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Ballistic transport is studied for relatively narrow channels formed in a high-mobility modulationdoped heterostructure. Sharvin resistance is observed in the constricted channel, whose length is extended to 20 pm as the effective width is decreased to 1.5 pm. This is related to the effect of lateral restriction imposed on the high-mobility structure. We also find a strong reduction of the differential channel resistance at a small applied voltage.Lateral constriction of modulation-doped heterostructures provides one-dimensional (1D) electron channels.When both the channel width and length are much shorter than the electron ballistic length, the channel conductance becomes length independent, and is defined by the number of occupied 1D modes in the constricted channel.A well-defined 1D channel shows quantized conductance steps of 2e /h . ' When the channel is not narrow enough to provide a 1D subband separation much greater than the thermal energy kT, the conductance is called Sharvin conductance.These characteristics have been demonstrated mostly for short submicrometer channels because the channel length must be less than the electron mean free path, which is generally 10 pm or less in modulation-doped heterostructures.The electron mean free path in most narrow channels reported is significantly reduced mainly due to diffusive scattering at the side walls. Recently, quantized conductance steps have been observed in 2to 4-pm-long channels prepared by chemical etching. This etching is very conventional, but is least damaging for making the lateral constriction.Recently developed modulation-doped Al Ga, "As/ GaAs heterostructures have an electron mobility exceeding 10X 10 cm /V s in the two-dimensional electron gas (2DEG), which corresponds to an electron mean free path of 100 pm. The modulation-doped structure generally has a thick undoped spacer of Al"Ga, As to suppress scattering of ionized impurities. Transport experiments on relatively large ballistic devices with such a high-mobility structure have revealed interesting properties ofballistic electrons in the classical model. ' A detailed study on the bend resistance of four-terminal large square structures has shown that the ballistic length is equivalent to the elastic mean free path of the large 2DEG system. '" The long ballistic length enables us to study transport characteristics of long ballistic channels on the scale of several tens of pm. We expect a fairly strong lateral confinement which is relevant for the observation of a Sharvin resistance regime in the long ballistic channels. In this paper we report the observation of Sharvin resistance and its breakdown in 4to 20-pm-long ballistic channels formed in a high-mobility (8 X 10 cm /V s) modulation-doped heterostructure.These are the longest ballistic channels ever reported. Wet etching is used to fabricate ballistic channels with an effective width of 1.5 -7.5 pm. At this channel width, boundary scattering does not affect the channel resistance significantly.Our results show that the Sharvin resist...

The growth of the AlGaSb∕InAs high-electron-mobility transistor (HEMT) epitaxial structure on the Si substrate is investigated. Buffer layers consisted of UHV/chemical vapor deposited grown Ge∕GeSi and molecular beam epitaxy-grown AlGaSb∕AlSb∕GaAs were used to accommodate the strain induced by the large lattice mismatch between the AlGaSb∕InAs HEMT structure and the Si substrate. The crystalline quality of the structure grown was examined by x-ray diffraction, transmission electron microscopy, and atomic force microscopy. Finally, very high room-temperature electron mobility of 27300cm2∕Vs was achieved. It is demonstrated that a very-high-mobility AlGaSb∕InAs HEMT structure on the Si substrate can be achieved with the properly designed buffer layers.

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