Negative differential resistance devices and their circuit applications.

Abstract

As CMOS technology advances to its physical limits of feature size shrinking, it is important to investigate technologies that employ alternative device physics and transport phenomena. Among a host of nascent technologies, resonant-tunneling diodes (RTDs) are the most mature and promising for commercial introduction for the following reasons. First, RTDs can operate at room temperature with large peak-to-valley current ratio (PVCR) which enables a large noise margin in RTD-based logic circuits. Second, RTDs can be monolithically integrated with conventional technologies such as heterojunction bipolar transistors (HBTs), and high-electron mobility transistors (HEMTs). Third, the self-latching property of RTDs makes it possible to realize complex logic functionality with a few devices (RTDs and transistors). This can have a tremendous impact on logic circuit designs and greatly increases the IC packing density. This thesis encompasses many aspects of research on InP-based RTD-HBT circuits. Device properties were first studied in this work. The RTD characteristics can be designed by optimizing the layer structures. The fabricated RTDs showed a wide range of peak current density from 5.7 x 10<super>3</super> A/cm<super> 2</super> to 1.27 x 10<super>5</super> A/cm<super>2</super> on different layer structure designs. The smallest peak voltage obtained was 0.25V on the RTD with an InAs subwell design. The highest PVCR found was 30 at room temperature. The InAlAs/InGaAs material system was used for fabricating HBTs. The highest <italic> f_T</italic> and <italic>f_MAX</italic> obtained in this work were 60GHz and 87GHz, respectively on a 2.5 x 2.5 mum<super> 2</super>-HBT. A breakdown voltage greater than 5V has been obtained on the HBT with a 7500A-thick collector. Semiconductor PIN diodes for photodetectors have also been fabricated. The measured responsivity was 0.7A/W. Si-based tunnel diodes were also studied in this work because of the possibility of integrating tunnel diodes with the CMOS process. Negative differential resistance was measured on MBE-wafers. These wafers had delta-doping planes to provide extremely high doping concentration and form triangular potential wells. These wells helped confine electrons and increase the tunneling probability. The maximum PVCR found was 1.18 at room temperature and 1.2 at 80K. The PVCR should be improved by optimizing the growth conditions. Based on the measured device characteristics, RTD-HBT logic gates were designed and fabricated. Two different processes, the via-hole process and the air-bridge process, were developed and improved for this work. Both processes featured all wet-etching processing and self-aligned base contacts. Several digital circuits including a static inverter, concensus element, minority gate and the Monostable-Bistable transition Logic Element (MOBILE) inverting gate have been implemented with RTDs and HBTs. The logic function of these RTD-based circuits has been confirmed up to 10Gb/s which was the limit of our test instrumentation. Photoreceivers based on RTDs and PIN diodes have also been successfully fabricated and tested. Compared to a commercially available photoreceiver, the fabricated photoreceiver consumes very low power (only 0.21mW) and has a high conversion gain of 3000V/W. Optimization of the circuit design should greatly improve the sensitivity of the photoreceivers. These results indicate the potential of RTD-HBT circuits of digital gates and optical communications.