
			Mosaic-III Bipolar Device


This is an example on an npn transistor with the base contact on the 
top of the device instead of the side.  This example also shows how to
put in a buried collector to lower the resistance of collector current.
Because this is a vertical device and the base contact is from the top,
there is an extrinsic base region through which only the base current
flows and the intrinsic base region through collector and base current
flow.

The device measures 1.1 x 1.0 microns.  The device is defined by a file
called mosaic.msh.  A simple mesh is generated to grid the device.  The 
electrodes are defined for the emitter, base and collector.  The collector
is n-type and is epitaxially grown and doped at 2e16.  There is a buried
collector region which is a half gaussian doped at 2e19 at the bottom of
the device.  The intrinsic base region is doped at 1.25e18 and is p-type.
The extrinsic base region is doped at 5e19 to lower resistance.  The emitter
is n-type and is doped at 2e20.  Except for the epitaxially grown collector,
the rest of the diffusion profiles are gaussian.

A file called highbias.p2 is used generate the curve for the collector
current versus collector to emitter voltage.  This is done by solving
first for an initial starting point where the collector and base are
biased at 0.1 volts.  That solution is stored in a file called mosaic1.
The collector voltage is then ramped upto 6 volts.  The base voltage is
then incremented and the collector is again ramped upto 6 volts.  This
way a family of collector current curves are obtained at various base
voltages.  The contacts in this example were defined to be charge neutral
so that they act ohmic.  This is actually a default condition and as such
is optional.  The base contact was defined to have a resistance of 1e3 so 
that a 'hard' voltage is not put on the base emitter junction.  Using the
resistance, the amount of current can be controlled going into the base.
This example uses a two carrier newton method with the appropriate models.
The collector characteristics are plotted.
