Chao-Hsin Wu

Influence of separate confinement heterostructure layer on carrier distribution in InGaAsP laser diodes with nonidentical multiple quantum wells

The thickness of the separate confinement heterostructure (SCH) layer is found to have a significant influence on the carrier distribution among InGaAsP multiple quantum wells in laser diodes. When the SCH layer is 120 nm thick, the carrier distribution of the fabricated laser diodes favors quantum wells near the n-cladding layer. When the thickness of the SCH layer is reduced to 20 nm, the carrier distribution of the fabricated laser diodes favors quantum wells near the p-cladding layer. Our experiments indicate that the carrier distribution of a fabricated laser diode can be engineered using an SCH layer of appropriate thickness.

Experimental determination of the effective minority carrier lifetime in the operation of a quantum-well n-p-n heterojunction bipolar light-emitting transistor of varying base quantum-well design and doping

The authors show that the electrical characteristics of an n-p-n transistor structure can be used to determine experimentally, under dynamical operating conditions, the effective carrier lifetime of injected minority carriers in the quantum-well (QW) base region of a heterojunction bipolar light-emitting transistor. The carrier lifetime is progressively reduced from 134ps (no base QW) to ∼35ps by inserting single or double QWs of increasing width to enhance the effective capture cross section for injected carriers (electrons), and is further reduced to ∼10ps by increasing the p-type doping from 5×1018to4×1019cm−3.

4.3 GHz optical bandwidth light emitting transistor

We demonstrate a quantum-well base heterojunction bipolar light emitting transistor (HBLET) operating in the common collector configuration with a 3 dB optical response bandwidth f3 dB of 4.3 GHz. The HBLET has a current gain, β (=|ΔIC/ΔIB|) as high as 30, and can be operated as a three-port device to provide simultaneously an optical and electrical output with gain. The f3 dB of 4.3 GHz corresponds to an effective carrier recombination lifetime of 37 ps, and shows that “fast” spontaneous recombination can be harnessed for high-speed modulation.

Tilted-charge high speed (7 GHz) light emitting diode

We demonstrate a higher speed form of light emitting diode (LED), an asymmetrical two-junction tilted-charge LED, utilizing an n-type buried “drain” layer beneath the p-type “base” quantum-well (carrier and photon) active region. The drain layer tilts and pins the charge in the manner of a heterojunction bipolar light emitting transistor (HBLET), selecting and allowing only “fast” recombination (recombination lifetime τB of the order of base transit time τt). The tilted-charge LED, simple in design and construction, is capable of operation at low current in spontaneous recombination at a 7 GHz bandwidth or even higher with more refinement.

Electrical-optical signal mixing and multiplication (2→22 GHz) with a tunnel junction transistor laser

A tunnel junction is incorporated at the collector of a transistor laser to provide an effective method for voltage-controlled modulation via internal (intracavity) Franz–Keldysh photon-assisted tunneling. Electrical-optical signal mixing above threshold is made possible by the nonlinear coupling of the optical field to the base emitter-to-collector carrier transport and the base-to-collector electron tunneling. Microwave signal mixing with a common-emitter tunnel junction transistor laser is demonstrated with a pair of input sinusoidal signals: one (f1=2.0 GHz) at the base using current modulation and the other (f2=2.1 GHz) at the collector using voltage modulation, producing an optical output with harmonics of up to (4f1+7f2)=22.7 GHz, despite being limited by amplifier bandwidth.

Tunnel junction transistor laser

A transistor laser with a tunnel junction collector is demonstrated. Its optical output is sensitive to third terminal voltage control owing to the electron tunneling (photon-assisted or not assisted) from the base to collector, which acts in further support of resupply of holes for recombination in addition to the usual base Ohmic current, IB. Collector tunneling enhances laser operation even under a weak collector junction field and quenches it under a strong reverse-biased field. The sensitivity of the tunnel junction transistor laser to voltage control enables the tunnel junction transistor laser to be directly modulated by both current and voltage control.

Scaling of light emitting transistor for multigigahertz optical bandwidth

We report for an n-InGaP/p-AlGaAs/i-InGaAs-QW/n-GaAs heterojunction bipolar light emitting transistor (HBLET) the record spontaneous optical-signal bandwidth, at −3 dB, of 1.8–4.3 GHz (corresponding to an effective carrier recombination lifetime of 37 ps). Besides the improved circuit matching of three-terminal device operation, the extension in performance is achieved by the lateral reduction in the emitter aperture size DA from 13 to 5 μm to provide higher injection current densities and better confinement of the radiative recombination in the base region. By reducing the carrier loss to lateral extrinsic recombination, we obtain with HBLETs higher current gains β(=ΔIC/ΔIB>30) and simultaneously >4 GHz optical bandwidths.

Modulation of high current gain (β>49) light-emitting InGaN∕GaN heterojunction bipolar transistors

The electrical and optical characteristics of high-gain, small-area InGaN∕GaN heterojunction bipolar transistors (HBTs) grown by metal-organic chemical vapor deposition on sapphire substrate are reported. The common-emitter current-voltage characteristics of a 3×10μm2 emitter device demonstrates a current gain β=ΔIC∕ΔIB=49 at 3mA and breakdown voltage, BVCEO>70V. The radiative recombination spectrum of a large area 100×100μm2 emitter HBT is measured, showing a peak at 387nm and a full width at half maximum of 47nm. A 1kHz modulation input is applied to the HBT and both the optical and electrical outputs of a large area device is demonstrated.

The effect of microcavity laser recombination lifetime on microwave bandwidth and eye-diagram signal integrity

Vertical microcavity surface-emitting lasers employing quantum wells and small aperture buried-oxide current and field confinement are demonstrated with wider mode spacing and faster spontaneous carrier recombination (enhanced Purcell factor), lower threshold current, larger side mode suppression ratio (SMSR), and higher photon density and temperature insensitivity. The result is a microcavity laser that achieves higher microwave modulation bandwidth (f−3dB = 15.8 GHz) at ultra-low power consumption (1.5 mW) with a slope for the modulation current efficiency factor (MCEF) = 17.47 GHz/mA−1/2, as well as a better quality eye diagram in high-speed data transmission. The microwave behavior model for the microcavity laser is used to estimate the enhanced recombination and reduced lifetime.

Influence of separate confinement heterostructure on emission bandwidth of InGaAsP superluminescent diodes/semiconductor optical amplifiers with nonidentical multiple quantum wells

Experiments show that the layer of separate confinement heterostructure (SCH) has a significant influence on the emission spectrum of superluminescent diodes (SLDs)/semiconductor optical amplifiers (SOAs). Reducing the thickness of SCH layer at the p-side could improve the uniformity of carrier distribution among multiple quantum wells (MQWs). With three In/sub 0.67/Ga/sub 0.33/As/sub 0.72/P/sub 0.28/ QWs near the p-side and two In/sub 0.53/Ga/sub 0.47/As QWs near the n-side, when the thickness of the SCH layer changes from 120 to 30 nm, the operation current for SLDs/SOAs to exhibit the full-width at half-maximum spectral width of above 270 nm could be reduced from 500 to 160 mA.