The invention relates to electronic devices, and, more particularly, to quantum mechanical resonant tunneling devices and systems and fabrication methods.
The continual demand for enhanced transistor and integrated circuit performance has resulted in improvements in existing devices, such as silicon bipolar and CMOS transistors and gallium arsenide MESFETs, and also in the introduction of new device types and materials. In particular, scaling down device sizes to enhance high frequency performance leads to observable quantum mechanical effects such as carrier tunneling through potential barriers. This led to development of alternative device structures such as resonant tunneling diodes and resonant tunneling hot electron transistors which take advantage of such tunneling phenomena.
Resonant tunneling diodes are two terminal devices with conduction carriers tunneling through potential barriers to yield current-voltage curves with portions exhibiting negative differential resistance. Recall that the original Esaki diode had interband tunneling (e.g., from conduction band to valence band) in a heavily doped PN junction diode. An alternative resonant tunneling diode structure relies on resonant tunneling through a quantum well in a single band; see FIG. 1 which illustrates a AlGaAs/GaAs quantum well. Further, Mars et al., Reproducible Growth and Application of AlAs/GaAs Double Barrier Resonant Tunneling Diodes, 11 J.Vac. Sci. Tech. B 965 (1993), and Ozbay et al, 110-GHz Monolithic Resonant-Tunneling-Diode Trigger Circuit, 12 IEEE Elec. Dev.Lett. 480 (1991), each use two AlAs tunneling barriers imbedded in a GaAs structure to form a quantum well resonant tunneling diode. The quantum well may be 4.5 nm thick with 1.7 nm thick tunneling barriers. FIG. 2 illustrates current-voltage behavior at room temperature. Note that such resonant tunneling "diodes" are symmetrical. With the bias shown in FIG. 3a, a discrete electron level (bottom edge of a subband) in the quantum well aligns with the cathode conduction band edge, so electron tunneling readily occurs and the current is large. Contrarily, with the bias shown in FIG. 3b the cathode conduction band aligns between quantum well levels and suppresses tunneling, and the current is small.
U.S. Pat. No. 4,912,531 shows lateral resonant tunneling through quantum dots of GaAs surrounded by AlGaAs and with metal electrodes over the tunneling quantum dots to modulate the potential in the quantum dots analogous to MOSFET operation. Similarly, U.S. Pat. No. 5,234,848 discloses resonant tunneling diodes formed laterally in a semiconductor wafer which allows simple layout and interconnection of such devices.
In contrast to vertical resonant tunneling structures which may use planar grown layers for the very thin tunneling barriers, lateral resonant tunneling structures essentially must form tunneling barriers by lithography to define the location of the barriers followed by etching and filling with barrier material. Such lithography of lines 1-15 nm wide (tunneling barrier thickness) lies well beyond the capability of standard integrated circuit optical lithography, so special approaches have been taken, such as electron beam (e-beam) or ion beam lithography. However, the known methods are difficult to perform.
U.S. Pat. No. 4,599,790 discloses a method for fabrication of a microwave MESFET gate with gate length on the order of 0.1 .mu.m. The method uses oblique depositions of metal on an opening in a photoresist layer to define a sublithographic opening, and then etches a trench in the underlying layer using this sublithographic opening. Lastly, gate metal deposition fills the trench to form a recessed gate.