Carrier Transit Time Research Articles

A Disaggregate Analysis of Ocean Carriers' Transit Time Performance

Abstract This study provides a disaggregate analysis of ocean carrier speed and reliability. The findings produce direct evidence that dramatic differences in carrier transit time and transit time reliability, also referred to collectively as transit time performance, prevail across major trade lanes. These results bring attention to transit time performance, which previous studies have shown to receive little attention from shippers, especially for ocean carrier selection. Differences in transit time performance impact not only shippers' carrier selection decisions, but also their development of effective international ocean transportation strategies. Liner ocean carriers can also use these results as a benchmark to judge how well they perform relative to the competition.

Photoluminescence of single-walled carbon nanotubes in field-effect transistors

We have studied the photoluminescence (PL) of individual single-walled carbon nanotubes(SWNTs) placed in field-effect transistor structures. The SWNTs were suspended in the airso that strong PL was obtained. When an external bias voltage was applied to the device,no spectral changes could be detected, but the intensity drastically increased ordecreased. This behaviour is explained by the injection/extraction of carriersfrom/to the electrodes by the electric field. In the case of p-type FETs in the air,PL intensity increased due to hole injection by applying a small negative gatebias. After the device was heated in vacuum, the maximum PL intensity wasobtained at zero gate bias. This can be explained by the change in the interfacebetween the SWNT and contact electrodes. The drain field dependence of thePL intensity shows a monotonic decrease, which is explained by a competitionbetween the recombination lifetime and transit time of photo-excited carriers inthe SWNT. Using analytical steady-state rate equations on carrier density inthe SWNT, the relation between the recombination lifetime and carrier transittime has been evaluated. The carrier saturation velocity has also been estimated.

Design of Intrinsic and Extrinsic Base Process of HBT using Blanket SiGeC Epitaxial Growth

The profile design methodology of a blanket SiGeC epitaxial layer for hetero junction bipolar transistor (HBT) is presented. Two important factors, Ge profile and Si cap are designed to minimize the carrier transit time in the intrinsic base region. Concurrently, a BF2 ion implantation and a cobalt silicide process for the extrinsic base were optimized to accommodate the Si cap thickness without relaxing a strain in SiGe layers caused by a reaction between Ge and Co. This design resulted in HBTs with 94-GHz cut-off frequency (fT), 81-GHz maximum oscillation frequency (fmax), and 0.45 dB minimum noise figure at 2 GHz, which could be utilized for consumer electronics products operating within a 2-5 GHz frequency range. The HBT was successfully implemented into 0.25-μm CMOS process with various kinds of passive devices to produce LSIs for consumer wireless electronics.

Application of admittance spectroscopy to evaluate carrier mobility in organic charge transport materials

We examine the feasibility of admittance spectroscopy (AS) and susceptance analysis in the determination of the charge-carrier mobility in an organic material. The complex admittance of the material is analyzed as a function of frequency in AS. We found that the susceptance, which is the imaginary part of the complex admittance, is related to the carrier transport properties of the materials. A plot of the computer-simulated negative differential susceptance versus frequency yields a maximum at a frequency τr−1. The position of the maximum τr−1 is related to the average carrier transit time τdc by τdc=0.56τr. Thus, knowledge of τr can be used to determine the carrier mobility in the material. Devices with the structure ITO/4,4′,4″ -tris[N, -(3-methylphenyl)-N-phenylamino] triphenylamine/Ag have been designed to investigate the validity of the susceptance analysis in the hole mobility determination. The hole mobilities were measured both as functions of the electric field and the temperature. The hole mobility data extracted by susceptance analysis were in excellent agreement with those independently obtained from time-of-flight (TOF) measurements. Using the temperature dependence results, we further analyzed the mobility data by the Gaussian disorder model (GDM). The GDM disorder parameters are also in good agreement with those determined from TOF.

Development of a charge-carrier drift velocity measurement system in diamonds by using a UV pulse laser

There are continuing efforts of developing faster FETs and diamond is one of the strong candidates as a base semiconductor. Since the upper-limit-frequency of diamond FETs determines saturated drift velocities of charge-carriers, we need to first characterize diamond to develop better FETs. It is, however, not easy to measure the velocities with response time of less than 20 ns. Therefore, we developed a drift velocity measurement system using a time-of-flight (TOF) technique with a UV laser with 100 ps pulse width. In order to realize response times faster than 20 ns, we employed a 50 Ω coaxial cable as a load, with which we could effectively reduce the stray capacitance and inductance, and also, suppress reflections in the signal which gives false signals. As a result, we can measure carrier-transit times shorter than 10 ns.

Miniaturized waveguide-integrated p-i-n photodetector with 120-GHz bandwidth and high responsivity

A low-capacitance waveguide-integrated photodiode on InP with a minimized absorber length is presented. In order to maintain a high quantum efficiency, an optical matching layer exploiting mode beating effects is employed. Its optimization leads to a twofold enhanced external responsivity of 0.5 A/W at 1.55-μm wavelength in accordance with simulation. The reduced p-n junction capacitance enables 3-dB bandwidths up to 120 GHz mainly limited due to carrier transit time effects.

Tensile strained Ge p-i-n photodetectors on Si platform for C and L band telecommunications

We demonstrate a 0.25% tensile strained Ge p-i-n photodetector on Si platform that effectively covers both C and L bands in telecommunications. The direct band edge of the Ge film has been pushed from 1550 to 1623 nm with 0.25% tensile strain, enabling effective photon detection in the whole L band. The responsivities of the device at 1310, 1550, and 1620 nm are 600, 520, and 100mA∕W under 0 V bias, which can be further improved to 980, 810, and 150mA∕W with antireflection coating based on calculations. Therefore, the device covers the whole wavelength range used in telecommunications. The responsivities at 1310 and 1550 nm are comparable to InGaAs photodetectors currently used in telecommunications. In the spectrum range of 1300–1650 nm, maximum responsivity was already achieved at 0 V bias because carrier transit time is much shorter than carrier recombination life time, leading to ∼100% collection efficiency even at 0 V bias. This is a desirable feature for low voltage operation. The absorption coefficients of 0.25% tensile strained Ge in the L band have been derived to be nearly an order of magnitude higher than bulk Ge. The presented device is compatible with conventional Si processing, which enables monolithic integration with Si circuitry.

Crossover from capacitive to pseudoinductive charge-relaxation in organic∕polymeric light-emitting diodes

In this work we propose a phenomenological microscopic approach to deal with pseudoinductive charge-relaxation processes (named also as negative capacitance phenomena) in organic molecules (tris-8-hydroxyquinoline-aluminum) and polymeric [poly(2-metoxy-5-(2′-etil-hexiloxy)-1,4-phenylene vinylene)] light-emitting diodes (OLEDs and PLEDs, respectively). The approach is based mainly on the fact that the recombination rate is higher than the slower carrier transit time to reach the recombination zone. The approach is supported by the fact that in both PLEDs and OLEDs, the strong pseudoinductive relaxation process was mainly observed when electron-hole recombination takes place, suggesting this is a recombination dependent phenomenon. Besides, the negative branch, in the real part of the complex capacitance representation as a function of the frequency, was not observed in PLED homopolar devices.

Monte Carlo investigation of current voltage and avalanche noise in GaN double-drift impact diodes

By Monte Carlo simulations, we investigate the current voltage characteristics and the current noise in GaN homojunction double-drift impact avalanche diodes. We have found that a suppression of avalanche noise from the standard excess noise factor starts when the dielectric relaxation time becomes comparable or less than the carrier transit time. The suppression reaches values down to three orders of magnitude when the current approaches the electrical breakdown regime. The negative feedback between fluctuations of the space charge and of the number of electron-hole pairs generated under avalanche conditions is found to be responsible for this giant suppression of noise.

High-Speed Light Modulation in Avalanche Breakdown Mode for Si Diodes

The light emission process from a p-n junction in the forward-bias region is slow to respond to modulation signals due to the indirect band structure of silicon. Experimental results for a reverse-bias region showing light modulation in the range of tens of gigahertz are observed for the first time. For such a light emitter, the limiting speed of light modulation is shown to be determined by the transit time of the minority carriers across the junction during the filament formation of breakdown currents, which has been demonstrated by simulation of the propagation of a shockwave-like pattern in the breakdown field.

Traveling-Wave Photodetector with Asymmetrically Heterostructured Intrinsic Region

Bandwidth limitation by carrier transit time was analyzed mathematically when traveling-wave photodetector's intrinsic region consists of heterostructure. Because of the smaller hole's velocity than electron's, the hole transit length is crucial for determining the large bandwidth. In order to compensate the small transit velocity of hole, we propose that the intrinsic region should be designed asymmetrically. In this study we found that appropriate asymmetric structure enhanced bandwidth by about 100% of symmetric structure and proved experimentally the effect of asymmetric intrinsic region.

DTRMC, a probe of transverse transport in microcrystalline silicon

Diffusion-induced time resolved microwave conductivity is a method for transverse transport studies in semiconductors deposited on crystalline silicon. In this paper, by measuring layers with different thickness, we show that the transit time of carriers is controlled by diffusion. Moreover, measurements as a function of flux allow us to deduce the diffusion coefficients of electrons and holes and to separate surface and bulk recombination, depending on transport being in unipolar or bipolar regime. We finally compare the diffusion coefficient deduced from this method with TRMC mobility.

Characterization of organic semiconductors by a large-signal capacitance–voltage method at high and low frequencies

ITO/semiconductor/metal structures, with metal-free tetraphenylporphyrin (H 2-TPP) as organic semiconductor, are investigated by means of a capacitance–voltage ( C– V) method employing a large-signal triangular voltage waveform, which allows C– V measurements to be performed from 100 kHz to 100 mHz. The semiconductor layer, 60 nm in thickness, was deposited by molecular beam epitaxy on an ITO substrate and covered by aluminum back contacts. By performing the C– V measurements at low frequencies, transport and trapping of carriers are studied. On the contrary, when the signal frequencies are high in comparison with the carrier transit time in the device, metal–semiconductor interactions at the contacts are investigated.

Numerical analysis of traveling-wave photodetectors' bandwidth using the finite-difference time-domain method

We present full-wave analysis of traveling-wave photodetectors (TWPDs) using the finite-difference time-domain (FDTD) method. Impulse response in the frequency domain is obtained after time-domain data are calculated by the FDTD method. The impulse response includes the optical field profile, carrier transit time, microwave loss, microwave dispersion, and velocity mismatch all together. Three-decibel bandwidth is analyzed with the thickness of an i-layer and waveguide width as the design parameters. It is shown how transit time and microwave characteristics affect the bandwidth according to the TWPD's length. Three-decibel bandwidth is dominated by carrier transit time in case the device length is shorter than 300-500 /spl mu/m under the conditions given in this paper. However, if the device length gets longer, microwave characteristics affect the bandwidth.

Analysis of high speed p-i-n photodiode S-parameters by a novel small-signal equivalent circuit model

A novel small-signal radio frequency (RF) equivalent-circuit of the side-illuminated input tapered waveguide-integrated p-i-n photodiodes (WG PIN PD) is proposed. The proposed RF equivalent-circuit involves both the carrier-transit effect and the external resistance-capacitance (RC) time constant limitation on the frequency response of the p-i-n PD. The carrier-transit effect is realized by adding an RC circuit to an ideal voltage-controlled current source as the input opto-RF equivalent circuit. The carrier transit-time effect is equivalently represented by the time-constant of this input RC circuit. This new equivalent circuit model fits well with both the measured reflection and optoelectronic conversion parameters of the WG PIN PD in a broad frequency range from 45 MHz to 50 GHz.

Charge transport at low electric fields inπ-conjugated polymers

In low-mobility materials the charge carrier mobility is usually measured using time-of-flight (TOF) methods. However, the use of TOF methods calls for a necessary condition to be fulfilled: viz., that the dielectric relaxation time must be bigger than the charge carrier transit time, ${\ensuremath{\tau}}_{\ensuremath{\sigma}}\ensuremath{\gg}{t}_{\mathrm{tr}},$ through the interelectrode distance d. If this condition is not fulfilled, then the equilibrium charge carrier concentration ${p}_{0}$ is sufficient to redistribute the electric field inside the sample during a time interval shorter than the small-charge drift time. Consequently, in the low-field region, the apparent charge carrier mobility estimated by the TOF method increases when the electric field decreases, while in reality the mobility does not increase. We show how to estimate the real mobility values, using extraction current transients [G. Ju\ifmmode \check{s}\else \v{s}\fi{}ka et al., Phys. Rev. Lett. 84, 4946 (2000)] for which the fulfillment of the ${\ensuremath{\tau}}_{\ensuremath{\sigma}}>{t}_{\mathrm{tr}}$ condition is unnecessary. We demonstrate this effect in regioregular poly(3-octyl thiophene) using both the TOF and carrier extraction by a linearly increasing voltage methods and show that the true mobility does not increase as a function of decreasing field.

Applications of blow-up theory to thyristor turn-on

An analysis of a symmetric double-gated thyristor is performed. By further imposing that the carrier transit times depend on the drift field, we arrive at a nonlinear partial differential equation. It describes the transverse behavior and admits blow-up-type solutions leading to current filamentation, i.e., a possible explanation for hot spots. The shortening of the bases during turn-on, associated with high drift fields expanding from the central junction toward the cathode junction is also predicted. These results could provide a simple interpretation for experimental data in GTOs published by other authors.

Large-volume thallium bromide detectors for gamma-ray spectroscopy

Thallium bromide (TlBr) is a wide-bandgap compound semiconductor characterized with high photon stopping power. In this study, large-volume /spl gamma/-ray detectors (approximately 3.8 /spl times/ 3.8 /spl times/ 3.8 mm/sup 3/) have been fabricated from TlBr crystals grown by the horizontal travelling molten zone (TMZ) method using zone-purified material. In order to extract the energy information of the incident radiation from the thick detectors, the short charge-collection time technique has been applied to the detectors. By this technique, the induced charge on the electrode is measured for a period of time that is short compared to the carrier transit time in order to minimize the deviation of the measured induced charge. The large-volume TlBr detectors irradiated with /sup 22/Na and /sup 137/Cs /spl gamma/-ray sources at room temperature have exhibited energy resolutions of 18.7% and 17.4% (FWHM) for the 511- and 662-keV peaks, respectively. To our knowledge, this is the first time that full-energy peaks have been obtained from TlBr detectors several millimeters thick. The time stability of the detector operation is also studied.

Theoretical consideration for carrier transport noise in nonequilibrium steady-state operation of metal–oxide–semiconductor field-effect transistor

This article describes a theoretical approach to carrier-density-fluctuation-induced high-frequency transport noise in n-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) under the condition of nonequilibrium steady-state operation. A partial differential equation describing the charge density fluctuation is formulated assuming the use of the current continuity condition. The model presented herein is applied to the analysis of the high-frequency transport noise in MOSFETs in a higher drain-to-source electric field range than is possible with the state-of-the-art model because the current continuity condition is taken into account. In contrast to the past result, it is shown that the normalized fluctuation power decreases or increases as the drain voltage increases, and it saturates at a certain value. When the drain voltage increases, the ratio of the carrier transit time to the effective relaxation time of fluctuation increases, which promotes the averaging of fluctuation. As a result, the increase in fluctuation is suppressed comparing with past theoretical results. In addition, the dependence of the carrier-density-fluctuation-induced transport noise on gate voltage is considered because both the local channel potential and the local carrier density inside the channel are taken into account in formulating the normalized fluctuation power. It is demonstrated that the normalized fluctuation power increases with gate voltage in the low gate voltage range and saturates at the certain value in the high gate voltage range. When the gate voltage increases, the ratio of the carrier transit time to the effective relaxation time of fluctuation decreases, which suppresses the averaging of fluctuation.

Optimization procedure for the design of ultrafast, highly efficient and selective resonant cavity enhanced Schottky photodiodes

An expression of quantum efficiency for high-speed resonant-cavity-enhanced (RCE) Schottky photodiodes is derived. This expression includes the structural and the physical parameters of the photodetector and takes into account the parameters of the metallic Schottky mirror and the wavelength dependence of the reflectivities. The metal layer thickness sets the maximum achievable quantum efficiency as it decays exponentially with it. The antireflection coating layer, on the other hand, determines the photodetector selectivity and the optimum absorption layer thickness that maximizes its quantum efficiency. An algorithm for the design and optimization of RCE Schottky photodetectors has been developed. Theoretical values of 647 GHz and 129 GHz were obtained, respectively, for the carrier-transit time limited 3-dB bandwidth and bandwidth-efficiency product for an RCE Schottky photodetector with a 0.02 /spl mu/m gold layer.