Silicon-on-Insulator Technology:FD SOI Transistors

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FD SOI Transistors

In SOI MOSFETs (Fig. 3.1b), inversion channels can be activated at both the front Si–SiO2 interface (via gate modulation VG ) and back Si–BOX interface (via substrate, back-gate bias VG ).

Full depletion means that the depletion region covers the whole transistor body. The depletion charge is constant and cannot extend according to the gate bias. A better coupling develops between the gate bias and the inversion charge, leading to enhanced drain current. In addition, the front- and back-surface potentials become coupled too. The coupling factor is roughly equal to the thickness ratio between the gate oxide and BOX. The electrical characteristics of one channel vary remarkably with the bias applied to the opposite gate. Owing to interface coupling, the front-gate measurements are all reminiscent of the back-gate bias and quality of the BOX and interface.

Totally new ID(VG) relations apply to FD SOI–MOSFETs whose complex behavior is controlled by both gate biases. The typical characteristics of the front-channel transistor are schematically illustrated in Figure 3.4, for three distinct bias conditions of the back interface (inversion, depletion, and accumu- lation), and will be explained next.

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In the above equations, Csi, Cox, and Cit are the capacitances of the FD film, oxide, and interface traps, respectively; Qsi is the depletion charge, FF is the Fermi potential, and Ffb is the flat-band potential. Subscripts 1 and 2 hold for the front- or the back-channel parameters and can be interchanged to account

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The extension to p-channels or accumulation-mode SOI–MOSFETs is also straightforward [1].

In FD MOSFETs, the threshold voltage decreases in thinner films (i.e., reduced depletion charge), until Csi prevails or quantum effects arise and lead to the formation of a 2–D subband system. In ultrathin films (tsi � 10 nm), the separation between the ground state and the bottom of the conduction band increases with reducing thickness: a VT rebound is then observed [18].

Subthreshold Slope

For depletion at the back interface, the subthreshold slope (Fig. 3.4b) is very steep and the subthreshold accounts for the influence of back-interface trapsCit and BOX thicknessCox on the front-channel current [19].

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turn, makes a1 tend to unity (as in bulk-Si or PD MOSFETs), causing an overall degradation of the swing.

It is worth noting that the above simplified analysis and equations are valid only when the BOX is thick enough so that substrate effects occurring underneath the BOX can be overlooked. The capacitances of the BOX and Si substrate are connected in series. Therefore, the swing may depend, essentially for thin BOXs, on the density of traps and surface charge (accumulation, depletion, or inversion) at the third interface: BOX-Si substrate. The general trend is that the subthreshold slope improves for thinner silicon films and thicker BOXs. Film thinning leads to a lower subthreshold swing only in the case of a few states at the silicon layer/BOX interface.

Transconductance

For strong inversion and ohmic region of operation, the front-channel drain current and transconduc- tance are given by

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where m1 is the mobility of front-channel carriers, and q1,2 are the mobility attenuation coefficients. Coefficient q2 reflects the surface roughness scattering and is relevant for ultrathin gate oxides.

The complexity of the transconductance curves in FD MOSFETs (Fig. 3.4c) is explained by the influenceof the back-gate bias via V (V ). The effective mobility and transconductance peak are maximum for depletion at the back interface, owing to combined effects of reduced vertical field and series resistances.

An unusual feature is the distortion of the transconductance (curve I, Fig. 3.4c) which reflects the possible activation of the back channel, before the inversion charge build-up is completed at the front channel [20]. While the front interface is still depleted, increasing VG reduces the back threshold voltage

and eventually opens the back channel. The plateau of the front-channel transconductance (Fig. 3.4c) can be used to derive directly the back-channel mobility.

Volume Inversion

In thin and low-doped films, the simultaneous activation of front and back channels induces by continuity (i.e., charge coupling) the onset of volume inversion [21]. Unknown in bulk Si, this effect enables the inversion charge to cover the whole film. Self-consistent solutions of Poisson and Schrödinger equations indicate that the maximum density of the inversion charge can be reached in the middle of the film, away from the interface. For double-gate operation the electric field cancels in the middle of the film enabling the carrier mobility to increase further. This results in higher current drive and transconductance, attenuated influence of interface defects (traps, fixed charges, and roughness), and reduced 1/f noise. Note that some degree of volume inversion subsists in single-gate MOSFETs if the film is ultrathin.

Multiple-gate MOSFETs (DELTA, FinFETs, and GAA transistors), designed to take full advantage from volume inversion, also benefit from reduced short-channel effects.

Defect Coupling

In FD MOSFETs, carriers flowing at one interface may sense the presence of defects located at the opposite interface. Defect coupling is observed as an apparent degradation of the front-channel properties, which is actually induced by the BOX damage. This unusual mechanism is notorious after back-interface degradation via radiation or hot-carrier injection.

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