Silicon-on-Insulator Technology:PD SOI Transistors

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PD SOI Transistors

In PD SOI MOSFETs, the depletion charge controlled by one or both gates does not extend from an interface to the other. A neutral region subsists and, therefore, the interface coupling effects are disabled. When the body is grounded (via independent body contacts or body-source ties), PD SOI transistors behave very much like bulk–Si MOSFETs and most of the standard ID(VG, VD) equations and design concepts apply. If body contacts are not supplied, the so-called floating-body effects arise, leading to detrimental consequences.

Classical Floating-Body Effects

The kink effect is due to majority carriers, generated by impact ionization, which collect in the transistor body. The body potential is raised which reduces the threshold voltage. This feedback gives rise to extra drain current (kink) in ID(VD) characteristics (Fig. 3.5a), which is annoying in analog circuits.

In weak inversion and for high drain bias, a similar positive feedback (increased inversion charge ®

more impact ionization ® body charging ® threshold voltage lowering) is responsible for negative resistance regions, hysteresis in log ID(VG) curves, and eventually latch (loss of gate control; Fig. 3.5b).

The foating body may also induce transient effects. A drain current overshoot is observed when the  gate is turned on (Fig. 3.5c). Majority carriers are expelled from the depletion region and collect in the neutral body increasing the potential. Equilibrium is reached through electron–hole recombination which eliminates the excess majority carriers, making the drain current to decrease gradually with time. A reciprocal undershoot occurs when the gate is switched from strong to weak inversion: the current now increases with time (Fig. 3.5d) as the majority carrier generation allows the depletion depth to shrink gradually. In short-channel MOSFETs, the transient times are dramatically reduced because of the additional contribution of source and drain junctions to establish equilibrium.

The high-frequency switching of integrated circuits may prevent the transistor body from reaching equilibrium. The charging and discharging of the body is an iterative process which may cause “history”

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effects and dynamic instabilities. In a ring oscillator, the switching delay of an inverter is governed by the amount of available current, which can be higher (overshoot) or lower (undershoot) than at equi- librium. The switching speed depends on the number of previous switches.

An obvious solution to alleviate floating-body effects is to sacrifice chip space for designing body contacts. The problem is that, in ultrathin films with large sheet resistance, the body contacts are far from being ideal. Their intrinsic resistance does not allow the body to be perfectly grounded and may generate additional noise. A floating body is then preferable to a poor body contact.

An exciting PD device is the dynamic-threshold DT-MOS transistor. It is simply configured by inter- connecting the gate and the body. As the gate voltage increases in weak inversion, the simultaneous raise in body potential causes the threshold voltage to decrease. DT-MOSFETs achieve perfect gate-charge coupling, maximum subthreshold slope, and enhanced current, which are attractive features for low- voltage, low-power circuits.

Gate-Induced Floating-Body Effects

In MOSFETs with ultrathin (<2 nm) gate oxide, the body is charged by the tunneling current giving rise to gate-induced floating-body effect (GIFBE). GIFBE occurs even at low drain voltage and is not related to impact ionization. Typical features are a second peak in transconductance (Fig. 3.6) [22,23] and an excess low-frequency noise [24].

The body potential is defined by the balance between the incoming gate tunneling current (body charging) and the outgoing current (body discharging via junction leakage and/or carrier recombination). In PD MOSFETs, the increase in body potential directly lowers the threshold voltage [23], giving rise to a “kink” in the drain current [25] and a second gm peak (Fig. 3.6a). GIFBE may also occur in FD MOSFETs, in particular when the back interface is biased in accumulation (Fig. 3.6b) [26]: the GIFBE peak gradually distorts and eventually offsets the mobility-related peak. It is clear that the mobility extracted from such a curve is totally meaningless.

GIFBE is a dimensional effect which decreases in shorter MOSFETs because the tunneling current is reduced, whereas the junction leakage is rather constant. GIFBE also decreases in narrower MOSFETs where the carrier lifetime is degraded and the source-body barrier is lower. In addition, GIFBE depends

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on the scanning speed of the gate voltage: for slower measurements, the second peak of the transcon- ductance appears at a lower VG. The asymmetry between gradual body charging (for increasing VG) and body discharging (for decreasing VG) is summarized by a hysteresis in ID(VG) curves [22,23].

The transient effects and history effects are dramatically modified because GIFBE enables a faster recovery of the equilibrium body charge.

From Partial to Full Depletion

Partial depletion occurs if the vertical depletion region wD , controlled by the gate, does not cover the whole body (wD < tsi). This old definition does not apply to very short devices, where the lateral depletion regions of the source and drain junctions enhance the overall depletion [27]. The junctions cause a lowering of the effective doping seen by the gate, allowing the vertical depletion region to extend deeper. The interesting consequence is that the transition from PD to FD operation is also controlled by the channel length, not only by the doping/thickness ratio. For this reason, PD technology would require excessive doping levels and will be hard to defend for very advanced CMOS nodes (<65 nm).

The discussion above is valid for SOI MOSFETs without pockets. In case of higher doping levels localized near the source and drain (pockets), the transition from PD to PD can show an opposite trend: shorter transistors exhibit a higher effective doping making them more PD [28].

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