Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Transistors
MOSFET: Transfer Characteristics
Introduction:
The transfer characteristics plot the output drain current as a function of the input gate bias, for fixed drain bias.
Transfer Characteristics:
- Clearly, in the linear region, I_{D} versus V_{G} should be a straight line. The intercept of this line on the V_{G} axis is the linear region threshold voltage, V_{T} (lin.) and the slope (divided by the applied V_{D}) gives us the linear value of k_{N}, k_{N}(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, g_{m} (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 g_{m} (lin.) is plotted as a function of V_{G }in Fig.(b). It may be noted that the transconductance is zero below V_{T} because there is little drain current.
- It goes through a maximum at the point of inflection of the I_{D}- V_{G} 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.
(Fig. a)
- For the transfer characteristics in the saturation region, we know that a quadratic dependence of I_{D }on V_{G}, we get a linear behavior by plotting not the drain current, but rather the square root of I_{D,} as a function of V_{G }that is shown in figure given below.
- In this case the intercept gives us the threshold voltage in the saturation region, V_{V(sat.)}. We know that due to effects such as drain-induced barrier lowering (DIBL), for short channel length MOSFETs the V_{T(sat)} can be lower than V_{T(lin)}, while the long channel values are similar.
- Similarly, the slope of the transfer characteristics can be used to determine the value of k_{N }in the saturation region, k_{N(sat.) }for the n-channel MOSFET, which can be different from k_{N(lin.)} for short channel devices.
(Fig. b)