Solid-State Electronics Vol. 32, No. 3, pp. 191-198, 1989 0038-1101/89 $3.00 + 0.00 Printed in Great Britain. All rights reserved Copyright © 1989 Pergamon Press pie DIFFUSION EFFECTS IN SHORT-CHANNEL GaAs MESFETs P. A. SANDBORN, J. R. EAST and G. I. HADDAD Center for High-Frequency Microelectronics,Solid-State ElectronicsLaboratory, Department of Electrical Engineering and Computer Science, The University of Michigan, Ann Arbor, MI 48109-2122, U.S.A. (Received 26 December 1987; in revised form 16 September 1988) Abstract--Diffusion effects in short-channel GaAs MESFETs are studied using a two-dimensional electron temperature simulation. A structure that consists of two n +-n contacts on a thin channel region was used as a test vehicle for the study. This structure was found to have a higher cutoff frequency than conventionally doped devices with the same gate length due to decreased gate capacitance and a selective modulation of the device transconductance. These results suggest that the performance of short-channel microwave MESFETs may be influenced strongly by electron diffusion. I. INTRODUCTION It is now possible to fabricate semiconductor struc- tures whose dimensions are comparable to Debye lengths and mean free paths of charge carriers. Present-day electron-beam lithography and photo- resist technology is capable of producing line widths of the order of 250 A[I] and GaAs MESFETs with gate lengths of 0.1/~m have been reported recently[2]. Devices with small dimensions and very short channels may have many potential advantages including high-frequency, high-speed operation and low power consumption[3]. The extrinsic Debye length represents the charac- teristic length over which the stud. unbalanced charge in a semiconductor decays: L D ~- ~/q2 n (1) where E = the permittivity, k = the Boitzmann con- stant, T = the electron temperature, q = 1.6 x 10 -19 C and n = the carrier concentration. For 1 x 1017cm-3 doped GaAs at 300 K, the Debye length is approxi- mately 140 A. As channel lengths become equal to a few Debye lengths, diffusion from highly doped contacts into the more lightly doped channel of short MESFETs becomes important. Previous studies of "ballistic" short-channel devices indicated that diffusion of carriers from contact regions into lower-doped channels may be a dominant factor in determining device operation under steady-state conditions[4,5]. In this study, these diffusion effects are evaluated using a two-dimensional computer simulation. The simulation consists of simultaneous solutions of Poisson's equation, the current con- tinuity equation, and a simplified form of the energy conservation moment of the Boltzmann transport equation. In Section 2 the device structure is intro- duced and an explanation of its proposed operation is given. In Section 3 a brief outline of the electron temperature model used to simulate the diffusion effects and in Section 4, the results for a range of different structures are presented. The results are summarized in Section 5. 2. THE GaAs DEVICE STRUCTURE The structure to be studied consists of two n +-n junctions self-aligned to a narrow channel region under the gate. This transistor structure is an extension of typical present transistor structures with ion implanted self-aligned source and drain contacts. A "conventional" transistor and the present structure both have an n + nn + configuration. The difference is that present structure has a channel distance between the more heavily doped contacts that is only a few Debye lengths long. The structure with parallel source and drain contacts on the ends of the device is shown in Fig. 1. If the channel region is only a few Debye lengths long, then diffusion of carriers from the contacts into the channel causes the actual carrier concentration in the channel to be higher than the doping in that area. The ratio of transconductance to gate capacitance represents the cutoff frequency, a common figure of merit for transistor operation. Both the gate capacitance and the transconductance of the short channel device depend on the electron distribution within the structure. The diffusion of the electrons into the channel from the two contact regions increases the channel electron density above the doping level and should increase the current and the transconductance. The purpose of this paper is to study these diffusion effects for a range of transistor structures with varying low doped region positions and lengths under the gate contact to see if they can be used to improve the high-frequency device operation. 191