[转载]Vdsat、Vov、Vds联系与区别
2018-07-16 10:34阅读：
Vov：过驱动电压overdrive voltage，Vov=Vgs-Vth，过驱动电压也用Vod表示
Vdsat:饱和漏源电压或夹断时漏源电压（刚出现夹断）saturation drain voltage
在长沟道下，vdsat=vgs-vth=vov，在短沟道下，由于二阶效应，vdsat小于vgs-vth，但这个值，spice也好，spectre也好，都是用来判断管子工作区间的。
vds>vdsat管子工作在饱和区
vds
Vov=Vgs-Vth,用MOS的Level 1
Model时，不考虑短沟道效用，Vdsat=Vov=Vgs-Vth,当Vds>Vdsat时，MOS的沟道就出现Pich-off现象，这时候电流开始饱和。（长沟道器件）
但是考虑到短沟道效应的模型里，沟道里的多子因为速度饱和效应（Velocity
saturation），Vds不需要到达Vov，只要到达Vdsat，Ids就会饱和，不会再上升。但是此时在物理上，沟道并未达到Pinch-
off，直到Vds=Vov，沟道的Pinch-off现象才会出现。也就是说在短沟道模型中，器件在沟道Pinch-off之前就会达到速度饱和，电流
不会再增加（短沟道器件）
Vds-Vdsat要留一定余量，一般200mv，差分输入对一般为100多mv，一般来说vdsat<50mV管子基本就工作在线性区；一是怕管子由于工艺进入线性区；二是饱和区边缘rds较小。
附英文解释：
Q:
We can see the parameter Vdsat in Cadence Spectre after we
perform. the DC simulation. But I can't figure out what's the
physical meaning of it?
I guess the vdsat is the overdrive voltage at first, but
it's not exact the same as Vgs-Vth. The Vds
at is smaller than Vgs-Vth. So anyone has the idea what is Vdsat is
the saturation drain voltage.
for a long channel device, Vdsat almost equal to
Vgs- Vth ( Vdsat = Vgs -Vth).
in strong inversion region(Vgs>Vth):
NMOSFET works in linear region when
0<<Vgs -Vth ), the inversion channel behaves like a simple
resistor. The drain current Ids increases linearly as the drain
voltage Vds increases. However, when Vds is larger it will cause an
increase of the voltage in the inversion layer at all points along
the channel (except for the singular point at the source edge).
This reduces the voltage across the gate capacitor and the
inversion charge density is reduced. The smaller amount of mobile
inversion charges results in a decrease in channel conductance,
which leads to a smaller slope in the Ids -Vds characteristics as
Vds increases. Eventually, Vds reaches the saturation voltage Vdsat
, at which point the mobile carriers at the drain side disappear in
this first order model, and the channel is 'pinched off' at the
drain side [2.4, 2.6]. The condition of no mobile carriers at the
pinch-off point has traditionally been used to obtain the
analytical saturation voltage expressions for long channel compact
models.
NMOSFET works in saturation region when VdsatVdsat,
the pinched-off region of the channel increases and extends towards
the source. The excess drain voltage beyond Vdsat will drop across
this pinched-off region and the drain current remains approximately
constant. However, we need to point out that the constant
saturation current behavior. is only an approximation. The small
but non-zero slope of the Ids -Vds characteristics in the
saturation region is very important to analog circuit performance
and must be accurately modeled by a compact model. In addition to
the finite length of the pinch-off region (channel length
modulation), drain induced barrier lowering and substrate current
induced body effect must be accounted for in modeling the current
in the saturation region.
for a short channel device, Vdsat = (Vgs
-Vth)//(L*Esat)
so the device enters saturation before Vds reaches Vgs – Vth
and operates more often in saturation.
critical gate voltage, at which an inversion layer is formed,
is called the threshold voltage (Vth ).When the voltage between the
gate and source, Vgs , is larger than Vth by several times the
thermal voltage vt (KBT/q), the device is said to be in the strong
inversion regime. When Vgs=Vdd (the power supply voltage), the
device is in the “on” state. When Vgs is less than Vth , the device
is in the subthreshold (or weak inversion) regime. When Vgs = 0,
the device is in the “off” state. When Vgs is biased near Vth , the
device operates in the moderate inversion egime,which is an
important operation region in low power analog
applications.
REFERENCE1. MOSFET Modeling and
BSIM3 user's Guide, 1999.
