S. Goswami

Wavelength selective detection using excitonic resonances in multiquantum-well structures

The wavelength and voltage dependence of photocurrent near excitonic resonances are used to study the wavelength selectivity of p-i(multiple quantum well, or MQW)-n photodiode structures with a parallel sequence of optical bits, each with a different wavelength. The selectivity is considered good if the state of a lambda /sub i/ wavelength bit can be detected regardless of lambda /sub j/(j not=i) state of the bits. Photocurrent is found to have very good selectivity only if lambda /sub j/ bits are all zero, i.e., the optical information is serial, but it is also found that differential photocurrent ( Delta I/sub ph// Delta V) provides a good selectivity for random states of lambda /sub j/ bits (i.e., parallel input). Four channel selectivity is demonstrated at 200 K. Specifically designed quantum-well structures can greatly improve this selectivity. >

Low-power exciton-based heterojunction bipolar transistors for thresholding logic applications

The principles of an integrated optoelectronic controller-modulator device, based on excitonic transitions and the enhanced Stark effect in quantum wells, are outlined. The device consists of a controller and a modulator as components. The controller is a heterojunction phototransistor with multiquantum wells incorporated in the base-collector depletion region. The amplified output of the controller enables switching of the modulator for low optical power levels. Experimental results on GaAs-AlGaAs based devices, realized by one-step molecular beam epitaxy and selective etching, are presented. The bipolar devices have current gains of approximately 35-40. The integrating-thresholding properties of the device are demonstrated and switching characteristics for 10 mu W input to the controller are measured. Cascadability, optoelectronic amplification, and multistage operation are demonstrated in terms of a fan out of eight devices. >

Temporal response characteristics of a In/sub 0.53/Ga/sub 0.47/As/InP quantum well phototransistor

An investigation of multiple-quantum-well heterojunction phototransistors with InGaAs/InP quantum wells in the collector and InGaAsP base is discussed. The design of the structure ensures that light is absorbed only in the quantum-well region, thus providing a way to study the correlation between quantum well and phototransistor carrier dynamics. Moreover, since the operation of a n-p-n phototransistor is governed by hole injection into the base, the transient behavior of the device reflects the hole dynamics in the multiple-quantum-well region. The response of the device to picosecond optical pulses shows strong dependence on bias conditions: from device response determined by minority carrier recombination time ( approximately 2 ns) at high base-emitter bias, to current time constant dominated response ( approximately 50 ps) at low base-emitter bias. The field dependent escape times of carriers from the quantum wells under different bias conditions are obtained (10-100 ps) and are seen to affect the risetime of the transistor to pulsed photoexcitation. >

A voltage tunable threshold logic gate based on multiquantum-well heterojunction bipolar transistor for multifunction logic

The properties of a heterojunction bipolar transistor with a multiquantum-well collector region for its application as a voltage tunable logic element are examined. The quantum confined Stark effect gives rise to a strong negative differential resistance in the photocurrent-voltage characteristic of the device, which allows the device to be switched optically and/or electronically. This permits the realization of a circuit where the NAND, INVERSE CARRY, and NOR logic functions can be implemented by simply changing the biasing. >

Photocurrent and intrinsic modulation speeds in P-I(MQW)-N GaAs/AlGaAs stark effect modulators

Abstract Theoretical and experimental studies on photocurrent are presented in p-i-n GaAs/Al 0.3 Ga 0.7 As MQW modulators and detectors. Field dependent excitonic spectra is calculated to obtain carrier generation and recombination rates using full band mixing effects in the valence band. A tunneling formalism is used to self consistently study the electric field dependence of the electron and hole collection efficiency. At low electric fields and low temperatures the carrier collection efficiency is very poor and we show that the collected photocurrent has a sublinear dependence on the impinging light intensity. This non-linearity is not due to band filling or exciton screening effects normally observed, but arises from the high recombination rates compared to tunneling rates. At higher electric field the collection efficiency is ∼ 100% resulting in a linear response and a negative differential resistance region. Consequences of our studies on intrinsic device speeds are discussed for p-i(MQW)-n modulators based on Stark effect.

Integrated Quantum Well Bipolar Devices for Tunable Detection and Optical Logic Applications

The properties of exciton-based quantum well devices suitable for low-power photonic switching and logic applications and wavelength selective detection will be described. The basic device is a heterojunction bipolar transistor with MQW collector region monolithically integrated with a p-i(MQW)-n modulator, realized by single-step epitaxy. The integrated controller-modulator device, by virtue of its unique photocurrent-voltage characteristics, exhibits integrating-thresholding properties. These properties have been exploited to demonstrate cascadability, fan-out, optoelectronic amplification, special logic functions, programmable optical/electronic memory capabilities, and wavelength selective detection with GaAs/AlGaAs heterostructure devices.

GaAs/AlGaAs based multiquantum well device for learning and decision making in optical neuro-computers

Theoretical and experimental examinations are made of two classes of devices which have the requisite properties for learning and neuronlike decision making. These properties involve responding to a time sequence of optical pulses (for learning) and integrating the thresholding and are realized in GaAs/AlGaAs multiquantum wells (MQWs) using the quantum confined Stark effect. The device is compatible with heterojunction bipolar transistor (HBT) technology since the controller is simply an HBT structure with a built-in MQW structure. Therefore, it should be possible to make use of the advances in HBT technology and develop large neuron arrays. Also, due to the current gain in the controller-modulator (C-M) device, the optical power requirements (~10 μW per device) are consistent with semiconductor laser structures, and one does not require high-power lasers. Based on the estimation of optical power dissipation (⩽10 W/cm2) with current technology it should be possible to achieve a 1000-b array with each C-M device being about 20 μm in diameter

Integrated multiquantum well heterojunction bipolar transistors for optical switching and thresholding applications

The integrating-thresholding properties of an integrated bipolar device realized by molecular beam epitaxy are demonstrated, and switching characteristics for 10 mu W input to the controller are measured. The device is based on the quantum confined Stark effect in multiquantum wells. The controller, which is a bipolar transistor with a MQW collector region, provides an amplified photocurrent and feedback voltage to switch the modulator. Cascadability, optoelectronic amplification, and multistage operation are demonstrated in terms of a fan-out of eight devices. The integrating-thresholding properties also lend themselves to the implementation of neurons and to the realization of decision-making processes. The controller-modulator device can form a versatile basic module for optical computation architectures.<<ETX>>

Wavelength selective detection using excitonic resonances in GaAs/AlGaAs P-I-(MQW)-N structures

Abstract The quantum confined Stark effect causes a strong wavelength and voltage dependence of photocurrent near excitonic resonances which is used to study the wavelength selectivity of p-i(MQW)-n photodiode. For a parallel input of optical bits each coming at a different wavelength, the selectivity is considered good if the state of a λ i wavelength bit can be detected regardless of the λ j (j ≠ i) state of the bits. Photocurrent is found to have very good selectivity if λ j bits are all zero, i.e. the optical information is serial. However, we find that differential photocurrent (Δ I ph /ΔV) provides a good selectivity for random states of λ j bits (i.e. parallel input). Four channel selectivity is demonstrated at 200K. Specially designed quantum well structures can greatly improve this selectivity.