Small-signal distributed FET model consistent withdevice scaling

Abstract

A distributed modelling approach for micro-and millimetre-wave FETs is presented. Model identification is directly carried out on the bases of S-parameter measurements and electromagnetic analysis of the device layout, without requiring cumbersome optiinisation techniques. Experimental results confirm that the model is consistenl with device scaling. Itztruducnc.tioiz:The development of high performance monolithic microwave integrated circuits (MMICs) requires global design procedures where not only the values of passive components, but also the active device geometry (e.g. number of fingers and gate width) represent available design parameters. In this context, robust scaling procedures for FET models are of key importance.In this Letter, a new approach to FET model scaling for MMIC design is proposed which is based on an empirical distributed modelling approach [I -31. In particular, model identification is carried out by means of accurate electromagnetic (EM) simulation and scattering parameters measured for a limited number of FET structures. On this basis, a characterisation of the 'active area' of the device is obtained which is consistent with simple scaling rules.[ CQ?? Fig. 1 Structure of distributcd niodelEmpiricul distributed model: The electron device is assumed to consist of an 'extrinsic passive structure' connected with a finite, suitable number of elementary 'intrinsic active slices' as shown in Fig. 1 for a two-finger FET where a single active slice per finger has been considered. As far as model identification is concemed, the extrinsic structure is characterised through its scattering matrix S (see Fig. I ) computed by means of EM simulation. This kind of analysis enables device geometry and material characteristics to be taken into account, for any given device structure and size, by means of a multiport S-matrix 'distributed' description. Thus, since electromagnetic propagation and coupling effects are accounted for by the passive structure, all the intrinsic devices are described by the same scattering matrix BA, which can be identified once the scattering matrix _a of the electron device has been measured. This identification procedure is well justitied by the experimental results provided in the following.The identification of the 3 x 3 matrix B A , which characterises the active slice, will be described for a two-finger FET where a single active slice per finger is considered as shown in Fig. 1. This choice, which does not limit the validity of the approach, has the advantage of simplifying the mathematical development and has been found suitable for-FET modelling up to 5OGHz. In particular, by denoting with S the 5 x 5 matrix obtained from the 8 x 8 scattering matrix S of the extrinsic part, after the application of symmetry conditions deriving from the FET structure, it can be shown that with ?L=l,z,,=l ,.., 3 = sz=1,2;~=3,..,5where the port indexes are defined according to Fig. 1. In eqn. 1, _I is the 3 x 3 identity matrix while _a is the 2 x 2 measured scattering matrix of the electr...