Date of Award

Fall 1994

Document Type

Thesis

Degree Name

Master of Science in Applied Physics - (M.S.)

Department

Physics

First Advisor

Chin, Ken K.

Second Advisor

Farmer, Kenneth Rudolph

Third Advisor

Feng, G. H.

Abstract

In this thesis, a physical model including the subthreshold conpensation properties is presented. The Poisson equation is solved analytically in one dimension for GaAs MESFET's with undoped substrates in the subthreshold region. The solution is then used to derive expressions for subthreshold drain current and subshreshold swing in MESFET's with undoped substrates. Very good agreement between experimental and analytical results is achieved.

Two key parameters (Nilo and Iso) that determine the subthreshold Characteristics have been analyzed as a function of residual acceptor concentration Na, deep level EL2 concentration Nt, channel doping concentration Nd and threshold voltage Vt. It is shown that Mo increases with Na and Nt increase, but decreases with Nd and Vt increase. For Iso, the results show it increase with Nd, Nt and Vt increase, but decreases with Na increases. The results also show that Nt has much smaller effect on subthreshold characteristics than Nd, Na and Vt. According to the results, very useful design rules are presented for the design of devices with good subthreshold leakage characteristics.

In addition to providing quick evaluation expressions, the analytical model presented in this thesis also gives us simple explanation for the observed subthreshold characteristics and offering, a useful basis for accurate analysis, simulation and fabrication of GaAs FET's with ultra low leakage current.

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