Simulation of GaAs MESFET

Simulation of GaAs MESFET

Characteristics

The development of microwave Gallium Arsenide Metal semiconductor Field Effect Transistors (GaAs MESFET) devices has enabled the miniaturization of cellular phones, pagers and other electronic devices. With these MESFET devices comes the need to model them. In this work, we have extracted a small signal equivalent circuit model for a GaAs MESFET device, where the elements are directly related to the physical design parameters of the device. The model is simulated using SPICE to give the GaAs MESFET characteristics.

Introduction

The inclusion of accurate GaAs MESFET models into circuit simulation routines is essential for achieving the successful and efficient fabrication of GaAs monolithic circuits. For GaAs monolithic microwave integrated circuits (MMIC's), a simple lumped-element equivalent circuit is commonly used, even for gate lengths down to 0.25 microns.

In the present work, the transmission line nature of the gate region is emphasized by introducing a II – equivalent representation for the neutral channel. In order to obtain manageable expression for the elements, the channel is taken to be uniform, although it is known to possess non uniformities owing to the action of the drain bias. This approximation is well justified.

The MESFET equivalent circuit

At high frequencies, the gate channel has the character of an RC transmission line, for which a II – equivalent representation can be used. Under saturated current conditions, a stationary may high-field domain may be present in the conducting channel, for. For which an equivalent is derived in turn of a small signal admittance. The partial circuits representing the domain and the neutral channel under the gate can now be assembled into a complete extended equivalent circuit for the MESFET. The complete circuit is shown in Fig. 1, including the parasitic

elements conventionally associated with the terminals and with the fringes of the gate depletion zone.

The usefulness of the small signal equivalent circuit can be evaluated by performing optimisation against experimental S-parameters. Table 1 lists the circuit parameters derived from this optimization. From experiments, value of iD is found to be around 40 m Amp. Using Win SPICE 3 software, the MESFET equivalent circuit was simulated and device output characteristics were obtained as shown in Fig. 2. The basic structure of the model is relevant for other FET designs such as hetrostructure FETs; high electron mobility transistors (HEMTs) and devices with sub micrometer gate lengths.

Conclusion

A new small signal equivalent circuit has been simulated for the GaAs MESFET, where the transmission line properties of the neutral channel have been emphasized and where the effects of a high field domain (in saturation) have been included. The basic features of MESFETs have been high lighted and implementation of the model in Win Spice 3 has been done. The basic structure of the present model is also applicable for other FET designs, and for sub-micrometer FETs.

Fuente:  http://www.oldcitypublishing.com/FullText/JAPEDfulltext/JAPED4.1-2fulltext/159-162%20Kumar.pdf

Luiggi Marquez C.I 17677911 CRF

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