830 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 5, MAY 2001 Schottky Barrier Depletion Modification—A Source of Output Conductance in Submicron GaAs MESFETs Mansoor M. Ahmed, Senior Member, IEEE Abstract—This investigation offers a new explanation for the output conductance in submicron GaAs MESFETs characteris- tics. Prior to the avalanche breakdown a sharp rise in the reverse Schottky barrier current, is observed at a potential where drain-to-source current, saturates. This could be attributed to the fact that after the onset of current saturation there is an increase in the effective channel height of the device as a function of drain-to-source voltage, . Experimental data suggest that by increasing there are more unbalanced positive ionic charges in the gate depletion toward the drain-side of the Schottky barrier. The electric field lines originated by these uncompensated charges induce an opposite charge density in the gate electrode. This modifies the gate biasing and hence the Schottky barrier depletion. As a result there is a wider available channel crossection for the flow of and consequently the current–voltage ( - ) characteristics exhibit a positive slope after saturation. Index Terms—MESFET, output conductance, Schottky barrier. I. INTRODUCTION G aAs FETs are used for microwave applications because of their superior high frequency characteristics [1]–[3]. The high frequency performance of these devices depend on the Schottky barrier response [4]. The simplest way to enhance the performance of the device is to reduce the gate length [5]–[10] and to improve the quality of the Schottky barrier [1]–[15]. The reduction in gate length originates the short channel effects namely: 1) shift in the threshold voltage, , 2) compression in the transconductance, of the device and 3) increase in the output conductance, in the saturation region of opera- tion [4]. The value of is the most unpredictable parameter in the device characteristics and is considered one of the major variables which generates discrepancy in simulated and the ob- served characteristics, especially in submicron devices [16]. Ac- cording to the Eastman and Shur model [17] could be asso- ciated with the channel widening into the substrate under the intense electric field. Ahmed [18], [19] showed that for a con- stant channel doping, the magnitudes of is a function of , and also , because the electric field inside the channel is dependent on these parameters. Manuscript received October 10, 2000. The review of this paper was arranged by Editor M. F. Chang. The author is with the Faculty of Electronic Engineering, GIK Institute of Engineering Sciences and Technology, Topi, Swabi, N-W.F.P, Pakistan. Publisher Item Identifier S 0018-9383(01)02484-4. II. FET REVERSE LEAKAGE CURRENT The component of electric field which is perpendicular to the flow of current could modify the Schottky barrier and hence the associated current–voltage ( - ) characteristics. It is thus possible to relate the output characteristics of a FET with its Schottky characteristics. Assuming a clean interface, the reverse current density crossing a Schottky barrier of a GaAs FET may be estimated by the thermionic emission theory and is given by [20] (1) where barrier height; electronic charge; Boltzmann constant; absolute temperature; applied voltage; effective Richardson constant. It is assumed that the increase in the junction current in the re- verse direction is due to the reduction in the Schottky barrier height [20], [21] which is defined as (2) where applied electric field; permittivity of GaAs material; Schottky barrier height at 0 V; constant with the dimension of thickness. The value of can be explained either by: a) a high density of interface states [22], [23]; b) an inhomogeneity in metal-semi- conductor interface [24]; and c) overlapping of electron wave function from the metal to the semiconductor [24], [25]. The third term in (2) represents the image force induced barrier low- ering. Its magnitude decreases in the forward direction and in- creases in the reverse direction [20], [26]. The Schottky barrier thermionic emission reverse leakage current accounting for barrier quality and image force induced barrier lowering may then be written as (3) 0018–9383/01$10.00 © 2001 IEEE