J. Allam

Ultrafast characterization of an in‐plane gate transistor integrated with photoconductive switches

An in‐plane gate field‐effect transistor is characterized by ultrafast electro‐optic sampling. The transistor is monolithically integrated with photoconductive switches in coplanar waveguide and <0.5 ps measurement time resolution is achieved. The gate‐drain capacitance of the transistor is obtained as 1.8 fF at zero drain voltage from displacement current transients. The gate‐drain capacitance is dominated by parasitic capacitance and the intrinsic gate‐drain capacitance is estimated as less than 0.2 fF.

Mode-discriminating electrooptic sampling for separating guided and unguided modes on coplanar waveguide

Mode-discriminating electrooptic sampling (MEOS) of coplanar waveguides was shown to discriminate between the symmetric quasi-TEM guided mode and asymmetric field distributions including unguided electromagnetic radiation. Radiation generated in a photoconductive switch and reflected from the back of the substrate was unambiguously identified. Ultrafast sampling of devices showed features in the transmitted pulse due to multiple substrate reflections. These features are removed using MEOS, leading to increased accuracy in determination of S-parameters.

Monolithically-integrated optoelectronic circuit for ultrafast sampling of a dual-gate field-effect transistor

An integrated optoelectronic circuit for ultrafast sampling of multi-terminal devices is described. This is achieved using optimized photoconductive switches fabricated from low-temperature-grown GaAs, monolithic integration of the device with the sampling circuit, control of the electromagnetic modes propagating on the coplanar waveguide using microfabricated airbridges, and discrimination of guided and freely-propagating modes using a novel electrooptic sampling method. As an example, the scattering parameters associated with the propagation of a picosecond pulse through one of the gates of a dual-gate heterojunction field-effect transistor are obtained at frequencies up to 300 GHz. The inter-gate capacitance is determined by measuring the electromagnetic transient coupled between the gates.

Optical second‐harmonic generation in laterally asymmetric quantum dots

We investigate laterally asymmetric quantum dots (LAQDs) as second‐order nonlinear optical elements in which symmetry‐forbidden intersubband transitions become allowed due to lateral asymmetry. The susceptibility for second‐harmonic generation (χ2ω(2)) of a two‐dimensional LAQD with three equispaced energy levels was studied by calculating the product of the intersubband transition dipole moments. The product was as large as 0.95×10−3 Lx3 for a dot of length Lx confined by an infinite‐potential barrier. The product was increased to 3.0×10−3 Lx3 by varying the aspect ratio (Ly/Lx) of the LAQD and decreasing the barrier height. The lateral asymmetry can be controlled by a gate electrode in semiconductor devices, leading to a device with tunable wavelength and nonlinear coefficients, suitable for quasi‐phase matching in nonlinear optical waveguides.

Influence of hot carrier dynamics on pulse propagation in semiconductor lasers

Abstract We time resolve the dynamic response of InGaAsP quantum-well semiconductor diode lasers after injection of a femtosecond pulse, using ultrafast upconversion. The injection of a single off-resonant pulse leads to a very fast generation of picosecond ‘dark pulses’, which then generate bright pulses again, leading to dark-pulse–bright-pulse oscillations with a change-over period of 500xa0ps. We suggest that nonlinear gain compression causes these oscillations.

Ultrafast characterization of in-plane-gate field-effect transistors: parasitics in laterally gated transistors

In-plane-gate field-effect transistors are probed by femtosecond electrooptic sampling. Ultrafast response of the transistors is dominated by a displacement current induced by parasitic gate-drain capacitance. Intrinsic and parasitic gate-drain capacitances of various transistor structures are obtained from displacement-current characteristics and are in quantitative agreement with the calculation of planar capacitances. Intrinsic gate-drain capacitances are in the order of 100 aF, while parasitic gate-drain capacitances are between 1.7 and 4.8 fF, more than ten times that of intrinsic gate-drain capacitances. Reduction in parasitic capacitance by a factor of two is achieved by means of grounded shields and is confirmed by calculation. The grounded-shields screen parasitic electric fields and transform parasitic coupling into a part of the waveguide coupling. This reduction in parasitic capacitance is the first demonstration that the parasitic field effect is controlled artificially by nanometre-scale device technology.

Dynamics of Trapping, Trap-Emptying, and Breakdown in LT GaAs

Photoconductors fabricated from annealed low-temperature-grown GaAs (LT GaAs) represent the state-of-the-art in ultrafast photodetectors due to sub-ps carrier trapping, high responsivity and high breakdown voltage.1 Much attention has been paid to measurement of the trapping time. However, the properties of the trapped charges and their influence on carrier transport has not been studied. In this paper, we investigate the dynamics of trapping, trap-emptying and breakdown in LT GaAs interdigitated photoconductors, and obtain clear evidence for field screening by trapped carriers and for avalanche breakdown. The effect of the trap-emptying time on the transport is revealed by comparison of the dynamical results with the temperature dependence of the dark current.

Ultrafast Sampling of a Dual-Gate Field-Effect Transistor

Measurement of semiconductor devices at frequencies ≥1THz will allow direct access to the dynamics of non-equilibrium carrier transport. At present, the fastest devices such as high electron-mobility transistors operate at frequencies <1THz but significantly greater than the ≈100GHz bandwidth of conventional electronic measurements.1 The reliability of extrapolating low-frequency measurements into the near-THz regime is unproven, and there is an increasing need for direct measurement methods in the 100GHz to 1THz frequency range.