Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
1994-06-21
2001-03-13
Prenty, Mark V. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S026000
Reexamination Certificate
active
06201258
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to hot carrier transistors and methods of manufacture therefor, and more particularly to hot carrier transistors utilizing quantum well injectors for high current gain.
BACKGROUND OF THE INVENTION
Unipolar hot-electron transistors (HETS) have received little attention when compared with the level of effort focused on bipolar transistors. The HET has long shown promise as a high frequency device, but its development has been impeded by stringent fabrication requirements, immature growth techniques, and the lack of design tools for optimizing electron-wavelength-scale devices.
In recent years, many of these difficulties have been surmounted. At 77K, resonant-tunneling hot-electron transistors (RHET) have been demonstrated with common emitter current gain on both gallium arsenide and indium phosphide substrates. See N. Yokoyama, et al.,
Solid State Electronics
31 (1988) 577.
In a conventional RHET, an injector structure is interposed between the emitter and the base. This injector structure includes a quantum well having a relatively low conductance band minimum electron energy level and a width that is on the order of an electron wavelength integer or half-integer multiple. The quantum well is bounded by high conductance band energy level barriers that are sufficiently thin that the electrons may tunnel through them. Electrons tunneling through the barrier from the emitter into the quantum well will resonate therein. The electron flux or current density passed through the quantum well will depend upon the energy of the electrons; electrons of certain energies will “couple” with the well better than others. Electrons resident in the quantum well are thus likely to be quasi-monoenergetic. These electrons are injected through the remaining barrier into the base.
However, the energy of injected electrons in conventional RHETs tends to be too high, and these electrons tend to be transferred from the lower-energy &Ggr; mode to the X and L modes through internal scattering, then to be lost to the collector. Room-temperature gain for conventional RHETs has therefore been difficult to achieve.
To this date, only two instances of room temperature current gain in a HET have been reported: an InP-based RHET (T. Mori et al.,
Extended Abstract of the Conf. of Solid State Devices and Materials
(1988) 507), and a device using AlGaAsSb heterojunctions (A. F. J. Levi and T. H. Chiu,
Applied Physics Lett.
51 (1987) 985). However, no InP-based RHETS have been reported to operate with gain better than 2.5 at room temperature.
SUMMARY OF THE INVENTION
The present invention provides a quantum transistor having an emitter, an injector structure coupled to the emitter, a base coupled to the injector structure and a collector coupled to the base. The injector structure includes a quantum well having a general conductance band minimum energy level. At least one notch of the quantum well has a conductance band minimum energy level that is lower than the general energy level of the quantum well. This notch is operable to lower the energy of electrons disposed within the quantum well. A barrier having a relatively high conductance band minimum energy level is formed adjacent the quantum well and opposite the emitter, and allows the tunneling of electrons resident in the well across the barrier into the base. Because the notch in the quantum well is capable of lowering the energy of electrons injected by the injector structure into the base, these electrons are more available in contributing to the current gain of the device. Devices using the notched injector structure of the invention have exhibited a current gain in excess of 12 at room temperature.
In the preferred embodiment, the injector structure includes a second barrier interposed between the emitter and the quantum well, in which electrons resonate. The provision of a second barrier allows the injection of electrons into the base at one or more discrete energies and makes the operation of the transistor possible at more than two states.
In a particularly preferred embodiment, the transistor of the invention is built on an iron-doped indium phosphide substrate, and has an emitter, base and quantum well formed of indium gallium arsenide. The barriers of the injector structure are formed from aluminum arsenide, while the quantum well notch is formed of indium arsenide. The collector of the transistor is formed of indium aluminum gallium arsenide. The width of the well is preferably about 4 nanometers, with 2 nanometers of this being the notch. The width of the barriers is 1.5 nanometers, while a preferred width of the base is 40 nanometers.
Another aspect of the invention concerns a preferred method for manufacturing the transistor, in which the emitter/injector stack is recursively back-etched and tested until a resonant tunneling diode IV characteristic is obtained.
REFERENCES:
patent: 4712121 (1987-12-01), Yokoyama
Brady W. James
Hoel Carlton H.
Prenty Mark V.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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