Physics and behavior of asymmetrically recessed InP-based MODFET´s fabricated with an electron beam resist process

Despite their superior high frequency performance, InAlAs/InGaAs modulation-doped field effect transistors (MODFETs) consistently exhibit high output conductances and low gain-to-drain breakdown voltages. Two main cap designs have been used when fabricating these devices to improve these characteristics. The first design is the use of an undoped or depleted cap which has the adverse effect of increasing both the drain and the source resistances. In order to preserve the high frequency characteristic of these devices, an alternate design, often called a “double-recess” design, may be used. This design features a wide recess trench in a doped cap which extends farther on the drain side of the gate than on the source side and is therefore asymmetric about the gate. This has the advantage of preserving a low source resistance while increasing the drain resistance to the desired value. However, this design often requires a separate lithography step to define the asymmetric recess trench. Previously, a unique electron beam process was developed and presented which is to date the only reported process which enables both the asymmetric recess to be made and a submicron T-shaped gate to be metallized in a single lithography step. The process, which utilizes multiple layers of electron beam resist, allows a great deal of control of the extent of the cap recess on the drain side of the gate without the need for an alignment between two separate lithography steps. In this paper, MODFETs are fabricated on OMVPE-grown InAlAs/InGaAs structure both with and without the use of this electron beam resist process in order to investigate the effects of asymmetric recess on device performance. The DC and high frequency characteristics of these devices are reported as a function of the extent of the gate recess toward the drain. Values for the small-signal equivalent circuit model elements extracted from high frequency S-parameter measurements are also presented