Transistor scaling, a major driving
force in the industry for decades, has been responsible for the dramatic
increase in circuit complexity. Shorter gate lengths have required lower drain
voltages and concurrently lower threshold voltages. Recent CMOS evolution has
seen a dramatic reduction in operating voltage as transistor size is reduced.
This was due to the maximum field limit on the gate oxide needed to maintain
good long-term reliability. Proper selection of the gate material can produce
low threshold transistors with off-state performance parameters equivalent to
high threshold devices.
The
Buried Channel Accumulation device, currently being used for p-type transistor
processes has the Fermi level at a considerable depth from the gate thereby
making it difficult to shut the device off. Attempts to bring the Fermi level
up result in severe degradation of device performance. Need for optimization of
existing BCA technology arose and Thunderbird Technologies, Inc. delivered! The
‘incredible’: Fermi-FET.
The Fermi-FET technology brings the Fermi level
nearer to the gate. This technology merges the mobility and low drain current
leakage of BCA devices as well as the higher short channel effect immunity of
SCI devices. This paper highlights aspects of the technology in a
non-mathematical presentation to give a sound general understanding of why the
technology is the most promising avenue for advanced very short devices.
Fermi-FET
technology can lead to significant improvement in circuit performance, layout
density, power requirements, and manufacturing cost with only a moderate
alteration of traditional MOSFET manufacturing technology. This technology
makes use of a subtle optimization of traditional buried channel technology to
overcome the known shortcomings of buried channel while maintaining large
improvements in channel mobility.
Fermi-FET
can optimize both the N-Channel and P-Channel devices with a single gate
material, provided the work function is near the mid-range between N and P-type
polysilicon. Materials that have been used in MOSFET technology with a suitable
work function include Tungsten, Tungsten Silicide, Nickel, Cobalt, Cobalt
Silicide, P-type Ge:Si and many others. There is about a 30% reduction in
junction capacitance relative to traditional MOSFET devices. This fact alone
gives a significant speed advantage to the Fermi-FET in large scale circuits.
The total speed improvement produced by both the lowered threshold and lowered
gate and junction capacitances is very substantial.
In order to illustrate the impact of lowered
threshold voltages via work function engineering, the large-signal transient
response of two inverter structures was simulated. A comparison of conventional
CMOS and metal-gate Fermi-FET structures was performed. It is seen that the
Fermi-FET inverter displays significantly improved rise and fall times compared
to the MOSFET. The different delay characteristics are evident. It is seen that
the Fermi-FET inverter displays significantly improved rise and fall times
compared to the MOSFET
.
The
individual device DC characteristics were already well-known from the device
simulations. For each inverter, the supply voltage was ramped up to Vd
with a delay sufficient to allow the circuit nodes to settle to their
initial DC state with the input low. The input was then pulsed high, then low;
again with a delay time long enough to guarantee all nodes reach steady state.
The corresponding outputs obtained give a comprehensive view of the device
performance as compared to the traditional technology and thus acts a primary
assessment of the feasibility of the new technology in lieu of existing ones.
The
output of the mixed-mode simulations is shown in the figure. Even at 0.4 mm
gate length the low threshold Fermi-FET is almost twice as fast as the MOSFET
in this simple circuit.
Simple
circuits such as this underestimate the benefit of the lowered capacitance
associated with the source/drain junctions, but they virtually ignore the
capacitance associated with the extended wiring in large circuits. The
Fermi-FET is the emerging technology in the ever-expanding empire of
electronics circuits and devices and is slated to be crowned the king in foreseeable
future.
