MOSFET: Transfer Characteristics
The transfer characteristics plot the output drain current as a function of the input gate bias, for fixed drain bias.
- Clearly, in the linear region, ID versus VG should be a straight line. The intercept of this line on the VG axis is the linear region threshold voltage, VT (lin.) and the slope (divided by the applied VD) gives us the linear value of kN, kN(lin.), of the n-channel MOSFET.
- If we look at actual data, however, we see that while the characteristics are approximately linear at low gate bias, at high gate biases the drain current increases sub-linearly.
- The transconductance, gm (lin.), in the linear region can be obtained by differentiating the right-hand side of Eq. of the drain current with respect to gate bias.
- The gm (lin.) is plotted as a function of VG in Fig.(b). It may be noted that the transconductance is zero below VT because there is little drain current.
- It goes through a maximum at the point of inflection of the ID- VG curve, and then decreases. This decrease is due to two factors that will be degradation of the effective channel mobility as a function of increasing transverse electric field across the gate oxide, and source/drain series resistance.
- For the transfer characteristics in the saturation region, we know that a quadratic dependence of ID on VG, we get a linear behavior by plotting not the drain current, but rather the square root of ID, as a function of VG that is shown in figure given below.
- In this case the intercept gives us the threshold voltage in the saturation region, VV(sat.). We know that due to effects such as drain-induced barrier lowering (DIBL), for short channel length MOSFETs the VT(sat) can be lower than VT(lin), while the long channel values are similar.
- Similarly, the slope of the transfer characteristics can be used to determine the value of kN in the saturation region, kN(sat.) for the n-channel MOSFET, which can be different from kN(lin.) for short channel devices.