Quantum Well Injection Transit Time Research Articles

Millimeter wave QWITT diode oscillator

The quantum well injection transit time (QWITT) diode charecteristics, small signal and large signal device models are used. A peak output power density of 35–5 KW/Cm2 in the frequency range of 5–8 GHZ hasx been obtained from a planar QWITT Oscillator

A generalized analytical model for the quantum well injection transit time diode

A generalized small-signal model of the quantum well injection transit time (QWITT) diode derived from the authors' previous large-signal model (see. ibid., vol.35, p.2315-2322, Dec. 1987), which includes not only the carrier space-charge effects but also the velocity transient effects and the carrier diffusion effects is presented. Simple closed forms for the device impedance have been obtained for efficient computation, where only one-dimensional integrations are required. It can be applied to any fashion of time dependence of the velocity transient and diffusivity transient, adopting a Gaussian form for the spatial profile of injected carriers. Using the formulas, the small-signal behavior and the design criteria for the QWITT diode are analyzed. Large-signal impedance of the device can also be estimated by the formulas. >

State-space analysis of the quantum-well injection transit time diode

A state-space linear model of the quantum-well injection transit time (QWITT) diode is developed. The resulting system of equations is suitable for time- and frequency-domain analysis of the QWITT diode with its external circuit, and since the eigenvalues (complex resonant frequencies) are an integral part of the formulation, the method is extremely useful for the design of oscillator circuits and for the study of stability problems that are associated with supplying bias to the diode. The model includes the effects of velocity overshoot and carrier diffusivity as well as the physical geometry of the devices being studied. It is tested by comparing the predicted small-signal impedance with other well-known models for similar devices. Using state-space analysis, it is predicted that long diodes with a positive injection conductance will not have an input impedance with a negative real part at any frequency.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Improved small-signal analysis of the quantum-well injection transit time diode

Two improved methods for calculating the small-signal impedance of the quantum-well injection transit time diode are discussed. The first extends the traditional analysis to include carrier velocity transient effects, producing an analytical expression from which optimum injection conductance and drift angle can be found. The second method includes the effects of both nonuniform carrier velocity and diffusion, casting the expression for impedance in terms of a double integration that is evaluated numerically. A lumped-element model that closely duplicates the impedance found by the numerical method is developed. A graphical means that allows the generation of the lumped-element model from the DC negative resistance and knowledge of the diode structure is provided. The more sophisticated models developed should be useful in device design, in evaluation of mounting and packaging schemes, and for suppression of unwanted oscillations. >

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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Microwave frequency operation of quantum-well injection transit time (QWITT) diode

In the letter we present DC and microwave characteristics of three different QWITT structures. A peak output power of ∼ 1 mW in the frequency range 2–8 GHz has been obtained. This is the highest output power obtained from any quantum-well oscillator at any frequency. We will show that, for the same quantum-well structure, by systematically increasing the length of the depletion region (i.e. the drift region) on the anode side of the device, a corresponding increase in the specific negative resistance and output power is obtained. The significant increase in output power clearly suggests that the intrinsic device characteristics have been improved through the use of a drift region. This is in keeping with small-signal analysis for the QWITT diode, which predicts an improvement in the RF performance of the device with the use of an appropriate drift region length for a particular frequency of operation.

Analysis and simulation of the quantum well injection transit time diode

The quantum-well injection transit time (QWITT) diode is simulated for two different injection phase angles (90 degrees and 270 degrees ) at 60, 90, 200, and 300 GHz. Quantitative analysis of the output power and efficiency is carried out by including the velocity transient effect, the diffusion effect, and the carrier space-charge effect. The diffusion effect and the carrier space-charge effect degrade the output power and efficiency of the device. The velocity transient effect enhances the device performance for a 270 degrees injection phase mode, but it renders the device useless for a 90 degrees injection phase mode. In comparison with other microwave devices, a simple QWITT diode is a very promising device for millimeter-wave frequency application when it is used with a 270 degrees injection phase angle. This is due to fast intrinsic frequency response time and extremely localized carrier injection mechanism as well as high transient velocity at a small distance. Because of the good efficiency of the QWITT diode, it is feasible to increase output power by integration of many QWITT diodes. >

Efficiency and power output of the quantum-well injection transit-time device

We report a theoretical analysis of the efficiency and power capability of the newly proposed quantum-well injection transit-time (QWITT) device. When compared with traditional semiconductor sources such as TED or IMPATT, our results show that the QWITT is a far better source at millimeter (mm) frequencies. The improved performance of the QWITT is due to the high intrinsic frequency response time, as well as to the extremely localized carrier injection mechanism and the utilization of the high transient velocity of carriers at small distances. Computer simulations with special consideration of these effects clearly confirm these merits. For example, at 300 GHz, a single QWITT can provide an output power of 1.2 mW at 7.4-percent efficiency when it is matched to a resonant circuit with about 1-Ω resistance.