Review of transistor modes of operation¶
$\large \texttt{FOLLOW THE CHANNEL}$
To have a channel (inversion mode), the voltage magnitude between the gate and the point beneath the gate insulator must be at least $|V_{TN}|$ or $|V_{TP}|$. For an N-channel device, the gate needs to be more positive than the channel to attract electrons. For a P-channel FET, the gate potential needs to be less than the channel in order to attract a surplus of holes to the surface.
The animation here is a nice illustration of charge in a MOSFET channel: (clickable link!) Wikipedia: Threshold Voltage
Channel on the X side?
| G-S , G-D | yes | no |
|---|---|---|
| yes | triode | saturation |
| no | saturation | cutoff |
- Note: linear mode is a synonym with triode mode
Because the physical structure of a MOSFET is symmetric, the mode table is symmetric. This is in contrast to the BJT whose emitter region is more highly doped than the collector to make it work better (larger $\beta$).
Translation table between the mode names and uses:
| BJT | MOSFET | typical use |
|---|---|---|
| cutoff | (cut)off | OFF/open switch |
| forward active | saturation | amplifier |
| saturation | triode | ON/closed switch |
| reverse active | saturation | FETs can be symmetric |
When using a MOSFET as a switch, the circuit is designed so that the transistor operates in either triode (ON) or cutoff (OFF). Saturation mode is FET amplifier mode and similar to active mode for a BJT. While it is unfortunate that "saturation" means different things for FETs and BJTs, the term is a good word within the context of the each device's semiconductor physics behavior.
Even though it is the gate-source and gate-drain voltages which set the operation mode, the gate-drain voltage $v_{GD}$ is not commonly used. The drain-source voltage $v_{DS}$ is used instead, from which it is easy to extract $v_{GD}$ when necessary as
\begin{equation} v_{GD} = v_{GS} - v_{DS} \end{equation}
Finally, the amount of charge in the channel (which affects current $i_D$) is varied by the voltage magnitude greater than the minimum required to invert the channel. This "overdrive" voltage $v_{OV}$ is frequently used as a separate term, but is merely
\begin{equation} v_{OV} = v_{GS} - V_{TN} \end{equation}
Fill in the table for an N-type MOSFET if $V_{TN} = 2.1\,\mathrm{V}$ (the nominal threshold of the BS170 in ECE Lab stock):
$V_S$ | $V_G$ | $V_D$ | $\hspace{2em}V_{GS}\hspace{1em}$ | $\hspace{1em}V_{OV}\hspace{1em}$ | $\hspace{1em}V_{DS}\hspace{1em}$ | $\hspace{1em} mode$ ---------:|----------:|-----------:|:----------:|:----------:|:----------:|:-----------| 0.0 | +5.0 | 0.0 | | | | | | | | | | | | 0.0 | +5.0 | +1.0 | | | | | | | | | | | | 0.0 | +5.0 | +4.0 | | | | | | | | | | | | 0.0 | +5.0 | +8.0 | | | | | | | | | | | | 0.0 | +5.0 | +2.0 | | | | | | | | | | | | +2.0 | +5.0 | 0.0 | | | | | | | | | | | | +2.0 | 0.0 | +2.0 | | | | | | | | | | | | 0.0 | +3.4 | +1.0 | | | | | | | | | | | | 0.0 | +3.4 | +1.3 | | | | | | | | | | | | 0.0 | +3.4 | +3.4 | | | | | | | | | | | | -15.0 | -9.4 | -3.7 | | | | |
Fill in the table for the BSS138 N-type MOSFET whose nominal threshold voltage is $V_{TN}=1.3\,\mathrm{V}$:
$V_S$ | $V_G$ | $V_D$ | $\hspace{2em}V_{GS}\hspace{1em}$ | $\hspace{1em}V_{OV}\hspace{1em}$ | $\hspace{1em}V_{DS}\hspace{1em}$ | $\hspace{1em} mode$ ---------:|----------:|-----------:|:----------:|:----------:|:----------:|:-----------| +3.3 | +3.3 | +5.0 | | | | | | | | | | | | +3.3 | +3.3 | 0.0 | | | | (1) | | | | | | | | 0.0 | +3.3 | 0.0 | | | | | | | | | | | | 0.0 | +3.3 | +5.0 | | | | (2) | | | | | | | |
(1) In a 3-terminal device (where S and B are connected), the D-B pn-junction is forward-biased and this state is temporary while the S side node is "dragged" down to the D voltage.
(2) Are there any forward-biased pn-junctions in this condition?