Abstract
ABSTRACT The invention of Silicon transistor brings the electronic industry to the great success. In order to get more circuit density and operate at low power, the dimension of transistors goes to be scaled down. However, the supply voltage (VDD) used to drive the transistors has not proportionately scaled down with transistor density. The root cause is that the scaling down in VDD is attributed to the non-scalability of the MOSFET threshold voltage (VTH) and also the limitation of the subthreshold swing (S). As the limitation of subthreshold swing in MOSFET exists, there needs a new device with different carrier mechanism and steeper subthreshold swing are needed. A steep subthreshold swing allows for lower supply voltage VDD and low power device operation because power scales is the square of VDD. Devices with steeper subthreshold swings behave less gate voltage and give out high ON to OFF currents ratio (ION/IOFF). Therefore, the mainstreams of nowadays technology are higher ION, lower IOFF and lower supply voltage. To overcome the limitation of subthreshold swing in MOSFET, a new switch with dramatically different carrier injection mechanism needs to be explored. In this work, it explores the Tunnel Field-Effect Transistors (TFET) as an alternative switching device to overcome the fundamental limit of the subthreshold swing in CMOS. TFETs rely on carrier injection via Band-to-Band Tunneling (BTBT) and have the absence of thermal (kT) dependence, which allows for the subthreshold swing to be steeper and less than the value of 60 mV/dec. Besides new devices and carrier injection mechanism, it will be shown that by employing low bandgap material, Germanium (Ge), only in the source region can be greatly enhanced ON current and ION/IOFF ratio. In addition to Ge source TFET, gate overlaps structure also used in this work to change types of tunneling, from point to line tunneling, in order to have more tunneling area from source to channel. Furthermore, different gate oxide thickness (3～5 nm), spacer thickness (10～20 nm), doping flows (15～45 sccm) and doping concentrations (1E15～5E15 cm-2) are applied in this work to improve Band-to-Band tunneling which induced to higher ON current and ION/IOFF ratio. From the measured ID-VG or calculated subthreshold swing results of this experimental work, it can conclude that thinner in gate oxide or spacer thickness or high flow rates in doping will result higher ON current and steeper the subthreshold swing. With higher drain voltage, their measured ON current will also be higher. For W/I and W/O gate overlap, the subthreshold swing value with W/I overlap has under 60 mV/dec whereas in W/O overlap has over 60 mV/dec. For ON current, W/I overlap has higher 2 orders than W/O overlap and their ION/IOFF ratio is about 6 orders.