| The advent of new technologies such as electron beam lithography (EBL) and molecular beam epitaxy (MBE) has presented the opportunity for exploring a new generation of devices based on quantum effects, as well as allowing the investigation of size limits of more conventional devices. Many novel physical effects are manifested by electrons in structures whose dimensions are on the order of the mean free path. EBL is the only technique which demonstrates the versatility of producing complex patterns on the nanometer scale, i.e. features below 100 nm. This is the scale to which device dimensions must be reduced in order to observe many properties of electron transport. To this end, an EBL system was developed in order to fabricate both novel quantum devices and short gate GaAs metal semiconductor field effect transistors (MESFETs) on the nanometer scale.;The effects of velocity overshoot in the short-gate MESFETs was observed. While the transconductance decreases with decreasing gate length for lengths above about 50 nm, below this level, transconductance increases are found for decreased gate length. Calculations show that the average velocity in the channel is above the saturation-limited velocity, and that overshoot is contributing to the increase in transconductance.;The BlochFET exhibited negative differential conductance (NDC) in the drain characteristics which was indicative of the possible presence of Bloch oscillations, although other explanations, such as sequential resonant tunneling are possible. Control devices with solid gates did not show any NDC. Calculations show that although tunneling is possible, conditions for Bloch oscillations existed which strongly indicate that possibility.;The thrust of this research has been three-fold. An EBL system, based on a commercial scanning electron microscope (SEM) has been used to investigate the dimensional limitations of such systems. Secondly, MESFETs with gate lengths as short as 25 nm, and with reasonable transconductances for gate lengths down to 35 nm, have been fabricated. These are the shortest gate lengths yet reported. Thirdly, a novel device called a BlochFET, which is based on a grid pattern of 40-nm lines on a 170-nm pitch was studied. This latter device is structured to investigate a previously theorized phenomenon known as Bloch oscillations. |
