Robert C. Potter

A novel A/D converter using resonant tunneling diodes

A large-signal resonant tunneling diode (RTD) model is used to simulate the performance of a 2-b A/D converter. Results from the theoretical analysis, the breadboard circuit demonstration, and the SPICE3 simulation are discussed. It is shown that the unique folding characteristics of the vertically integrated RTD greatly reduce the complexity of the A/D converter circuit, making analog-to-digital conversion at tens-of-gigahertz rates possible. >

Three‐dimensional integration of resonant tunneling structures for signal processing and three‐state logic

We have developed structures with two well‐defined negative differential resistance (NDR) regions by sequentially growing two resonant tunneling devices separated by an n+ connecting layer. Devices fabricated from these structures exhibited three stable operating points for multilevel logic circuits and were used in circuits which multiplied the input signal frequency by 3 or 5. This approach can be extended to obtain more than two NDR regions by vertical integration of additional resonant tunneling structures.

Combining resonant tunneling diodes for signal processing and multilevel logic

A device with multiple negative differential resistances was obtained by combining two AlInAs/InGaAs based resonant tunneling diodes in series. Equal peak currents and large current peak‐to‐valley ratios were demonstrated at room temperature. Three stable operating points were identified for trilevel logic applications and a multiply‐by‐three circuit was demonstrated.

A self-latching A/D converter using resonant tunneling diodes

A high-speed analog-to-digital (A/D) converter based on the resonant tunneling diode (RTD) is described. This A/D converter takes advantage of the folding characteristic of the RTD to reduce circuit complexity. The speed of the A/D converter is improved by the fast latching action of the RTD digitizer. Simulations show that the 4-B A/D converter can have a sampling rate of several gigahertz. >

Bias circuit effects on the current‐voltage characteristic of double‐barrier tunneling structures: Experimental and theoretical results

Using the stable, dc current‐voltage (I‐V) curve measured from a double‐barrier resonant tunneling structure, we have studied the effects of external circuit elements on device oscillations. A simulation, using the experimental I‐V and a simple circuit model for the biasing arrangement, showed that hysteresis and vertical jumps appear in the current‐voltage curve when the circuit oscillates. This observation is supported by experimental results obtained on the same device with external circuit elements intentionally added to the biasing configuration.

Observation of electron quantum interference effects due to virtual states in a double‐barrier heterostructure at room temperature

Strong electron quantum interference effects have been observed at room temperature in the current‐voltage characteristics of a double‐barrier, wide‐well, lattice‐matched In0.52Al0.48As /In0.53Ga0.47As heterostructure tunneling device. A total of 22 oscillations was observed in the differential conductance. A model is proposed which attributes ten of the oscillations to the usual resonant tunneling via the subbands and the rest to the presence of virtual states that exist in the well and second barrier regions.

Three and six logic states by the vertical integration of InAlAs/InGaAs resonant tunneling structures

Double‐barrier/single‐well resonant tunneling structures based on InAlAs/InGaAs were vertically integrated on an InP substrate to obtain devices with multiple negative‐differential resistance (NDR) regions. These devices, with either two or five tunneling structures, exhibited uniform current peaks and valleys and also had NDR regions about equally spaced in bias voltage. The devices were used in simple circuits to demonstrate three or six stable logic levels which could be set with current pulses.

Interface roughness scattering in AlAs/InGaAs resonant tunneling diodes with an InAs subwell

We present simulation results on the current‐voltage (I‐V) characteristics of an InP‐based AlAs/InGaAs resonant tunneling diode (RTD) with InAs subwell. Space‐charge limited transport is accounted for using a self‐consistent electrostatic potential calculated using the Hartree approximation. Three‐dimensional scattering is simulated using the recently developed multiple sequential scattering theory. Interface roughness scattering is found to be dominant over polar phonon scattering in the devices studied. Of particular interest is interface‐roughness (IR) scattering at the InGaAs/AlAs and InGaAs/InAs interfaces and its impact on the valley current. The existence of a critical terrace size that maximizes IR scattering is identified through simulation. The origin of the asymmetry commonly measured in the RTD I‐V characteristic is discussed with respect to asymmetries in interface scattering. The use of the InAs subwell and associated interface roughness scattering to tune the peak current while keeping a ne...

Analysis of the hysteresis in the I-V characteristics of vertically integrated, multipeaked resonant-tunneling diodes

The hysteresis (extrinsic) and current‐voltage (I‐V) characteristics of the multiwell, vertically integrated, resonant‐tunneling diode are analyzed. Our analysis shows that hysteresis in the vertically integrated diode I‐V can result from interchanging the order in which the devices switch, depending if the bias is increasing or decreasing. Experimental results are presented that support this analysis.

Multiple-valued counter

The use of resonant tunneling diodes (RTDs) in multivalued counters, which greatly simplify the counter circuitry, is described. In order to achieve this implementation, a unique state-dependent current source is used to successively trigger RTD-based counter. >