Electrotopic.com

Electronics & Software – Tips & Guide

general

# Why does not fet field effect transistor have input charectetstics?

## Why does not fet field effect transistor have input charectetstics?

In all cases, the current consumed by the door is extremely low, small enough to interfere with the operation of the connected circuits. the grid current versus voltage plot shows almost zero current variation, except in the case of a fetal gate junction that is forward biased.

So while it is possible to draw such characteristics, it is not very useful.

a characteristic curve is a line between two quantities (or variables). the two most commonly used variables for drawing a characteristic curve are voltage and current. in most cases, we plot the voltage along the x-axis (which makes it the independent variable) and the current along the y-axis (the dependent variable).

The output characteristic of a fet is a graph of the output current versus the output voltage. the output current is id (drain current) and the output voltage is vds (drain voltage at the source).

the way of drawing the output curve is to vary the drain-source voltage and at each value of vds, the id of the drain current is plotted. these points, when connected together, form a curve. (For each output curve, we keep the value of the input voltage vgs constant.)

Now let’s look at the input terminals of the fet. the input voltage is vgs. What about the input current? the fet may be a jfet or a mosfet. in the case of the jfet, at the input, we have a reverse bias pn junction. how much current does a reverse bias pn junction conduct? ideally, it does not conduct any current (or a negligible current). whatever input voltage we apply, the input current is (almost) zero.

In the case of a mosfet, at the input terminal, we have an oxide layer, which is a nonconductive layer. How much current does a non-conductive layer allow? again, ideally zero. therefore, regardless of the input voltage applied to the mosfet, the input current will be zero.

Remember that to draw a characteristic curve, we need two quantities (voltage and current). but we have seen that, at the input terminals of a fet, we can have a voltage but no current. and, with zero current in both cases, our curve will simply coincide with the x-axis (voltage axis). therefore, we do not draw the input curves for the fets.