Welcome back to Electronics, this is Dr. Robinson. In this lesson, we'll look at MOSFET characteristics. In your previous lesson, you were introduced to CMOS logic gates. Our objectives for this lesson are to introduce MOSFET characteristic curves, and to introduce DC biasing. Let's look at some characteristic curves for an in-channel enhancement mode MOSFET. Now in a previous lesson, you saw how the underlying physics of the MOSFET results in a set of characteristic curves known as the MOSFET output characteristics, a plot of the MOSFET drain current versus the drain to source voltage for different values of gate to source voltage. Now implicit in this set of curves, is another characteristic curve known as the MOSFET transfer characteristic. A plot of the MOSFET drain current versus the gate-to-source voltage. Now remember, the parameter that's changed to generate each one of the curves on the output characteristic is VGS, and in this plot I varied VGS. From 1.5 up to a value of four, in steps of 0.5 Volts. So this bottom curve is for VGS = 1.5 or less, and the top curve is VGS = 0.5. So to generate a transfer characteristic from an output characteristic, we choose a VDS. And in this case, you can see that I chose VDS equal to 7 volts. So I draw a vertical line at VDS equals 7 volts, and then work my way up the line reading off the VGS versus ID pairs so here we have a drain current of zero at a VGS of 1.5. So we come to a VGS of 1.5 and we have a drain current of zero. We then move to the next curve, a VGS of 2 and we have some slightly higher drain current. And we continue that to generate the transfer characteristic curve. Now it's apparent from looking at this curve, the Transfer Characteristic Curve, that the relationship between Drain Current and Gate to Source voltage is not a linear relationship. You can also see that here on the Output Characteristics Curve. Between each one of these curves is a VGS step of 0.5 but as the VGS increases the distance between each consecutive curve increases. So on the Transfer Characteristic, in moving from 1.5 to 2 VGS, we get this small change in ID, but moving that same distance in voltage, from say 3.5 to 4, we get a much larger change in drain current. Now let me draw on this set of MOSFET Output Characteristics curves wherein channel enhance mode MOSFET a boundary line and this boundary is define by the equation VDS=VGS-VTO. Now MOSFET's quiescent point or Q point or bias point is defined by the relationship of it's drain current, it's drain to source voltage and it's gate to source voltage. If this key point lies to the left of this boundary line, we say that the MOSFET is operating in it's saturation region. So for example, if it had a, the MOSFET had a drain to source voltage of five volts and a drain current of three milliamps, that keypoint would place the MOSFET in it's saturation region. Now, if those three quantities combine such that the q-point is to the left of this boundary, we say that the MOSFET is operating in it's linear region. Now associated with each MOSFET is a parameter known as the threshold voltage or turn on voltage, VTO. This is the minimum value of VGS for which current flows through the MOSFET. So if VGS is less than this threshold voltage value ID is equal to zero and the MOSFET is off or it's set to be operating at it's cut off region. So on the output characteristics curves this line at the bottom where ID is equal to zero would define the cut off region. Where VGS is less than VTO. Now let's define the MOSFET regions of operation, that we examine graphically in the previous slide, in terms of equations. Remember the MOSFET operates in its cutoff region when it's gate to source voltage is less than the threshold voltage, and in that region, no current flows through the MOSFET. It's drain current is equal to 0. Now, if the gate to source voltage is greater than VTO and current is flowing, then the region of operation is determined by the value of VDS. If VDS is such that we lie to the left of that boundary on the previous slide, or in other words, if VDS is less than VGS- VTO, then the drain current is related to the gate to source voltage in a linear way. If VGS is greater than VTO but the drain to source voltage is such that we lie to the right of that boundary on the previous slide, VDS is greater than VGS- VTO then you can see that the drain current is related to the gate to source voltage as a square law relationship. It's proportional to VGS squared. Now in these equations for ID in the linear regions and ID in the saturation region, you can see that we have two intrinsic MOSFET parameters. K, known as the transconductance parameter, which has units of amps per volts squared and VTO, the thresh hold voltage or turn on voltage. Now let's look at how the different regions of operation affect the shape of the MOSFET transfer characteristics. Here we're examining two sets of transfer characteristics. One where VDS is equal to seven volts and one where VDS is equal to one volt. For a MOSFET having a threshold voltage of 1.5 volts. Now, we can see that when VGS is less than VTO of 1.5 volts. There's no drain current and the MOSFET is off in its cutoff region. Now, let's examine this point here where VGS is equal to two volts. Now we know that the region of operation is determined by the relationship of VDS to VGS- VTO. So where VGS is equal to two volts that implies that VGS- VTO is equal to zero point five volts. So for both this condition, where VDS is equal to one volt and this condition where VDS is equal to seven volts, both seven and one are greater than zero point five. So the MOSFET is operating in the saturation region, For a VGS of 2. But let's look at a VGS of 2.5. Here we have VGS minus VTO is equal to 1. And the MOSFET that has a VDS of 7 is still operating in its saturation region because 7 is greater than 1. But the MOSFET that has a VDS of 1 volt is operating at the transition between the saturation region and the linear region. Because 1 is equal to 1, or VDS is equal to VGS minus VTO. Then as we continue to increase the gate to source voltage moving in this direction, the MOSFET with a VDS of 1 will continue to be in its linear region. And you can see that in this region ID is related to VGS in a linear way. But for the MOSFET that has a VDS of 7 as VGS continues to increase, it remains in its saturation region where ID is related to VGS in a parabolic way. Or ID is related to the square of VGS. Now when a MOSFET is operating in it's saturation region, it can be used as an amplifier. When a MOSFET is operating in it's liner region it's used as a voltage controlled resistor where the resistance is controlled by the gate to source voltage So, in summary, during this lesson we introduced MOSFET characteristics, and we introduced DC biasing. In our next lesson, we'll look at the common source amplifier, a particular type of circuit that uses a MOSFET, and we'll concentrate on the DC analysis of this amplifier. So thank you, and until next time.
