23-12-2010, 03:50 PM
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INTRODUCTION
Since the fabrication of MOSFET, the minimum channel length has been shrinking continuously. The motivation behind this decrease has been an increasing interest in high speed devices and in very large scale integrated circuits. The sustained scaling of conventional bulk device requires innovations to circumvent the barriers of fundamental physics constraining the conventional MOSFET device structure. The limits most often cited are control of the density and location of dopants providing high I on /I off ratio and finite subthreshold slope and quantum-mechanical tunneling of carriers through thin gate from drain to source and from drain to body. The channel depletion width must scale with the channel length to contain the off-state leakage I off. This leads to high doping concentration, which degrade the carrier mobility and causes junction edge leakage due to tunneling. Furthermore, the dopant profile control, in terms of depth and steepness, becomes much more difficult. The gate oxide thickness tox must also scale with the channel length to maintain gate control, proper threshold voltage VT and performance. The thinning of the gate dielectric results in gate tunneling leakage, degrading the circuit performance, power and noise margin.
Alternative device structures based on silicon-on-insulator (SOI) technology have emerged as an effective means of extending MOS scaling beyond bulk limits for mainstream high-performance or low-power applications .Partially depleted (PD) SOI was the first SOI technology introduced for high-performance microprocessor applications. The ultra-thin-body fully depleted (FD) SOI and the non-planar FinFET device structures promise to be the potential “future” technology/device choices.
In these device structures, the short-channel effect is controlled by geometry, and the off-state leakage is limited by the thin Si film. For effective suppression of the off-state leakage, the thickness of the Si film must be less than one quarter of the channel length.
The desired VT is achieved by manipulating the gate work function, such as the use of midgap material or poly-SiGe. Concurrently, material enhancements, such as the use of a) high-k gate material and b) strained Si channel for mobility and current drive improvement, have been actively pursued.
As scaling approaches multiple physical limits and as new device structures and materials are introduced, unique and new circuit design issues continue to be presented. In this article, we review the design challenges of these emerging technologies with particular emphasis on the implications and impacts of individual device scaling elements and unique device structures on the circuit design. We focus on the planar device structures, from continuous scaling of PD SOI to FD SOI, and new materials such as strained-Si channel and high-k gate dielectric.
PARTIALLY DEPLETED [PD] SOI
The PD floating-body MOSFET was the first SOI transistor generically adopted for high-performance applications, primarily due to device and processing similarities to bulk CMOS device.
The PD SOI device is largely identical to the bulk device, except for the addition of a buried oxide (“BOX”) layer. The active Si film thickness is larger than the channel depletion width, thus leaving a quasi-neutral “floating” body region underneath the channel. The V T of the device is completely decoupled from the Si film thickness, and the doping profiles can be tailored for any desired VT .
The device offers several advantages for performance/ power improvement:
1) reduced junction capacitance,
2) lower average threshold due to positive V BS during switching.
3) dynamic loading effects,in which the load device tends to be in high VT state during
switching
The performance comes at the cost of some design complexity resulting from the floating body of the device, such as
1) parasitic bipolar effect and
2) hysteretic VT variation.
Parasitic Bipolar Effect
In PDSOI an n-p-n transistor is formed with source and drain as emitter & collector respectively and body as the base. The topology typically involves an “off” transistor with the source and drain voltage set up in the “high” state (hence body voltage at“high”) When the source is subsequently pulled down, large overdrive is developed across the body-source junction, causing bipolar current to flow through the lateral parasitic bipolar transistor.This may result in circuit failure.
In SRAM bitline structures, the aggregate parasitic bipolar effect of the unselected cells on the selected bitline disturbs the read/write operations and limits the number of cells that can be attached to a bitline pair
Hysteretic VT Variation
The hysteretic VT variation due to long time constants of various
body charging/discharging mechanisms.
A commonly used gauge for hysteretic VT variation (or “history effect” as it is known in the SOI community) is the disparity in the body voltages and delays between the so-called “first switch” and “second switch” . The “first switch” refers to the case where a circuit (e.g., inverter) starts in an initial quiescent state with input “low” and then undergoes an input-rising transition. In this case, the initial dc equilibrium body potential of the switching nMOSFET is determined primarily by the balance of the back-to-back drain-to-body and body-to-source diodes. The “second switch” refers to the case where the circuit is initially in a quiescent state with input “high.” The input first falls and then rises (hence, the name “second switch”). For this case, the preswitch body voltage is determined by capacitive coupling between the drain and the body.
