Download EE105 Spring 2006 Discussion 3 PSpice Introduction (Download

EE105 Spring 2006
Discussion 3
PSpice Introduction
(Download: ‘http://www.orcad.com’ > Download > ‘OrCAD 10.5 Demo Software’)
PSpice is a student version of the Spice circuit simulator.
Input files for Spice are called netlists. The netlist translates the circuit diagram into a
computer readable format. In PSpice, the netlist file is always given the extension .cir.
Example netlist:
Simple Circuit
*Circuit Description
Vin 1 0 pulse (0 5 2ns 2ns 2ns 20ns 40ns)
R1 1 2 1k
C1 2 0 100pF
*Circuit Analysis
.tran .1ns 100ns
.probe
.end
The first line of any Spice file is the “name” of the simulation and is always skipped by
the simulator. The simulator also skips any lines preceded by an asterisk, which denotes
comment lines.
The section labeled Circuit Description is the computer description of the circuit
components. The first letter of each of these lines tells which component the line
describes, so, for example, Vin refers to a voltage source and R1 refers to a resistor.
After the component name, the first two numbers (since resistors, voltage sources, and
capacitors are two terminal devices) give the node numbers, which allows you to define
where each component is connected in the circuit. Everything with the same number can
be treated as if they are connected together with ideal wires. In other words, since Vin
and R1 both have the number 1, they are connected to each other. The number 0 always
refers to ground in any Spice file.
After the node numbers, we give the component values, so for R1, the 1k means it is a
1kΩ resistor.
After we give all the circuit components, we tell the simulator what functions we would
like to simulate. These simulator commands are indicated by adding a period before the
command, so for example, a transient analysis, which tells how a circuit performs as a
function of time, is indicated by a .tran statement. The command .probe instructs the
simulator to save these values in a format that can be plotted on a graph.
Finally, all Spice netlists must end with the command .end, which instructs the simulator
to stop reading the netlist.
PSpice has a built-in waveform viewer, so as long as the netlist includes the .probe
command, you can select a node and see the output.
MOS Capacitor
Example (p+ gate / p-type substrate)
Heavily boron-doped polysilicon (referred to as p+ polysilicon) is sometimes used as a
gate material. Its thermal potential is φp+ = -550 mV. Assume the gate oxide is tox = 150
angstroms and the substrate doping is Na = 6.5 x 1016 cm-3.
(a) Find the flatband voltage VFB. What state is the capacitor in for the case of thermal
equilibrium?
(b) Find the threshold voltage VTn.
(c) Sketch the charge density, electric field, and potential through the MOS capacitor in
thermal equilibrium.
(d) Sketch the charge density, electric field, and potential through the MOS capacitor for
VGB = VTn.
- Solution
a)
$N
" p = #Vth ln& a
% ni
$ 6.5 *1016 '
'
=
#26mV
ln
) & 1010 ) = #407.9mV
) (
(
%
(
VFB = #(" p + # " p ) = #(#0.55 + 0.4079) =142.1mV
Capacitor is in accumulation mode in thermal equilibrium.
!
b)
" ox 345.3 fF /cm
=
= 230.2nF /cm 2
tox 150 #10$8 cm
1
VTn = VFB $ 2% p +
2q" sN a ($2% p )
Cox
1
= 0.1421+ 0.8158 +
(2) 1.6 #10$19 1.036 #10$12 6.5 #1016 (0.8158) =1.53V
230.2n
Cox =
(
!
)(
)(
)
c)
The charge on the gate will be:
(
)
QG = Cox (VGB "VFB ) = "CoxVFB = "230.2nF /cm 2 (0.1421V ) = "32.7 #10"9 C /cm 2
so the electric field in the oxide will be:
E=
!
QG #32.7nC /cm 2
=
= #94.7kV /cm
" ox
345.3 fC /cm
!
d)
Since VGB = VTn, the device is on the onset of inversion, and the depletion region has just
hit Xdmax without storing any inversion charge on the silicon surface:
Xd max =
2" s (#2$ p )
!
=127.4nm
(1.602 %10 )(6.5 %10 )
= (1.602 %10 )(6.5 %10 )(127.4 %10 ) =132.7nC /cm
qN a
QG = qN a Xd max
=
(2)(1.035 %10#12 )(0.8158)
16
#19
#19
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#7
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