It is shown theoretically that the relaxation oscillation in the direct modulation of the injection laser is reduced by transverse carrier diffusion when the stripe width is comparable to the carrier diffusion length. Modified rate equations are derived by considering the distribution of the light intensity, the carrier density, and the diffusion of the carrier. By small-signal analysis of the rate equations it is found that the height of the resonancelike peak in the modulation characteristics is a minimum when the transverse width in which both the light and the carriers are confined is around the diffusion length.

We proposed a vertical InGaAs channel metal–insulator–semiconductor field effect transistor (MISFET) with an ultranarrow mesa structure, an undoped channel, and a heterostructure launcher. With the aim of obtaining a narrow mesa structure, we proposed the concept of performing selective undercut etching after dry etching. We fabricated the proposed device with a 60-nm-long and 15-nm-wide channel mesa structure. In the fabricated device, the observed drain current density was 1.1 A/mm. Because the channel mesa width was 15 nm, the drain current density per unit area was 7 MA/cm2. Thus, a high current density was achieved for an ultranarrow mesa structure.

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The high‐speed characteristics and off characteristics of a novel transistor with a hot‐electron launcher and a transit layer of an intrinsic semiconductor whose current is controlled by an insulated gate are evaluated by Monte Carlo simulation. The dependences of the cutoff frequency fT on the gate insulator thickness, collector bias voltage, gate length and transit layer length are clarified. For the optimum structure consisting of InP/InGaAs/W as the emitter/transit layer/collector, Au as the gate and bisbenzocyclobutene (BCB) as the gate insulator and the surrounding material, fT is 1.4 THz at the collector current density of 1.6 MA/cm2. The collector current can be controlled over four orders of magnitude by a gate voltage swing of 0.6 V. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)