The University of Texas System

Ben G Streetman

Terry J Mattord

Sanjay K Banerjee

Dean P Neikirk

Aloysious F Tasch, Jr

Ben G Treetman

Vijay P Kesan

Craig W Farley

A MODFET device has highly doped source and drain regions separated by an undoped semiconductor alloy in which the mole fraction is graded between the source and the drain and with a conduction (and/or valence) band discontinuity at the heterojunction between the source and semiconductor alloy channel region of the device. Due to the graded mole fraction, the bandgap of the undoped semiconductor alloy decreases along the channel from the source to the drain and creates a built-in electric field. The higher bandgap in the source compared to that in the channel permits high energy carrier injection into the channel, with the built-in longitudinal electric field increasing carrier drift velocity and reducing transit time between the source and drain. In a preferred embodiment, the MODFET device has a vertical structure with the source and semiconductor alloy layers stacked on a drain substrate. The channel is defined by selectively etching the stacked structure with the exposed edge of the semiconductor alloy layer forming the channel. An undoped semiconductor layer and a modulation doped semiconductor layer are formed over the exposed edge with the undoped layer forming a spacer for the modulation doped layer. Carriers from the modulation doped layer spill over into the undoped channel and form a two-dimensional (or hole) electron gas. The devices can be implemented as p-channel and as n-channel devices fabricated in III-V compound and in Column IV-based semiconductor materials.

Modulation doped field effect transistor having built-in drift field

A sublimating and cracking device which may be used to produce a collimated beam of molecules from a solid source. The device includes an elongated sublimating tube and an elongated cracking tube adapted to be axially oriented off axis to each other. The sublimating and cracking tubes are interconnected and heated in use.

Sublimating and cracking apparatus

A furnace having a sublimating section, a cracking section oriented off axis to the sublimating section, and a valve for controlling flux between the sections. The valve includes an annular plug having at least one longitudinal slot. The plug is retractable from a fully closed position where the slot is completely covered, to a fully open position where the slot is completely exposed. The slot becomes increasingly exposed as the plug is moved from the fully closed position to the fully opened position, thereby increasing flux from the sublimating section to the cracking section.

A device and method for producing an ultra pure molecular beam of elemental molecules utilizing a reduced thermal gradient filament construction in an effusion cell is provided. The effusion cell comprises a crucible having an open and a closed end and at least one heating filament distributed immediate the crucible. The at least one heating filament or filaments are provided having a first portion having a first pitch in proximal relationship to the open end of the crucible and having a second portion having a second pitch wherein the first pitch is of a higher spatial frequency than the second pitch. The heating filament is non-inductively wound about the crucible and positioned in conformity to the outside structure of the crucible. The heat shield is positioned proximate and about the crucible and heating filament or filaments. The heating filaments are connected to a controllable electric power supply producing a near constant temperature along the long axis of the crucible. By placing source material in the crucible and placing the crucible heating filaments and heat shield in a reduced pressure atmosphere and powering the heating filaments, a substantially constant temperature is produced along the crucible long axis resulting in an ultra pure molecular beam of elemental molecules of the source material.

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Austin, TX, United States of America

Ben G Streetman