can exceed 2×10¹³ cm⁻², which is more than an order of magnitude higher than for traditional GaAs/AlGaAs heterostructures. The mobility-sheet carrier concentration product for these two dimensional systems might also exceed that for GaAs/AlGaAs heterostructures and can be further enhanced by doping the conducting channels and by using “piezoelectric” doping. Current densities over 20 A mm⁻¹ can be reached in GaN-based high electron mobility transistors (HEMTs). These high current values can be combined with very high breakdown voltages in high-power HEMTs, which are expected to reach several thousand volts. Recent Monte Carlo simulations point to strong ballistic and overshoot effects in GaN and related materials, which should be even more pronounced than in GaAs-based compounds but at much higher electric fields. This should allow us to achieve faster switching, minimizing the power dissipation during switching events. Self-heating, which is unavoidable in power devices, raises operating temperatures of power devices well above the ambient temperature. For GaN-based devices, the use of SiC substrates allows to combine the best features of both GaN and SiC technologies; and GaN/SiC-based semiconductors and heterostructures should find numerous applications in power electronics.     

Modeling multi-band effects of hot electron transport in silicon by self-consistent solution of the Boltzmann transport and Poisson equations

This paper presents a self-consistent numerical technique for the solution of the multi-band Boltzmann transport equation (BTE) and the Poisson equation in silicon. The effects of high energy bands ( 3 eV) are modeled in the formulation. The numerical technique utilizes a new curvilinear boundary-fitted coordinate grid which is tailored for self-consistent calculations. A new Scharfetter-Gummel like discretization of the BTE is presented. The numerical algorithm is tested on a n⁺ − n − n⁺ device structure.     

Overshoot saturation in ultra-submicron FETs due to minimum acceleration lengths

Ultra-submicron GaAs MESFETs have been fabricated with gate lengths ranging from 25 nm to 80 nm, using an electron-beam lithography process, in order to examine the operating characteristics as a function of gate length. The MESFETs were fabricated on wafers doped at 2×10¹⁷ cm⁻³ and 1.5×10¹⁸ cm⁻³ with active layer thicknesses of 250 nm and 60 nm, respectively. Measurements of the transconductance, and the inferred transit velocity, as a function of the effective gate length show a minimum in these quantities near 55 nm. The rise in transconductance below this gate length is attributed to the onset of velocity overshoot in the channel region, and both the inferred transit velocity and the variations between the lightly doped samples and the heavily doped samples support this interpretation. For gate lengths below about 40 nm, however, the transconductance again drops. We attribute this drop to the existence of a minimum acceleration length needed for the carriers to reach the high values of the overshoot velocity. We have investigated this behavior with a transient transport model based upon a parameterized velocity autocorrelation function incorporating both energy and momentum relaxation rates. The results of this model yield qualitative agreement with both the measurements and the interpretation given above.     

High-field transport with hot phonons in degenerate semiconductors

The effect of phonons in non-thermodynamic equilibrium (hot-phonons) on steady-state transport in degenerate bulk and two-dimensional (2D) semiconductor structures is studied. The phonon-drift effects are taken to be negligible, and, the formulation is confined to electrons in a single parabolic band interacting with a - population of polar-optic phonons, restricted to interface modes in the 2D case. Hot phonons in bulk semiconductors lead not only to a decrease in the energy relaxation rate, but, also, to a decrease in the mobility as the carrier concentration n_3D is increased. Hot phonons in 2D systems, on the other hand, cause the energy relaxation rate and mobility to go through a minima as the electron concentration n_2D is increased. The electron drift velocity, contrary to the bulk case, is seen to with n_2D for n_2D > 10¹² cm⁻², and reflects the difference in the nature of electron density of states in the two cases.     

Accurate modeling for submicrometer-gate Si and GaAs MESFET's using two-dimensional particle simulation

Device characteristics, including nonstationary carrier-transport effects such as velocity overshoot phenomena in submicrometergate Si and GaAs MESFET's, are presented in detail by two-dimensional full Monte Carlo particle simulation. Accurate current-voltage characteristics and transient current response are successfully obtained without relaxation time approximation. Moreover, the carrier dynamics influence on device operation is clarified in a realistic device model, compared with the conventional simulation. It can be pointed out that such nonstationary carrier transport is acutely important for accurate modeling of submicrometer-gate GaAs MESFET's, but is not as important for that of Si MESFET's.     

Numerical analysis of nonequilibrium electron transport inAlGaAs/InGaAs/GaAs pseudomorphic MODFETs

Nonequilibrium electron transport in InGaAs pseudomorphic MODFETs has been analyzed with the moment equations approach. In the model, the momentum and energy balance equations for the two-dimensional electrons in the InGaAs channel are solved with relaxation times generated from a Monte Carlo simulation. The two-dimensional electron wave functions and the quantized state energies in the InGaAs quantum well are calculated exactly from the Schrodinger equation along the direction perpendicular to the quantum well. Also included is a two-dimensional Poisson equation solver. In the calculation, all of the equations are solved iteratively until a self-consistent solution is achieved. The simulation results for a realistic device structure with a 0.5-μm recessed gate show a significant overshoot velocity of 4.5×10⁷ cm/s at a drain bias of 1.0 V. Electron temperature reaches a peak value of around 2500 K under the gate. In energy transport, the diffusive component of the energy flux is found to be dominant in the high-field region     

Impulse responses of submicron GaAs photodetectors

Metal–semiconductor–metal photodetectors with different submicron spacings (d = 100, 300, 500, 700 and 900 nm) were fabricated on GaAs with a carrier recombination time of 100 ps by electron beam lithography. Temporal responses of the detectors were measured by photoconductive sampling in order to identify factors which limits the response speeds. At a low excitation of <100 μW, the response speeds of 100, 300 and 500 nm spacing detectors are limited by parasitic capacitances of the submicron structures. The speeds of 700 and 900 nm spacing detectors are limited by an electron/hole transport in the semiconductor. At a high excitation of >100 μW, the response speeds of the all spacing detectors are limited by field screening caused by electron–hole plasma.