Transistor Laser Research Articles

Room temperature operation of electro-optical bistability in the edge-emitting tunneling-collector transistor laser

Optical bistable devices are fundamental to digital photonics as building blocks of switches, logic gates, and memories in future computer systems. Here, we demonstrate both optical and electrical bistability and capability for switching in a single transistor operated at room temperature. The electro-optical hysteresis is explained by the interaction of electron-hole (e-h) generation and recombination dynamics with the cavity photon modulation in different switching paths. The switch-UP and switch-DOWN threshold voltages are determined by the rate difference of photon generation at the base quantum-well and the photon absorption via intra-cavity photon-assisted tunneling controlled by the collector voltage. Thus, the transistor laser electro-optical bistable switching is programmable with base current and collector voltage, and the basis for high speed optical logic processors.

Electro-optical hysteresis and bistability in the ring-cavity tunneling-collector transistor laser

Bistable devices are fundamental to digital electronics as building blocks of switches, logic gates, and memory in computer systems. We demonstrate here transistor lasers exhibiting both electrical and optical hystereses with sharp square-corner current switching and different voltage thresholds in the collector IC–VCE and optical L–VCE family of characteristics. The electro-optical hysteresis is explained by the different switching paths of electron-hole (e-h) and photon interactions involving cavity coherent and incoherent energy states employing intracavity photon-assisted tunneling at the collector junction and spontaneous/stimulated e-h recombination at the base quantum-well. The electro-optical bistability of the transistor laser possesses a unique property of voltage (field) modulation and the basis for high speed optical logic processors.

Graded index separate confinement heterostructure transistor laser: analysis of various confinement structures

A new configuration of the confinement structure is utilized to improve optoelectronic performance, including threshold current, ac current gain, optical bandwidth, and optical output power of a single quantum well transistor laser. Considering the drift component in addition to the diffusion term in electron current density, a new continuity equation is developed to analyze the proposed structures. Physical parameters, including electron mobility, recombination lifetime, optical confinement factor, electron capture time, and photon lifetime, are calculated for new structures. Based on solving the continuity equation in separate confinement heterostructures, the threshold current reduces 67%, the optical output power increases 37%, and the −3 dB optical bandwidth increases to 21 GHz (compared to 19.5 GHz in the original structure) when the graded index layers of AlξGa1−ξAs (ξ:0.05→0 in the left side of quantum well, ξ:0→0.02 in the right side of quantum well) are used instead of uniform GaAs in the base region.

Tunneling modulation of a quantum-well transistor laser

Different than the Bardeen and Brattain transistor (1947) with the current gain depending on the ratio of the base carrier spontaneous recombination lifetime to the emitter-collector transit time, the Feng and Holonyak transistor laser current gain depends upon the base electron-hole (e-h) stimulated recombination, the base dielectric relaxation transport, and the collector stimulated tunneling. For the n-p-n transistor laser tunneling operation, the electron–hole pairs are generated at the collector junction under the influence of intra-cavity photon-assisted tunneling, with electrons drifting to the collector and holes drifting to the base. The excess charge in the base lowers the emitter junction energy barrier, allowing emitter electron injection into the base and satisfying charge neutrality via base dielectric relaxation transport (∼femtoseconds). The excess electrons near the collector junction undergo stimulated recombination at the base quantum-well or transport to the collector, thus supporting tunneling current amplification and optical modulation of the transistor laser.

Theory for voltage modulation of transistor lasers using Franz-Keldysh absorption in the presence of optoelectronic feedback.

Compared with typical diode lasers (DLs), transistor lasers (TLs) support not only current-controlled but also voltage-controlled modulation. In this work, we theoretically investigate the small-signal voltage modulation of TLs based on the Franz-Keldysh (F-K) absorption and related optoelectronic feedback. In addition to the conventional rate equations relevant to DLs, our model physically includes various F-K effects. An optically induced current due to the F-K absorption may dramatically alter the voltage response of TLs. A model composed of the intrinsic optical response and an electrical transfer function which is fed back by this optical response is proposed to explain the true behaviors of voltage modulation in TLs.

High current gain transistor laser.

A transistor laser (TL), having the structure of a transistor with multi-quantum wells near its base region, bridges the functionality gap between lasers and transistors. However, light emission is produced at the expense of current gain for all the TLs reported up to now, leading to a very low current gain. We propose a novel design of TLs, which have an n-doped InP layer inserted in the emitter ridge. Numerical studies show that a current flow aperture for only holes can be formed in the center of the emitter ridge. As a result, the common emitter current gain can be as large as 143.3, which is over 15 times larger than that of a TL without the aperture. Besides, the effects of nonradiative recombination defects can be reduced greatly because the flow of holes is confined in the center region of the emitter ridge.

Doping of Active Region in Long Wavelength InP-Based Transistor Lasers

The effects of doping in the multiquantum well (MQW) active region on the properties of InP-based long wavelength deep ridge transistor lasers (TLs) are numerically studied. Doping in the MQWs is shown to lead to a decrease of the slope efficiency and a notable increase of the current gain of the TLs, which makes MQW doping a useful tool for facilitating the design of TLs. When there are nonradiative recombination centers on the exposed MQW side walls of the TLs, doping in the MQWs is found to be able to enhance the performance of the TLs by reducing the threshold current and increasing the current gain greatly. With doping in the MQWs, the carrier diffusivity is decreased, making the defects less effective in consuming the carriers. The results show that doping in the MQWs helps to obtain high-quality TLs. The reduced carrier diffusion that results from doping also helps to improve the quality of a normal diode laser in case a deep ridge structure is needed.

Improvement in the current-gain of a 1.3-µm npn-AlGaInAs/InP transistor laser using a thin p-GaInAsP base layer

With the aim of increasing the current gain of a 1.3-µm-wavelength npn-AlGaInAs/InP transistor laser, we realized devices with a thin base layer to suppress unnecessary recombination between electrons and holes in the base layer. Consequently, we obtained a current gain of 0.18 with a base-layer thickness of 50 nm, which was 9 times higher than that previously reported with a base-layer thickness of 100 nm.

Intra-cavity photon-assisted tunneling collector-base voltage-mediated electron-hole spontaneous-stimulated recombination transistor laser

Optical absorption in a p-n junction diode for a direct-gap semiconductor can be enhanced by photon-assisted tunneling in the presence of a static or dynamic electrical field. In the transistor laser, the coherent photons generated at the base quantum-well interact with the collector field and “assist” optical cavity electron tunneling from the valence band of the base to the conduction band states of the collector. In the present work, we study the cavity coherent photon intensity effect on intra-cavity photon-assisted tunneling (ICPAT) in the transistor laser and realize photon-field enhanced optical absorption. This ICPAT in a transistor laser is the unique property of voltage (field) modulation and the basis for ultrahigh speed direct laser modulation and switching.

Theoretical estimation of optical gain in Tin-incorporated group IV alloy based transistor laser

We have calculated the electronic band structure and polarization dependent optical gain in a strain balanced SixGeySn1−x−y/GezSn1−z based transistor laser (TL) with GezSn1−z single quantum well (QW) in the base. Design consideration for QW is also addressed to ensure moderate carrier and optical confinement. A significant TE mode optical gain is obtained in mid infra red region for the transition of Г valley conduction band to heavy hole valence band. Optical gain in the QW plays an important role in determining the optical characteristics of Tin (Sn) incorporated group IV material based TL.

Optical pulse generation in a transistor laser via intra-cavity photon-assisted tunneling and excess base carrier redistribution

For a direct-gap semiconductor (e.g., a p-n junction), photon-assisted tunneling is known to exhibit a high nonlinear absorption. In a transistor laser, as discussed here, the coherent photons generated at the quantum well interact with the collector junction field and “assist” electron tunneling from base to collector, thus resulting in the nonlinear modulation of the laser and the realization of optical pulse generation. 1 and 2 GHz optical pulses are demonstrated in the transistor laser using collector voltage control.

Analysis of Tunable Internal Loss Caused by Franz–Keldysh Absorption in Transistor Lasers

The Franz–Keldysh (FK) absorption at the base-collector junction of a transistor laser (TL) is inevitable because of the reverse bias therein. The bias condition, thus, plays a crucial role in the determination of internal loss of TLs. In this study, effects from various facet coatings of edge-emitting TLs on the internal loss ( $\alpha _{{\rm int}}$ ), which is influenced by the FK absorption, are investigated. Experimental analyses on the electrical and optical characteristics of these TLs at various temperatures are presented. The $\alpha _{{\rm int}}$ of different coated devices at various bias voltages are then extracted from their light-versus-current curves. We demonstrate that the internal loss resulted from the FK absorption is bias dependent, and therefore tunable. By contrast, the intense field inside the cavity with highly reflective coatings on both facets may saturate the FK absorption and make it less bias controllable.

Transistor Laser With a Current Confinement Aperture in the Emitter Ridge

We propose a transistor laser with a current confinement aperture (a-TL) formed in a reverse junction layer in the middle of emitter ridge. The a-TLs are studied numerically in comparison with the deep ridge TLs. Data show that in a-TLs, the effects of nonradiative recombination centers on the side walls of the emitter ridge can be reduced greatly due to the confinement of carrier flow in the center region of the emitter ridge. Furthermore, the reverse junction layer can be used as an etching stop layer for the emitter ridge formation, which facilitates both the design and the fabrication of the device. Together with the fact that in a-TLs, the effects of p-type material are alleviated effectively by placing the multi-quantum wells above the base layer, high performance can be expected, especially for long-wavelength devices.

Structural nonlinear effects in In0.53Ga0.47As/GaAs heterostructure bipolar transistor lasers

The effects of carrier capture on nonlinear gain of In0.53Ga0.47As/GaAs heterostructure bipolar transistor (HBT) lasers have been investigated. Calculations show that the gain of transistor laser (TL) for this structure depends on the parameters such as the initial electron energy, structure size, capture time and pump photon energy.

Numerical analysis of distortion characteristics of heterojunction bipolar transistor laser

Numerical analysis of harmonic and third order intermodulation distortion of transistor laser is presented in this paper. The three level rate equations are numerically solved to determine the modulation and distortion characteristics. DC and AC analysis on the device are carried out to determine its power–current and frequency response characteristics. Further, the effects of quantum well recombination time and electron capture time in the quantum well, on the modulation depth and distortion characteristics are examined. It is observed that the threshold current density of the device decreases with increasing electron lifetime, which coincides with earlier findings. Also, the magnitude of harmonic distortion and intermodulation products are found to reduce with increasing current density and with a reduction of spontaneous emission recombination lifetime. However, an increase of electron capture time improves the distortion performance. A maximum modulation depth of 18.42dB is obtained for 50ps spontaneous emission life time and 1ps electron capture time, for 2.4GHz frequency at a current density of 2Jth. A minimum second harmonic distortion magnitude of −66.8dBc is predicted for 50ps spontaneous emission life time and 1ps electron capture time for 2.4GHz frequency, at a current density of 7Jth. Similarly, a minimum third order intermodulation distortion of −83.93dBc is obtained for 150ps spontaneous emission life time and 5ps electron capture time under similar biasing conditions.

Continuous-wave operation up to 20 °C of deep-ridge npn-InGaAsP/InP multiple quantum well transistor laser emitting at 1.5-μm wavelength.

We report continuous-wave (CW) operation up to 20 °C of 1.5-μm wavelength npn-InGaAsP/InP multiple quantum well (MQW) transistor laser (TL) with a deep-ridge structure. With CW laser emission, the common emitter current gain of the device can be over 3.5, which is significantly larger than those of the previously reported long wavelength TLs. It is found that at low base current, the laser operation occurs on the first excited state of the MQWs. At high base current, however, the device shows stimulated emissions on the ground state transition. The trend is contrary to what has been observed in the GaAs based TLs and is explained by the change of carrier flow at different base currents.

Single Quantum-Well Transistor Lasers Operating Error-Free at 22 Gb/s

Data are presented showing error-free transmission by a 200- $\mu $ m-long transistor laser at 15 °C. The device has a threshold current of 30 mA and exhibits a modulation bandwidth of $f_{\mathbf {-3dB}} = 10.4$ GHz at I $_{B}= 85$ mA and V $_{\mathrm {\mathbf {CE}}} = 1.5$ V. At this bias point, the device is able to transmit $2^{7}$ -1 Pseudo Random Binary Sequence (PRBS) data with no errors at 22 Gb/s. The effect of transistor laser input impedance on optical modulation amplitude is discussed and short cavity ( $200~\mu $ m) devices are shown to exhibit a better match to the 50 $\Omega $ impedance of the signal generator.

Metamorphosis of the transistor into a laser

Based on the invention and operation of the transistor, the alloy diode laser, the quantum-well diode laser and the high-speed heterojunction bipolar transistor (HBT), we have invented and realized now a transistor laser (TL). The transistor laser is a three-terminal technology providing coupling and the coherent light emission in the transistor. The quantum-well (QW) heterojunction bipolar transistor laser, inherently a fast switching device, operates by transporting a small minority base charge density over a nanoscale base thickness in picoseconds. The TL, owing to its fast recombination speed, its unique three-terminal configuration, and complementary nature of its optical and electrical collector output signals, enables resonance-free base current and collector voltage modulation. It is a compact source of electro-optical applications such as nonlinear signal mixing, frequency multiplication, negative feedback, and optoelectronics logic gates.

Performance Optimization of GaAs-Based Vertical-Cavity Surface-Emitting Transistor-Lasers

We report on the optimization of p-n-p-type vertical-cavity surface-emitting transistor-lasers based on the fusion between an AlGaAs/GaAs heterojunction bipolar transistor and an InGaAs/GaAs vertical-cavity surface-emitting laser (VCSEL) using an epitaxial regrowth process. It is shown how a proper design of the base region can extend the transistor active range of operation well beyond lasing threshold, thereby resulting in typical transistor laser operational characteristics including milliwatt-range output power, milliampere-range base threshold current, record low-power dissipation under laser operation, and continuous-wave operation up to at least 60 °C. A pronounced breakdown in the collector current characteristics in the limit of high base current and/or emitter-collector voltage accompanied by a quenching of the optical output power is interpreted as being related to quantum well band filling.

Low Power Operation of a Vertical Cavity Transistor Laser via the Reduction of Collector Offset Voltage

Data are presented on low-power operation in a vertical cavity transistor laser via reduced collector offset voltage by placing the emitter contact on the top of four pairs of the GaAs/AlGaAs DBR to reduce emitter resistance and subsequently depositing 11 SiO2/TiO2 DBR pairs to improve cavity quality. Consequently, the device demonstrates a reduced collector offset voltage of 1.65 V and a low-power dissipation of 6.32 mW under laser operation.