MOSFET, short for metal-oxide semiconductor field-effect transistor, is a type of transistor. This section introduces the N-channel enhancement-mode field-effect transistor, whose symbol is shown in Figure 1-29, and it has three pins: the gate (G), drain (D), and source (S). How do we determine if the transistor is an N-channel or P-channel? We can look at the arrow in the device symbol. Whether in MOSFETs or bipolar transistors, the arrow always points from P to N. The channel indicated by the arrow is the N-channel, and vice versa for the P-channel.
Figure 1-29: Circuit symbol for MOS.
Figure 1-30 shows a typical output characteristic curve for an NMOS transistor, where the horizontal axis represents the voltage difference between DS, and the vertical axis represents the current flowing through DS, IDS. Different curves correspond to different voltage differences between GS, VGS, increasing from bottom to top. MOSFETs have three operating regions: the variable-resistance region, the constant-current region, and the cutoff region. The curves in the figure mainly describe the relationship between VGS, IDS, and VDS. By carefully observing the curves, we can identify the following three important operating regions.
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| Figure 1-30: Output characteristic curve of N-channel MOS. |
1.Linear Region region.
In this region,
for a fixed VGS, IDS increases as VDS increases, but the ratio of △IDS/△VDS is
a constant value (fixed slope). This ratio is the reciprocal of resistance. At
this point, the MOSFET acts like a resistor, exhibiting different conductive
resistances for different VGS in this region. The output characteristic curves
in this region are different straight lines with different slopes. By adjusting
VGS, we can adjust the resistance of the MOSFET, so it is called the
variable-resistance region. For example, the slope of curve VGS1 in the
variable-resistance region corresponds to the reciprocal of resistance R1, and
similarly, the slope of curve VGS2 corresponds to R2, and the slope of curve
VGS3 is greater than VGS2 greater than VGS1, so the resistance of the MOSFET is
R1>R2>R3. In summary, the larger the VGS in this region, the smaller the
resistance, and macroscopically, VGS controls the conductive resistance of the
MOSFET. Figure 1-31 also shows the MOSFET resistance curve, where we can see
that the larger the VGS, the smaller the resistance. When used as a switch, the
MOSFET acts like an open switch with a very small resistance between DS when it
conducts.
2.C onstant-Current Region
The next region is the constant-current
region, where IDS is only related to VGS and is not affected by changes in VDS.
If VGS increases, IDS also increases, and we can control the current (or
amplify the current) by controlling the VGS voltage. This is a transconductance
amplifier (input voltage, output current, and the ratio of output current to
input voltage is the transconductance), so the MOSFET is a voltage-controlled
device, while the bipolar transistor is a current-controlled device.
3. cutoff region
When VGS is less than the threshold voltage
VGS(th), the MOSFET operates in the cutoff region and acts like it is closed.
The conductive resistance is very high, and there is basically no current
flowing through DS. As shown in Figure 1-31, the larger the VGS, the smaller
the conductive resistance and the larger the current. Conversely, the smaller
the VGS, the larger the conductive resistance, and the smaller the current.
This is the principle of using the MOSFET as a switch.
| Figure 1-31: N-channel MOS resistance curve. |
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