Numerical Simulation of Radiation Damage on the Device Performance of Gaas Mesfets

In this work, the effect of the radiation on the current-voltage characteristics of device GaAs metal Schottky field effect transistors (MESFET) at room temperature is investigated. Numerical Simulation tuned by means of a physics based device simulator. When the substrate of this transistor is subjected to radiations, structural defects, which are created, have undesirable effects and can degrade the performance of the transistors. These defects appear like deep traps. Results showed that in the presence of donor traps the current-voltage characteristics increases. However, acceptor traps have a significant effect on the current-voltage characteristics. In the presence of acceptor traps, the space charge zone in the channel increases, hence, reduces the current drain. The GaAs metal Schottky field effect transistors (MESFET) are one of important components used in electronic devices. Deep traps are believed to be responsible for many parasitic effects in GaAs FETs such as the gate lag and drain lag effects in which a slow transient is observed in the drain current following a voltage applied to the gate or the drain. The trap properties, energy and cross-section either measured by transient spectroscopy, voltage or optically excited deep level transient spectroscopy (DLTS). The transistor GaAs MESFETs is simulated for two dimensional on a semi-insulating substrate compensated by deep levels, and to clarify effect of impurity compensation by deep levels in the substrate [1]. In this work K. Horio et al, they simulate important case, that is, a case of GaAs MESFET on a Cr-doped semi-insulating substrate where deep Cr acceptors compensate shallow donors [1], and the compare the results with those obtained for a case with deep EL2 donors. The objective of our work is to make modeling GaAs MESFET using Silvaco ATLAS TCAD simulator. We will determine the electrical characteristics Ids-Vds, with the influence of traps in low and high-resistivity material for two sample of transistor MESFET presented below.

The GaAs metal Schottky field effect transistors (MESFET) are one of important components used in electronic devices.The transistor GaAs MESFETs is simulated for two dimensional on a semi-insulating substrate compensated by deep levels, and to clarify effect of impurity compensation by deep levels in the substrate [1].
In this work K. Horio et al, they simulate important case, that is, a case of GaAs MESFET on a Cr-doped semiinsulating substrate where deep Cr acceptors compensate shallow donors [1], and the compare the results with those obtained for a case with deep EL2 donors.
The objective of our work is to make modeling GaAs MESFET using Silvaco ATLAS TCAD simulator.We will determine the electrical characteristics Ids-Vds, with the influence of traps in low and high-resistivity material for two sample of transistor MESFET presented below.

PHYSICAL MODEL
A. Device structure that it gives the ability to visualize physical phenomena inaccessible and therefore observable [2,3].
The basic equations are the following: In this work, we will study two samples of transistors MESFET.As to a model for the semi-insulating substrates, we adopt a two level compensation model as described below.
Fig. 1.Devices structures simulated in this study.
In the semi-insulating substrate n-type, we assume that deep acceptors (NtA) compensate shallow donors (ND).In the ptype semi-insulating substrate, we assume that deep donors (NtD) compensate shallow acceptors (NA).

B. Numerical simulation
In this work, we used the simulator TCAD-SILVACO (two-dimensional ATLAS) to study the performance of transistors MESFETs GaAs in the presence of deep traps.The important advantage of this type of simulator is Poisson's Equation relates the electrostatic potential to the space charge density: Where ψ is the electrostatic potential, The continuity equations for electrons and holes are defined by equations: Where Gn and Gp are the generation rate for electrons and holes, respectively, and Un, Up are the recombination for electrons and holes respectively.
Where i n is the effective intrinsic concentration and T is the lattice temperature.These two equations may then be re-written to define the quasi-Fermi potentials:

RESULTS AND DISCUSSIONS
Activation energies and capture cross sections of the traps used in this work [1,4,5]. .
The final term accounts for the gradient in the effective intrinsic carrier concentration, which takes account of band gap narrowing effects.Effective electric fields are normally defined where by: Which then allows the more conventional formulation of drift-diffusion equations to be written see Eqs. 7.a and 7.b.
The density of deep acceptors and deep donors in the substrate are varied from (5 × 10 13 to 10 16  −3 ) [5] The conditions for high-resistivity material are given by relations following [6]: For n-type substrate: ND>NA then (NtA-NtD) > (ND-NA) For p-type substrate: NA>ND then (NtD-NtA) > (NA-ND) Vds (V)      The results showed that in the presence of donor traps the current-voltage characteristics increases because the number of free carriers Increases.However, the acceptor traps have a significant effect on the current-voltage characteristics.In the presence of acceptor traps, the load space area in the channel Increases, hence, Reduces the current drain.
Deep traps are believed to be responsible for many parasitic effects in GaAs FETs such as the gate lag and drain lag effects in which a slow transient is observed in the drain current following a voltage applied to the gate or the drain.The trap properties, energy and cross-section either measured by transient spectroscopy, voltage or optically excited deep level transient spectroscopy (DLTS).

0
 is the local permittivity, NA and ND are the shallow-acceptor density and shallow-donor density, respectively, p and n are the hole and electron densities, respectively,  tA N and  tD N are the ionized deep acceptors and deep donor's density, respectively.


By default, ATLAS includes both Eqs.2.a and 2.b In some circumstances, however, it is sufficient to solve only one carrier continuity equation.Derivations based upon the Boltzmann transport theory have shown that a drift-diffusion model may approximate the current densities in the continuity equations.In this case, the current densities are expressed in terms of the quasi-Fermi levels n are the electron and hole mobilities.The quasi-Fermi levels are then linked to the carrier concentrations and the potential through the two Boltzmann approximations:

( 5
.b) By substituting these equations into the current density expressions, the following adapted current relationships are obtain It should be noted that this derivation of the drift-diffusion model has tacitly assumed that the Einstein relationship holds.In the case of Boltzmann statistics this corresponds to: solved by the Newton method.Several models are activated in the simulation including: Shockley-Read-Hall recombination (SRH Model), electric mobility dependent parallel field (Fldmob model), concentration dependent mobility (Conmob model), and mpact ionization (impact of Selb.model).
Figs.1 and 2show the influence traps donor and acceptor in the substrates on the electrical characteristics (currentvoltage).

Figs. 2
Figs. 2 and 3 shows simulated results Ids-Vds characteristics of GaAs MESFETs on the n-type and p-type substrate, respectively.Two cases with different deep-acceptor densities in the substrate are shown Figs.2.a and 2.b.The results for a case with deep donors and shallow acceptors in the substrate are shown by Figs.3.a and 3.b.

Figs. 3 .
Figs. 3.a and 3.b shows Ids versus Vds plot at Vgs=0 with increasing density of donor traps, the current increases because the number of free carriers increases.