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MESFET: MESFET Application Examples
SILVACO International 11-7
• Id/Vds characteristics with Vgs=0
• AC analysis at a single DC bias at different frequencies
• conversion of capacitance and conductance data from ATLAS into s-parameters
• Smith Charts of MESFET s-parameters using TonyPlot
The geometry and doping of the MESFET device used in this example is described using the ATLAS
structural syntax. Initially the mesh is specified in x and y, using the following command format
x.mesh loc=aaa spac=bbb specifying that there is a mesh spacing of bbb and x location aaa. Region
statements are used to specify two GaAs regions which will later be doped differently. Electrode
statements specifies the names and positions of electrodes. The workfunction of the gate contact is
set using
contact. Each GaAs region is uniformly doped. An n-type active layer sits on top of a lowly
doped p-type substrate. GaAs mobility models for concentration and field dependence are set in the
models statement.
The numerical methods used at the initial stage of the simulation are conservative. The statement,
method gummel newton specifies that the decoupled (gummel) method is used at the start of the sim-
ulation for each bias point. This switches to the coupled (Newton) method if convergence is not ob-
tained. This is the most robust method for the initial bias steps and for complex devices. It does
consume more CPU time and can lead to problems at high current levels. If these problems occur
the statement
method newton can be used.
The DC simulation proceeds by ramping the drain voltage in the
solve statement until Vds=3.0V.
After this a log file is opened and AC analysis begins. The full sweep of frequency is done on the line:
solve ac.analysis direct frequency=1.e9 fstep=2.e9 nfsteps=20
The ‘ac’ parameter switches on the AC analysis. Direct specifies a robust AC solver for high frequen-
cies. Activating the ‘direct’ solver will cause more memory to be allocated than was used during the
DC simulation. If your system does not have enough virtual memory to run the AC application the
simulation will stop here with an error message. The exact amount of memory required depends
on the number of mesh points. The
frequency value is the initial frequency of the AC signal, fstep
is the step value added to this frequency and nfsteps is the number of steps.
The AC analysis in ATLAS calculates the real and imaginary current components from a small AC
signal on top of the existing DC solution. From these currents ATLAS calculates the conductance
and capacitance between each pair of electrodes. By plotting the log file, users can see graphs of
drain-gate conductance (C drain>gate) or total gate capacitance (-1*C gate>gate).
The s-parameters (or Z,Y and ABCD parameters) are calculated by setting the s.param (or y.param
etc) on the log statement preceeding the ac analysis. The most important parameters are the *port
parameters. s-parameter analysis assumes the device is a two port device with four terminals. The
inport and outport are used to assign the respective electrodes. Users should ensure the correct def-
inition of all ports. In this example, the input port is set to the gate and output port to the drain.
To load and run this example, select the Load example button while this text is displayed. The input
file and several support files will be copied to your current working directory. Once loaded into
DECKBUILD, select the run button to execute the example.