Keithley S530, User Manual

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S530 Parametric Test System Test Subroutine Library User's Manual Section 3: Test subroutine library reference
S530-907-01 Rev. A / September 2015 3-73
vtext
This subroutine estimates the extrapolated gate-source threshold voltage of a metal-oxide field-effect transistor
(MOSFET).
Usage
double vtext(int d , int g , int s , int sub , char type , double vlow , double vhigh ,
double vds , double vbs , double ithr , double vstep , int nmax , double *slope , int
*kflag )
d
Input The drain pin of the device
g
Input The gate pin of the device
s
Input The source pin of the device
sub
Input The substrate pin of the device
type
Input
Type of transistor:
"N"
or
"P"
vlow
Input
The start of the gate-source voltage (V
GS
) binary search, in volts
vhigh
Input
The end of the V
GS
binary search, in volts
vds
Input The forced drain voltage, in volts
vbs
Input Substrate bias, in volts
ithr
Input
Drain-source trigger current (I
DS
), in amperes
vstep
Input
V
GS
step size, in volts
nmax
Input The maximum number of steps
slope
Output The calculated slope
kflag
Output Return status flag:

0 = Normal operation

1 = The
ithr
parameter is too high; indicates that the
vlow
parameter is above the voltage threshold (V
T
) and the slope is
constantly decreasing

2 = Did not find peak slope; indicates that the
vhigh
parameter
was below V
T
(maximum slope was the last value)

3 = Binary search on V
GS
failed; may indicate that the
vlow
and
vhigh
parameters were below V
T
and the device never turned on

4 = The
type
parameter was not specified as "N" or "P"

5 = V
GS
step size is 0.0
Returns Output Gate-source voltage threshold, in volts.
Details
This subroutine estimates the extrapolated threshold voltage of a MOSFET using the maximum slope
method. Maximum slope refers to the common technique of numerically differentiating the I
DS
versus
V
GS
curve. Slope refers to the FET transconductance (g
m
).
This subroutine uses a two-step method of finding V
T
. First, a binary search is done on the V
GS
to find
a drain current (I
DS
) that is within 0.25 of the estimated threshold current (for most enhancement
devices
this value is 1 μA). If the measured I
DS
value is within tolerance, the routine continues with a
sliding five-point linear least-squares (LLSQ) analysis of the I
DS
-V
GS
curve.