Resonant tunneling and quantum integrated circuits

Abstract

Devices fabricated at the scaling limits of conventional technology must either contend with quantum-mechanical tunneling as a parasitic effect or incorporate tunneling into the device function. Taking the latter approach, the phenomenon of resonant tunneling between epitaxiallygrown double-barrier heterostructures shows significant promise for extending integrated circuit performance both before and beyond the post-shrink era. Recent progress in this technology is reviewed. Introduction Today, a wide range of applications are being explored for resonant tunneling technology including high speed and multistate memory [ 1-31, shift registers/correlators [4], and logic [5-91, where the number of interconnects and transistor delays are reduced by the use of the multi-state tunneling device. Other applications for tunneling devices include analog-to-digital convertors [lo, 111, optical receivers [12], samplers [13], and triggering circuits [14], all with microwave to millimeter wave bandwidths. In addition tunneling device architectures based on universal logic gates [15,16] could provide solutions to the interconnect bottleneck of post ULSI ICs. Resonant Tunneling Diode/"ransistor Technology The resonant tunneling diode (RTD), in its simplest form, consists of an epitaxial sandwich of two wide bandgap electronic barriers cladding an interior quantum well region [17-191. The total 0-7803-2442-0-8/95/$4.00 01995 IEEE 455thickness of the (double barrier structure is typically 10 nm or less. The current-voltage (I-V) characteristic is N-shaped, increasing from zero to a peak current determined by the thickness of the tunnel barriers, at a peak voltage determined by the width of the quantum well. Beyond the initial peak, the current decreases to a minimum given by the sum of parasitic transport processes across the double barrier: phonon scattering, interface roughness scattering, thermionic emission, tunneling through higher energy quantum-well states, and interband tunneling [20].The N-shaped I-V characteristic of the RTD can be used with transistors to increase the functionality of circuits. The combination of RTD and transistor occupies less space than the two devices fabricated separately since the epitaxy for the two devices, one above the other, allows vertical integration. Typically only a single extra mask level is required [6]. As an example, the logical XNOR function is readily contructed using 2 transistors and an integrated RTD/transistor pair, with only a single power supply. The same circuit using emitter-coupled logic requires 9 transistors and two power supplies.Beyond the single peak I-V characteristic, the RTD can be combined epitaxially in series to create multiple current peaks. The multiple current peaks of the series RTD combination can be translated into a multi-state transistor/RTD characteristic. For example, a 4-RTD stack in the emitter of a bipolar transistor can be used to produce a binary output characteristic in response to a multi-valued input voltage [7]. This Characteristic is naturally...