Elsevier

Microelectronics Reliability

Volume 83, April 2018, Pages 173-179
Microelectronics Reliability

A modified two dimensional analytical model for short-channel fully depleted SOI MESFET's

https://doi.org/10.1016/j.microrel.2018.03.004Get rights and content

Highlights

  • In this version of the article, the authors revised the manuscript according to the reviewers’ comments including adding a verification chart to compare analytical (modelling) results with experimental data and extending the analytical investigation to cover more device parameters’ range.

  • The analytical results match very well with simulation data.

  • The presented model displays the device performance for various device parameters and bias conditions.

Abstract

Analytical modeling of channel potential of SOI MESFET can be obtained by solving Poisson's equation to derive an expression for channel potential. Superposition method is an accurate technique for solving Poisson's equation, in which the solution of the 2-D Poisson's equation is represented as the sum of the 1-D Poisson's equation and a 2-D Laplace's equation solutions. In the existing models applying this method, the authors tried to solve 2-D Laplace's equation by using approximations [1] or modifying the boundary conditions [2] producing inaccurate results. In this report, a new methodology is applied to develop a modified analytical model for the channel potential of fully depleted SOI MESFET's, in which the drawbacks of the previous models are significantly eliminated. Using this model, the subthreshold performance of the device including channel potential, threshold voltage, drain current, and subthreshold swing under various conditions have been studied, plotted, and compared with TCAD simulation and experimental results. It is concluded the proposed model has been improved in term of accuracy compared to other existing models.

Introduction

The leading VLSI circuits are mostly based on MOSFET's which are the most common transistor currently used in integrated circuits. However, SOI MESFET's are considered as a good contender for operating at high power and high-speed applications such as military communications, satellites, aerospace, and data storage [3]. SOI MESFET's offer excellent channel mobility, enhanced radiation hardness, immunity to hot carrier aging, low junction capacitance, low noise, latch-up immunity, high frequency and high temperatures performance. Due to these advantages, MESFET's are able to bypass many MOSFET-related problems and can be used as a better choice for various electrical applications [4]. These devices also can be used in complementary Si MESFET (CMES) technology with the advantage of low power dissipation [5]. Moreover, SOI MESFET's can be integrated alongside SOI CMOS with no expensive modification to the CMOS process flow [6].

Several analytical models have been developed to describe the electrical performance of this device. Most of these models are based on the solution of 2-D Poisson's equation in the depletion region under the gate. Many mathematical techniques including Green function method [7], the approximation of 2-D potential by a parabolic function [8] [9], and superposition method [10] are used to solve Poisson's equation. Green function method involves with complex mathematical descriptions which are difficult to calculate. Approximating of potential distribution by a parabolic function leads to simple and less accurate model. Superposition method, which is more accurate than other methods, has been used by Marshal et al. for silicon MESFET [11] and Chiang et al. for SOI-MESFET [1]. It is reported that the solution technique used by Chiang et al. has drawbacks that led to imprecise results [9]. They used the superposition method and found that the eigenvalue kn satisfies the following equation.cotkntsi=coxcsi1kntsi

They approximate Eq. (1) as following.kntsi2;forn=1,2,3,,

Using this approximation they obtained Fourier coefficients of the channel potential but Jit et al. stated that this approximation is reasonable for tsi ˂˂ tox (i.e.(cox/csi) → 0) and is not accurate enough for all values of tsi and tox [2]. To prove this assertion, they solved Eq. (1) numerically for tox = 0.2 μm and two values of tsi = 0.01 and tsi = 0.08 μm. They found that k1tsi = 1.5812 and 1.6501, while if approximation of Eq. (2) is used, the value of kntsi is irrelevant of tsi and tox, which is not proper. This value is constant and equal to 1.5708. Another drawback reported for Eq. (2) is the discontinuity of Cotangent function at kntsi=2 for even integer values of n, which leads to an undefined answer [2]. Moreover, the property of orthogonality of eigen functions is ignored. Jit et al. offered solutions to address these problems. They ignored the effect of vertical electric field and modified the boundary condition at Si-SiO2 interface asUxyyy=tsi=0

Using this boundary condition, they stated thatkntsi2n1π2;forn=1,2,3,,

Although in Eq. (4), the discontinuity of cot(kntsi) for even values of n is eliminated but this equation is not sufficiently precise. Computing of k1tsi applying Eq. (4), still results in a value of 1.5708 for all values of tsi and tox. Therefore this equation is not a general solution for all values of tsi and tox. Since the effect of the vertical electric field is ignored, the outcome is not accurate enough for thin-oxide SOI MESFET's. In this paper, considering the orthogonality property of the sinusoidal functions in the Fourier series, we presented an exact technique for calculating the potential distribution. Using this method, we derived an analytical models for channel potential, threshold voltage, drain current, and subthreshold swing of SOI MESFET's. Making use of the presented model, the subthreshold performance of the device under various device parameters and bias conditions are plotted and discussed. The accuracy of the obtained analytical model is investigated using the TCAD simulations.

Section snippets

Description of the proposed model

Fig. 1 shows the cross-sectional view of an SOI-MESFET under analysis where L is the physical gate length, tsi, is the thickness of silicon layer and tox is the thickness of buried oxide layer. Also, VGS, VDS, and VSub represent the applied voltages to the gate, drain, and substrate, respectively.

Results and discussion

This section presents the results calculated by the analytical model with a wide variation of the device parameters and biasing conditions against the 2-D numerical device simulator, ATLAS from Silvaco [21]. By using the expressions derived in Section 2, the profile of bottom potential, threshold voltage, drain current, and subthreshold swing have been calculated and plotted. In order to demonstrate the enhancement of our model over the existing models in [1,2], the results are compared

Conclusion

A new methodology in superposition technique is successfully applied to solve the two dimensional Poisson's equation in order to analytically model the channel potential, threshold voltage, drain current, and subthreshold swing of fully depleted SOI MESFET's. In the proposed model, unlike the two previous models, there are no approximate method or change in boundary conditions which lead to inaccurate results. The model has been verified by comparing with simulation data, experimental results,

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