Two-dimensional numerical analysis of the narrow gate effect in MOSFET

Lateral variation of the local threshold voltage causes non-linearity in the drain conductance-gate voltage characteristics, resulting in a nonunique external threshold voltage which varies with gate voltage. Using a 16-bit minicomputer, a two-dimensional (2-D) finite-difference program for narrow gate MOSFET (NAROMOS), and an accurate and efficient new finite-difference boundary equation at the oxide-semiconductor interface, computations are carried out for the external threshold voltage and a measurable electrical channel width as a function of the applied dc gate and substrate voltages. The depletion approximation is employed in order to compare the 2-D results with the 1-D analytical solution of the depletion model. Computed curves are presented for the lateral variations of the depletion layer thickness, surface potential, normal surface electric field, local as well as external threshold voltages, and electrical channel width as a function of the device structure, material parameters, and bias voltages. Based on the 2-D results and device physics, an analytical approximation of the threshold voltage versus the gate width, simple enough for CAD of VLSI, is derived whose parameters may be determined from either a 2-D computation or experimental measurements on one test device of a known gate width. The computed increase of the external threshold voltage with decreasing gate width compares well with published experimental data.

[1]  P.P. Wang,et al.  Device characteristics of short-channel and narrow-width MOSFET's , 1978, IEEE Transactions on Electron Devices.

[2]  C. Sah,et al.  The effects of fixed bulk charge on the characteristics of metal-oxide-semiconductor transistors , 1966 .

[3]  F. Fang,et al.  Transport Properties of Electrons in Inverted Silicon Surfaces , 1968 .

[4]  T. Shibata,et al.  A new bird's-beak free field isolation technology for VLSI devices , 1981, 1981 International Electron Devices Meeting.

[5]  F. Stern,et al.  Electronic properties of two-dimensional systems , 1982 .

[6]  Kuang Yi Chiu,et al.  A bird's beak free local oxidation technology feasible for VLSI circuits fabrication , 1982 .

[7]  Hideo Sunami,et al.  Characteristics of a Buried-Channel, Graded Drain with Punch-Through Stopper (BGP) MOS Device , 1981, 1981 Symposium on VLSI Technology. Digest of Technical Papers.

[8]  F.Z. Custode,et al.  A model of a narrow-width MOSFET including tapered oxide and doping encroachment , 1981, IEEE Transactions on Electron Devices.

[9]  Kjell Jeppson,et al.  Influence of the channel width on the threshold voltage modulation in m.o.s.f.e.t.s , 1975 .

[10]  R.H. Dennard,et al.  1 µm MOSFET VLSI technology: Part II—Device designs and characteristics for high-performance logic applications , 1979, IEEE Transactions on Electron Devices.

[11]  E. M. Buturla,et al.  Finite-element analysis of semiconductor devices: the FIELDAY program , 1981 .

[12]  T. Shibata,et al.  An optimally designed process for submicrometer MOSFET's , 1982 .

[14]  D. Takacs,et al.  Electrical measurement of feature sizes in MOS Si2-gate VLSI technology , 1980, IEEE Transactions on Electron Devices.

[15]  C. Sah.  Characteristics of the metal-Oxide-semiconductor transistors , 1964 .

[16]  Threshold voltage of a narrow-width MOSFET , 1981 .

[17]  J. Brews.  Subthreshold behavior of uniformly and nonuniformly doped long-channel MOSFET , 1979, IEEE Transactions on Electron Devices.

[18]  T. Watson,et al.  Geometry effects of small MOSFET devices , 1979, 1977 International Electron Devices Meeting.

[19]  A. S. Grove,et al.  Electron and hole mobilities in inversion layers on thermally oxidized silicon surfaces , 1965 .

[20]  Gerard Merckel,et al.  A simple model of the threshold voltage of short and narrow channel MOSFETs , 1980 .

[21]  L. A. Akers,et al.  An analytical expression for the threshold voltage of a small geometry MOSFET , 1981 .

[22]  Chih-Tang Sah,et al.  New mobility-measurement technique on inverted semiconductor surfaces near the conduction threshold , 1979 .

[23]  Gerhard K. Ackermann,et al.  Threshold voltage of narrow channel field effect transistors , 1976 .