Intermodulation Distortion Simulation with a New MOSFET Model

Abstract

The second-and third-order intermodulation products of MOSFET amplifiers are investigated with a new MOSFET model, which has been implemented in SPICE3f4. Simulations performed with the new model agree well with measurements. G I INTRODUCTION This paper proposes a new MOSFET model that correctly predicts intermodulation (IM) products. The model applies a MESFET drain current description with a new continuous capacitance model. The nonlinear MESFET drain-source current model proposed in [1] is used for its high-order continuity in the drain current description and its derivatives. This feature is important for correct simulation of intermodulation (IM) products, which are determined by the derivatives [2][3] [4]. The validity of the model for predicting output power spectrum characteristics and intermodulation is examined. Figure 1: Lumped-element MOSFET model topology function with parameters IB and VB, are also included as in [1]: iBS = Is [eqVBS NkT _I -IB [e VBSIVB I] (1) II NEW NONLINEAR MOSFET MODEL The lumped-element topology of the MOSFET model is shown in Fig. 1. The intrinsic components of the new model are the drain-source current and charge models. Extrinsic bulk-source and bulk-drain diodes describe the bulk junction. Extrinsic gate-source, gate-drain and gate-bulk capacitances model the overlap capacitances. Source and drain access resistances, RS and Rd, are also added as extrinsic elements. The intrinsic capacitances are shown diagrammatically as a charge model in the figure. This section first describes the bulk diode model and then presents the intrinsic dc drain-source current model. The capacitance models that include intrinsic and extrinsic elements are reported subsequently. A Bulk Diode Model The standard diode equation is used to describe the bulkdrain and bulk-source currents with parameters Is and N. Reverse breakdown currents, described by a similar iBD =is [ qVBDINkT _ ' -IB [eVBD/VB -1 (2)where q is electronic charge [C], k is Boltzmann constant [J/K], Is is the junction saturation current [A], N is ideality factor, and IB and VB are bulk-junction breakdown current [A] and breakdown voltage [V] respectively. Together Is, N, IB and VB are the diode model parameters.The source-bulk junction charge, Q bs, and the drainbulk junction charge, Qbd, are calculated bywhere the model parameters are: CQ, the unit-area zerobias capacitance [F/m2]; Ob, the junction built-in potential [V]; and A, and Ad, the source and drain areas [m2 respectively.The source-bulk and the drain-bulk junction capacitances, Capbs and Capbd, are the derivatives of (3) and (4) with respect to Vb, and Vbd, respectively.

S-parameter simulation with a new MOSFET model

“…The validity of the MOSFET model for predicting output spectrum and intermodulation in power applications has been established in previous work [2]. This paper outlines the model and establishes its validity in small-signal applications by comparing measured and simulated S-parameters of a MOSFET.…”

“…To ensure charge conservation, terminal charges, Q gate , Q bulk and Q drn , are used as state variables. The intrinsic capacitances are derived from the expression of these terminal charges that are expressed as [2,3,4]:…”

“…1(c). This reduction is necessary to compensate frequency dispersion in the device, which is discussed in [2] for intermodulation distortion simulation.…”