V. P. Kesan

A new transit-time device using quantum-well injection

The use of such techniques as molecular beam epitaxy has allowed the fabrication of devices in which tunneling is the dominant transport mechanism. In this paper a new transit-time device which uses resonant tunneling through a quantum well is proposed and analyzed. Depending on the bias level, this device may permit injection of carriers into the drift region at more favorable phase angles (hence higher efficiencies) than other transit-time devices. The device promises low noise performance and should be capable of operating at high millimeter-wave frequencies with higher output power than other transit-time devices or pure quantum-well oscillators. Since the device uses quantum-well injection and transit-time effects, it is called a QWITT diode.

Microwave and millimeter-wave QWITT diode oscillators

The authors present DC, microwave, and millimeter-wave characteristics of different quantum-well-injection transit-time (QWITT) devices. Small-signal and large-signal device models are used to provide physical design parameters to maximize the output power density at any desired frequency of operation. A peak output power density of 3.5-5 kW/cm/sup 2/ in the frequency range 5-8 GHz has been obtained from a planar QWITT oscillator. This appears to be the highest output power density obtained from any quantum-well oscillator at any frequency. This result also represents the first planar circuit implementation of a quantum-well oscillator. Good qualitative agreement between DC and RF characteristics of QWITT devices and theoretical predictions based on small-signal and large-signal analyses is achieved. The device efficiency has been increased from 3% to 5% by optimizing the design of the drift region in the device through the use of a doping spike with optimized concentration, without compromising the output power at X-band. Self-oscillating QWITT diode mixers are also demonstrated at X-band in both waveguide and planar circuits. The self-oscillating mixer exhibits a conversion gain of about 10 dB in a narrow bandwidth and a conversion loss of about 5 dB if broadband operation is desired. >

Photoluminescence studies of pseudomorphic modulation‐doped AlGaAs/InGaAs/GaAs quantum wells

We discuss the photoluminescence (PL) properties of pseudomorphic modulation‐doped Al0.15Ga0.85As/In0.2Ga0.8As/GaAs quantum wells as a function of temperature. At 4.2 K, hole localization influences the PL linewidth; however, at higher temperatures (77 K) the thermal energy of photoexcited holes is sufficiently large to obtain a reliable measure of sheet carrier density from the PL linewidth. Our results also suggest that information about the interface quality can be obtained from an analysis of the PL linewidth at 77 and 4.2 K. The spectra taken from several samples clearly show that the PL transition energy exhibits a free‐carrier density dependence due to band‐gap renormalization and electric field effects.

Dependence of apparent barrier height on barrier thickness for perpendicular transport in AlAs/GaAs single‐barrier structures grown by molecular beam epitaxy

Current‐voltage characteristics of single‐barrier AlAs/GaAs heterostructures measured over a wide temperature range are used to elucidate the mechanisms governing electron transport through these barriers. Five barriers, ranging in thickness from 14.2 to 150 A, are examined. The results clearly illustrate Γ‐band, elastic‐tunneling‐dominated transport for the thinnest barrier (14.2 A) devices and thermionic emission characteristics for the thickest barrier (150 A) devices. However, devices with an intermediate barrier thickness exhibited tunneling‐like currents larger than calculated low‐temperature elastic tunneling currents. This effect is apparently due to inelastic tunneling from the GaAs Γ band through the AlAs X‐point barrier.

Photoluminescence and electroreflectance studies of modulation-doped pseudomorphic AlGaAs/InGaAs/GaAs quantum wells

In this study, we describe the correlations between the photoluminescence (PL) spectra and electrical properties of pseudomorphic modulation-doped AlGaAs/InGaAs/GaAs quantum wells (MDQWs) grown by molecular beam epitaxy. In MDQWs, the presence of a large sheet carrier density contributes significantly to the PL linewidth. At low temperatures (4.2 K), free carrier induced broadening of the PL linewidth is influenced by the material quality of the structure. At higher temperatures (77 K), differences in the material quality do not affect the linewidth significantly, and under these conditions the PL linewidth is a good measure of the sheet carrier density. The ratio of the 77 K to 4.2 K PL linewidths provides useful information about the crystalline quality of the MDQW structures as illustrated by the correlation with 77 K Hall mobility data and a simple model. We present results of Electron Beam Electroreflectance (EBER) to characterize MDQWs and undoped quantum wells in the AlGaAs/InGaAs/GaAs material system. Several transitions have been observed and fitted to excitonic Lorentzian lineshapes, providing accurate estimates of transition energy and broadening parameter at temperatures of 96 K and 300 K.

A Self Oscillating Qwitt Diode Mixer

A waveguide and planar self oscillating mixer using the Quantum Well Injection Transit Time (QWITT) diode have been tested at X-band. The waveguide mixer circuit can either be tuned to have a narrow band conversion gain of about 10 dB or broad band conversion loss of 4 dB. The planar mixer circuit exhibits a narrow band conversion gain of 4 dB or a broad band conversion loss of 8 dB. These are the first experimental results showing conversion gain from a self oscillating mixer using a quantum well diode.

Power-optimized design of quantum well oscillators

The use of quantum well devices for extremely high frequency oscillators has recently been proposed. Results up to 200 GHz have been reported, although the output powers have been extremely small. We present analyses of a new quantum well oscillator device, the quantum well injection transit time (QWITT) diode, which exploits transit time effects to improve rf performance of quantum well oscillators. The small and large signal analyses indicate that a QWITT diode may provide reasonable power levels at frequencies above 100 GHz.

Growth and rapid thermal annealing of AlGaAs/InGaAs pseudomorphic modulation‐doped structures

We have examined molecular beam epitaxial growth conditions required to obtain good surface morphology and excellent electrical properties from normal and inverted pseudomorphic Al0.15Ga0.85As/InyGa1−yAs (y=0.15–0.20) high electron mobility transistor (HEMT) structures. For the same spacer layer thickness, inverted pseudomorphic HEMT’s exhibit significantly higher sheet carrier concentrations than corresponding normal HEMT’s. In addition, we report the first study of the influence of rapid thermal annealing on the electrical properties of pseudomorphic HEMT’s, using both close‐contact and arsenic overpressure annealing. An improvement in two‐dimensional electron gas (2‐DEG) mobility with increasing anneal temperatures that is consistent with reduction in strain in the InGaAs channel is observed. This study also shows that pseudomorphic HEMT’s exhibit excellent thermal stability with no degradation in the electrical characteristics of the device for anneal temperatures up to 800 °C.

Microwave and millimeter wave QWITT diode oscillator

The authors present the DC, microwave, and millimeter-wave characteristics of different quantum well injection transit time (QWITT) diodes. Small-signal and large-signal device models are used to provide physical device design parameters to maximize output power density. A peak output power of 1 mW in the frequency range of 5-8 GHz has been obtained from a planar QWITT oscillator. In addition, millimeter-wave oscillations at 28-31 GHz in a full-height waveguide circuit with an output power of 30 mu W have been obtained. Results on improving device efficiency by optimizing the design of the drift region through the use of a doping spike are also presented. By optimizing the doping concentration, and width of the doping spike, an increase in efficiency from 2% to 5% is obtained, without compromising on output power at X-band.<<ETX>>

Time dependent simulation of the quantum well injection transit time diode

The quantum well injection transit time (QWITT) diode is a transit time device that couples a heterojunction barrier structure as an injector with a depleted epitaxial drift region, as shown in Figure 1. The device, which was originally proposed by Kesan et al. [1], is a promising solid state source with potential for operation at the higher millimeter wave frequencies. Both large and small signal analyses of the device have shown that the specific negative resistance due to transit time effects from the drift region are on the order of, or are significantly greater than, the intrinsic negative differential negative resistance of the quantum well double barrier diode. This paper presents results of a large signal simulation which uses a full set of semiconductor time dependent transport equations, and solves self-consistently for the electric field as well as for the electron and hole populations throughout the device.