Heterojunction, Vacuum-Glass Field Effect Transistors
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0891-EE07-11.1
Heterojunction, Vacuum-Glass Field Effect Transistors Michael W. Geis, Sandra Deneault, Keith E. Krohn, Michael Marchant, David L. Cooke1 and Theodore M. Lyszczarz Lincoln Laboratory, Massachusetts Institute of Technology Lexington, MA 02420-9108, USA, 1 U.S. Air Force Research Lab. AFRL/VSBX, Hanscom AFB, MA 01731-3010, USA. ABSTRACT This note reports on a surface field effect transistor, SFET, where the electron channel consists of the interface between vacuum and a Cs-doped glass, and an electrode on the back of the glass substrate is used as the gate. The device has a transconductance of 4x10-10 S cm-1. The transconductance is limited by the glass surface roughness, ~ 0.4 nm RMS. A reduction of surface roughness to 0.1 nm RMS is expected to increase device transconductance. INTRODUCTION Electrons can float in vacuum on the surface of certain materials [1-3] much like electrons can be restricted to the interface between two semiconductors in heterojunction devices. These electrons are bound by electrostatic attraction to the material, but are quantum mechanically forbidden to enter the bulk. Table I is a partial list of these materials, which include cryogenically condensed gases, metals, and insulators. The surface-electron quantum states are similar to hydrogenic quantum states for zero angular momentum, l=0 [1-3]. Figure 1 shows an example of the probability density for the ground and first excited states of the surface electron. Ground State, n=1
Vacuum
X
1
2
1
X
Bulk Material
Probability density
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First Excited Sate, n=2
0 0
1 2 Distance (nm)
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Figure 1 Typical examples of surface electron’s probably density in vacuum on a metal surface for the ground and first excited quantum states. X1 and X2 are the average distances the electron is above the bulk material for the two states. Since the electrons are decoupled from the material, the possibility exists that for some material systems high-mobility or ballistic-electron motion will be realized. Surface electron
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mobilities > 107 cm2 V-1 s-1 have been reported on liquid He [4] and electrons are calculated to have quantum coherence lifetimes of several milliseconds on frozen Ne [5]. Field emission cathodes using these surface states emit electrons at record low fields, 10 V cm-1 [6], more than 100 times less than the best field emission cathode using diamond [7] or electric field enhancement from carbon nanotubes [8]. EXPERIMENTAL RESULTS Surface electrons have been characterized on cryogenetically condensed gasses since the 1970’s and on metals since 1990, but technical difficulties have limited investigation of insulators, like diamond and Cs-doped glass. This note reports on a surface state field effect transistor, SFET, where the electron channel is the interface between vacuum and Cs-doped glass, and the gate is the silicon substrate, as shown in Fig. 2.
Figure 2 Schematic drawing of surface state field effect transistor, SFET. The drain and source electrodes, separated by 10 µm, consist of 150 nm of W on
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