Rikmantra Basu

Design and Modeling of GeSn-Based Heterojunction Phototransistors for Communication Applications

We propose the use of Ge1-xSnx heterojunction phototransistors (HPTs) as efficient optical receivers on Si substrates and analyze their performance. Our designs use n-Ge/p-Ge1-xSnx/n-Ge1-xSnx layers pseudomorphically grown on Si wafers via a Ge virtual substrate, which offers compatibility with complementary metal-oxide-semiconductor (CMOS) technology. By incorporating Sn into the Ge photon-absorbing layer to shrink the bandgap, the photodetection range can be significantly extended to the mid-infrared (MIR) region with a considerably enhanced optical response. The use of HPT structures provides optical conversion gain to further enhance the optical responsivity, thereby enabling efficient photodetection in the short-wave infrared region. We develop theoretical models to calculate the composition-dependent band alignments, the band structures (by taking into account the nonparabolic effect), the absorption coefficient, and the optical responsivity for the proposed GeSn HPTs. As the Sn content increases, the conduction band nonparabolicity becomes increasingly significant and considerably impacts the optical absorption coefficient. Moreover, analysis of the spectral response for the Ge1-xSnx HPTs shows that efficient photodetection covering the entirety of the fiber-optic telecommunication bands, as well as the emerging 2-μm MIR communication band, can be achieved. These results indicate that the proposed Ge1-xSnx HPTs are attractive for use as high-responsivity CMOS-compatible photodetectors in communication applications.

Simulated characteristics of a heterojunction phototransistor with Ge1-xSnx alloy as base

Abstract. Groups III-V compound semiconductors and their alloys are the main photodetecting elements for the entire fiber optic telecommunication band. However, the recent successful growth of Ge1-xSnx alloy on Ge virtual substrates on Si platform makes the group IV alloys a potential competitor. Ge1-xSnx alloy shows direct band gap and has an absorption coefficient almost 10 times higher than that of Ge. The photonic devices are complementary metal–oxide–semiconductor compatible. We have considered an n-Ge/p+-Ge1-xSnx/n-Ge1-xSnx heterojunction phototransistor (HPT) and studied the variations of terminal currents by considering the Gummel Poon model of HPT, and values of optical and current gains, photocurrent, and responsivity have been obtained. The performance of the device as a photodetector at fiber optic communication wavelengths seems quite encouraging to justify the use of GeSn-based HPTs as a replacement of III-IV semiconductor-based photodetectors.

Characteristics of heterojunction phototransistors with Ge 1−x Sn x multiple quantum wells in the base

Germanium-tin alloy (Ge<inf>1−x</inf>Sn<inf>x</inf>), due to its direct gap nature for x > 0.08, has higher absorption coefficient than that of pure Ge. Using COMSOL Multiphysics, we have simulated the characteristics of Ge/GeSn/Ge heterojunction phototransistor, the base of which incorporates a multiple quantum well (MQW) structure made of Ge<inf>0.87</inf>Sn<inf>0.13</inf> barrier and Ge<inf>0.75</inf>Sn<inf>0.155</inf> well at a wavelength of 1.55 μm. The 3-QW structure shows high current gain over a wider range of base-emitter voltage than a single QW structure and a structre without any QW.

On the performance of graphene based plasmonic biosensor with bimetallic combination on 2S2G prism

Surface plasmon resonance (SPR) sensors based on bimetallic combination of silver (Ag) and gold (Au) with grapheme in visible and in near infrared (IR) region of operation are studied. Five-layer sensors geometry is considered based on Kretschmann configuration having chalcogenide prism as substrate material. Proposed structure shows enhanced detection accuracy and intrinsic sensitivity at near IR wavelength.

Investigation of CaF 2 substrate based plasmonic gas sensor in IR

The performance of proposed surface plasmon resonance based methane gas sensor based on Kretschmann configuration in infrared (IR) is discussed. In this structure, calcium fluoride (CaF2) substrate is considered with gold (Au) layer deposited on its base and methane gas is used as a sensing layer. Performance of sensor is analysed in terms of sensitivity and figure of merit (FOM) with various metal layer thicknesses using angular interrogation method.

Phonon and alloy scattering limited electron mobility of direct gap Ge 1−x Sn x alloys

We have estimated the values of the mobility of electrons in GeSn alloy for Sn composition x > 0.08 for which the material shows direct bandgap. For Γ and L valleys, acoustic and intervalley phonon scattering, as well as alloy scattering are considered. The calculated values are one order of magnitude higher than in pure Ge. The effects of uncertainties in parameter values are discussed.

Design and construction of a laser switch based electrical control circuit

To facilitate people with disabilities and those who are old, a novel laser switch based electrical control circuit is proposed. The circuit operates on 230V, 50Hz A.C. and can be operated at large distances of up to 10m. The laser switch is realized by using a diode laser that emits radiations in the visible region of light and a phototransistor as a detector. The electrical control is achieved by using a power EMOSFET to perform switching “on” and “off” operations. To prevent accidental switching operations a minimum contact time of 2.79 seconds is ensured using an incident time analyzer. The electronic circuit is tested for different laser wavelengths with a constant power output of 1mW. The device is also tested for different light ambiences. Test results show that lasers of wavelengths ranging from 652nm to 750 nm can be used for the desired switch operations.

Signal-to-noise ratio for a Ge-GeSn-GeSn Hetero PhotoTransistors at 1.55 µm

Heterojunction Photo Transistors (HPTs) provide optical gain but no excess noise. Recently good quality alloys of GeSn are regularly grown on Si substrate, opening up the possibility of having lasers, photodetectors etc. using standard VLSI technology. In this paper signal-to-noise ratio (SNR) for an HPT having GeSn as base has been estimated at telecommunication wavelength of 1.55 µm. The calculated gain-bandwidth product indicates high values and satisfactory SNR.