Nitin J. Shah

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. >

Gate-length dependence of the speed of SSI circuits using submicrometer selectively doped heterostructure transistor technology

Frequency dividers and ring oscillators have been fabricated with submicrometer gates on selectively doped AIGaAs/GaAs heterostructure wafers. A divide-by-two frequency divider operated up to 9.15 GHz at room temperature, dissipating 25 mW for the whole circuit at a bias voltage of 1.6 V, with gate length ∼ 0.35 µm. A record propagation delay of 5.8 ps/gate was measured for a 0.35-µm gate 19- stage ring oscillator at 77 K, with a power of 1.76 mW/gate, and a bias voltage of 0.88 V. The maximum switching speed at room temperature was 10.2 ps/gate at 1.03 mW/gate and 0.8 V bias, for a ring oscillator with the same gate length. With a range of gate lengths on the same wafer fabricated by electron-beam lithography, a clear demonstration of gate-length dependence on the propagation delay was observed for both dividers and ring oscillators.

Scaling properties and short-channel effects in submicrometer AlGaAs/GaAs MODFET's: A Monte Carlo study

Scaling properties of n/sup +/-Al/sub x/Ga/sub 1-x/As/GaAs MODFETs with submicrometer gate lengths (L/sub G/=0.50 to 0.05 mu m) are examined, using Monte Carlo methods. High-frequency performance of MODFETs can be improved by scaling the gate lengths, but various studies suggest that there exists a lower limit for the gate after which no improvement should be expected. The lower limit is determined here to be approximately=0.10 mu m. Devices with smaller gate lengths than 0.1 mu m exhibit degraded transconductance (g/sub m/), large shift in threshold voltage due to poor charge control in the channel, and a sharp reduction in output resistance (R/sub o/). It is shown that the drain current saturation in MODFETs is not caused by the velocity saturation effect, but by channel pitch-off. Electron velocities calculated from Monte Carlo simulations and extracted from g/sub m/ and f/sub t/ measurements are reconciled. >

Short pulse transfer characteristics of Al/sub x/Ga/sub 1-x/As/GaAs and Al/sub x/Ga/sub 1-x/As/In/sub y/Ga/sub 1-y/As modulation-doped heterojunction FET's

Short-pulse drain current versus gate voltage transfer characteristics measured for modulation-doped HFETs (MODFETs) with four donor-layer-channel-layer combinations-(1) Al/sub 0.3/Ga/sub 0.7/As-GaAs, (2) Al/sub 0.2/Ga/sub 0.8/As-GaAs, (3) Al/sub 0.3/Ga/sub 0.7/As-In/sub 0.2/Ga/sub 0.8/As, and (4) Al/sub 0.2/Ga/sub 0.8/As-In/sub 0.2/ a/sub 0.8/As-are compared with the DC transfer characteristics. The measurements are relevant to high-speed switching in HFET circuits. Significant shifts in threshold voltage are observed between the DC and short-pulse characteristics for the structures with n/sup +/-Al/sub 0.3/Ga/sub 0.7/As donor layers, while the corresponding shifts for structures with n/sup +/-Al/sub 0.2/Ga/sub 0.8/As donor layers are relatively small or virtually nonexistent.<<ETX>>