Susanta Sen

Quantum functional devices: resonant-tunneling transistors, circuits with reduced complexity, and multiple valued logic

Recent advances in the area of quantum functional devices are discussed. After a discussion of the functional device concept, resonant-tunneling bipolar transistors (RTBTs) with a double barrier in the base region are described. Design considerations for RTBTs with ballistic injection and the first observation of minority-electron ballistic RT are presented. RTBTs using thermionic injection and exhibiting a high peak-to-valley ratio at room temperature in the transfer characteristics are also described. Multiple-state RTBTs and their DC and microwave performance are then discussed. Circuit applications of RTBTs also are discussed. It is shown that RTBTs allow the implementation of many analog and digital circuit functions with a greatly reduced number of transistors and show considerable promise for multiple-valued logic. Experimental results on frequency multipliers and parity bit generators are presented. Analog-to-digital converters are memory circuits are also discussed. Two novel superlattice-base transistors are reported. Negative transconductance is achieved by suppression of injection into minibands. Gated quantum-well RT transistors are also discussed. >

Resonant tunneling device with multiple negative differential resistance: Digital and signal processing applications with reduced circuit complexity

A new approach to obtain multiple peaks in the current-voltage characteristic of a resonant-tunneling (RT) device is demonstrated. The peaks are generated using only the ground state resonance of the quantum well rather than several states, as in conventional RT devices. The separation between the peaks is voltage tunable and also the peak currents can be made nearly equal, which is necessary to use the device in a variety of circuit applications. A functional device operating at 100 K, with two peaks in the I-V has been fabricated. The first practical demonstration of circuits for frequency multiplication by a factor of five, a three-state memory and a 4-bit parity generator, using a single functional RT device each, is also reported. The use of multiple-peak RT devices in these circuits results in an order of magnitude reduction in the number component per function over conventional techniques.

Quantum-well resonant tunneling bipolar transistor operating at room temperature

The first resonant tunneling bipolar transistor (RBT) is reported. The AlGaAs/GaAs wide-gap emitter device, grown by molecular beam epitaxy (MBE), contains a GaAs quantum well and two AlAs barriers between the emitter and the collector. In the common emitter configuration, when the base current exceeds a threshold value, a large drop in the collector current (corresponding to a quenching of the current gain β) is observed at room temperature, along with a pronounced negative conductance as a function of the collector-emitter voltage. These striking characteristics are caused by the quenching of resonant tunneling through the double barrier as the conduction band edge in the emitter is raised above the bottom of the first quantized subband of the well. Single-frequency oscillations are observed at 300 K. The inherent negative transconductance of these new functional devices is extremely valuable for many logic and signal processing applications.

Resonant tunneling devices with multiple negative differential resistance and demonstration of a three-state memory cell for multiple-valued logic applications

A new resonant-tunneling (RT) functional device with two peaks in the current-voltage (I-V) characteristic has been demonstrated. Contrary to conventional RT devices, the peaks are obtained using a single resonance of the quantum well. The peaks separation is voltage tunable and the peak currents are nearly equal, which is important for a variety of device applications. Using a single device, a three-state memory cell has been implemented.

Observation of resonant tunneling through a compositionally graded parabolic quantum well

We report the first observation of electron resonant tunneling through parabolic quantum wells, compositionally graded by means of short-period (15Å) AlxGa1-xAs/GaAs superlattices grown by molecular beam epitaxy. In one structure, comprising a 300Å wide well compositionally graded from AlAs to GaAs, five equally spaced resonances are observed in the current voltage characteristic (I-V), in good agreement with the theory. In another structure with 432Å wide wehs graded from Al0.30Ga0.70As to GaAs, up to sixteen resonances are observed in the I-V. The first ten correspond to resonant tunneling through the quasi-bound-states of the double barrier while the other ones are ascribed to electron interference effects associated with virtual levels in the quasi-continuum energy range above the collector barrier.

Multiple-state resonant-tunneling bipolar transistor operating at room temperature and its application as a frequency multiplier

A resonant-tunneling bipolar transistor with two peaks in the direct as well as in the transfer characteristics is presented. The multiple peaks are obtained by sequentially quenching resonant tunneling through the ground states of a series of double-barrier quantum wells, placed in the emitter of a Ga/sub 0.47/In/sub 0.53/As-Al/sub 0.48/In/sub 0.52/As bipolar transistor, thus obtaining nearly equal peak currents and peak-to-valley ratios. The transistor exhibits current gain of about 70 at room temperature and 200 at 77 K. Peak-to-valley current ratios at room temperature and at 77 K are as high as 4:1 and 20:1, respectively. Frequency multiplication by factors of three and five has been demonstrated using the multiple-peak transfer characteristics of the transistor.<<ETX>>

Multiple negative transconductance and differential conductance in a bipolar transistor by sequential quenching of resonant tunneling

The operation of the first multistate bipolar transistor is reported. Multiple peaks are obtained in the collector current‐voltage characteristics by sequentially quenching resonant tunneling through a series of double barriers placed in the emitter. GaInAs npn devices with two AlInAs/GaInAs double barriers exhibit two peaks in the direct and transfer characteristics, with peak‐to‐valley ratios in the latter as high as 4:1 and 20:1 at 300 and 77 K, respectively. The intrinsically multistate operation of this new transistor opens up exciting opportunities for digital and analog circuits with greatly reduced complexity and multiple valued logic.

New resonant-tunneling devices with multiple negative resistance regions and high room-temperature peak-to-valley ratio

A new resonant-tunneling (RT) device with multiple peaks in the current-voltage characteristics is presented. The multiple peaks are obtained by sequential quenching of resonant tunneling through the ground-state resonances of the quantum wells (QWs) of a number of double barriers (DBs) in series, thereby producing the peaks at nearly the same current level. Devices with three and five peaks operating at room temperature were implemented using Ga/sub 0.47/In/sub 0.53/As/Al/sub 0.48/In/sub 0.52/As heterostructures. The current-voltage characteristics of these devices indicate peak-to-valley ratios of 5:1 at room temperature and as high as 18:1 at 77 K. These devices are useful for multiple-valued logic and applications involving reduced circuit complexity.<<ETX>>

Negative transconductance resonant tunneling field‐effect transistor

The operation of a new resonant tunneling transistor is reported. The field‐effect transistorlike structure contains a double barrier in the gate. Resonant tunneling through the gate can be quenched by varying the drain or gate bias, leading respectively to negative conductance and negative transconductance in the drain current. Dramatic differences in the negative conductance and transconductance regions of the current‐voltage characteristic are observed for opposite bias polarities. This different behavior directly demonstrates the role, in controlling resonant tunneling, of the electron accumulation and depletion layers in the channel adjacent to the double barrier.

Resonant tunneling spectroscopy of hot minority electrons injected in gallium arsenide quantum wells

We have employed a new electron spectroscopy technique based on resonant tunneling to study hot minority‐electron transport in GaAs quantum wells, following ballistic injection. Direct information on the momentum and energy distribution perpendicular to the heterointerfaces is obtained from the measured resonant tunneling current without requiring derivative techniques. For injection energies of ≂0.2 eV, strong energy and momentum relaxation occurs over distances≲250 A making impossible the observation of minority‐electron ballistic transport in heavily doped (>1018 cm−3) GaAs. The energy distribution is found to be strongly non‐Maxwellian; from our data we can infer electron scattering times ≤10−14 which are consistent with recent evidence of strong electron‐hole scattering in GaAs quantum wells.