Designing bulk-driven MOSFETs in scaled technologies

Abstract

With the advent of system-on-a-chip integration, analog components are being forced to co-exist with their digital counterparts in scaled processes where the supply voltage, V DD , is expected to fall to 0.7 V by 2012 [1]. As a result of this integration, traditional analog topologies suffer from voltage headroom constraints since the threshold voltage, V T , can no longer scale proportionately with V DD due to sub-threshold leakage constraints. In 1996, Blalock described how the bulk terminal of a MOSFET could be used as a device input to enable analog design at very low supply voltages [2]. Using the MOSFET in this way, one could eliminate V T from the signal path allowing for a significant rise in the available voltage headroom. However, this device, called the bulk-driven (BD) MOSFET, suffered from intrinsic gain limitations because it depended upon the transconductance of the bulk, g mb , rather than the gate (g m ). Since Blalock's initial investigation, there has been no effort undertaken to make the BD MOSFET more adaptable to modern processes even though there have been numerous publications of low voltage analog designs incorporating the use of BD MOSFETs in these technologies. Therefore, this paper takes the first step towards creating a BD MOSFET that has been optimized for modern processes by proposing the use of delta doping [3] to improve g mb ; this claim will be verified through 2-D device simulations in ATLAS [4] and the results will be compared against haloimplanted devices which are currently being used in circuits utilizing the BD MOSFET. Furthermore, this paper will investigate the role that quantum mechanical (QM) confinement and gate oxide thickness, t ox , play in the performance of BD MOSFETs. Simulation results show that QM effects do not degrade g mb as aggressively as g m and that the t ox requirements necessary for gate-driven (GD) devices can be relaxed when using the bulk as an input. Figure 1 displays a cross-section of the delta-doped, n-type BD MOSFET simulated in this paper. The device utilized shallow trench isolation (STI) and had a t ox = 1.4 nm, a junction depth, x j = 30 nm, and a background doping level of: 1 x 10 16 cm -3 ; the source/drain regions and their extensions were doped to: 1 x 10 20 cm -3 and 2 x 10 19 cm -3 , respectively. The lightly-doped layer, t epi , had a thickness of 5 nm and a doping concentration of: 1 x 10 15 cm -3 ; it was designed to reduce ionized impurity scattering in the channel. The delta-doped region was 50 nm thick and had a doping level of: 1.75 x 10 18 cm -3 , 3.25 x 10 18 cm -3 and 7.5 x 10 18 cm -3 which yielded V T 's of: 0.3 V, 0.42 V and 0.6 V, respectively. The device simulations used the energy balance transport equations (hcte.el) to model carrier transport and the Bohm quantum potential model (bqp.n) to account for the movement of inversion layer carriers away from the Si/SiO 2 interface [4]; velocity saturation (fldmob) and impact ionization (toyabe) were also included in the simulation [4]. Fig. 1 Cross-section of a delta-doped...

Designing bulk-driven MOSFETs for ultra-low-voltage analogue applications

“…4 nm; the channel length, L = 400 nm and y epi = 5 nm) [23]. Note that normalization set g mb and g m equal to 1 at t ox = 1.4 nm.…”

“…The dependence of g mb and g m on t ox is depicted in figure 2 excluding the influence of polysilicon gate depletion [23].…”

“…As noted in the introduction, one major drawback of the BD MOSFET is that its transconductance is less than g m ; this causes the device to have an inferior intrinsic gain and cut-off frequency compared to its GD counterpart. To make g mb more comparable to g m , it is beneficial to use delta doping [13][14][15] (figure 1) to improve the control of the bulk terminal [23]. The delta-doped profile is able to enhance g mb by redistributing dopants away from the surface of the device and into a deltadoped region just below the channel in such a way that y d is reduced relative to a uniformly doped profile at a particular value of V T -i.e., at a constant effective background doping level.…”