Capacitance modeling of laterally non-uniform MOS devices

Abstract

In compact transistor modeling for circuit simulation, the capacitances of conventional MOS devices are commonly determined as the derivatives of terminal charges, which on their tmn are obtained from the so-called Ward-Dutton charge partitioning scheme [I]. For devices with a laterally non-uniform channel doping profile, however, it is shown in this paper that 1) no terminal charges exist for the description of capacitances. Instead, 2) a model is presented for the capacitances of such devices, including numerical results for a MOS transistor with a laterally diffused channel doping profile. Finally, 3) a method is given to incorporate such a capacitance model in circuit simulators which are traditionally based on terminal charge models. Introduction LDMOS devices are well-known examples of MOS deviceswith a laterally non-uniform channel doping profile. Accurate modeling of capacitances in high-voltage LDMOS devices is a prerequisite for integrated RF-design of, for instance, switchmode power supplies and power amplifiers. For high-voltage devices often the sub-circuit model approach is followed, but the effect of the lateral non-uniformity in the channel is usually neglected [2][3][4][5][6]. Efforts have been taken to incorporate the lateral channel non-uniformity in a terminal charge model 17-91,

New fundamental insights into capacitance modeling of laterally nonuniform MOS devices

“…In [10] we have shown for a three-terminal device with a laterally diffused doping profile that this method indeed yields for a closed voltage cycle in time a non-zero charging current through the source-and drain terminal and a zero charging current through the gate terminal. Furthermore, we have shown in [10] that this method yields exactly the same results as a circuit simulation using the segmentation approach. Thus, capacitance models can be successfully implemented into conventional charge-based circuit simulators.…”

“…Before turning to laterally non-uniform MOSFETs, we recall the charge-and capacitance modeling of uniform MOSFETs (see also [10]). As shown by [13], the total current I i through terminal i of a uniform MOSFET is given by…”

“…We have tested both implications (4) and (5) of the existence of terminal charges for a laterally non-uniform MOSFET, using device simulations as well as circuit simulations with the segmentation approach mentioned above (see also [10]). Again, we do not consider gate leakage, since LDMOS devices have an oxide that is sufficiently thick.…”

“…For capacitance modeling, efforts have been taken to incorporate the lateral channel non-uniformity in a terminal charge model [7]- [9]. However, we have shown [10] that incorporation of the lateral channel non-uniformity via a terminal charge model is incorrect. Instead, we have developed a new capacitance model, which takes into account the lateral non-uniformity.…”

“…In this paper, the results of [10] are elaborated, and the new capacitance model is derived in detail. Furthermore, we show additional, experimental results by comparing this capacitance model with high-frequency measurements performed on a MOSFET with a laterally diffused channel doping profile.…”