Junction FETs
Following Bipolar Junction Transistors came Field Effect Junction Transistors. They were the answer for a need of high input resistance in various circuits. Where Bipolar transistors work on drawing current from the Base FETs work from an applied voltage with very little current flow. The output can still be looked at as a varying resistance between two pins. Our input pin is called the Gate. The output current flows between the Source and Drain. Working from a positive voltage and ground the Source is usually connected to the power line and the Drain is connected to the load. This is not the only configuration but it can be used as a general rule most of the time.
Bipolar transistors have a Base resistance in the hundreds of Ohms. J-FETs have an input (Gate) resistance in the neighborhood of 1012 Ohms. FETs are made around a channel of one type of material. N-JFETs have a channel of N-Type material. P-JFETs have a channel of P-Type material. The gate is formed as a junction of the opposite material of the channel and is often described as being wrapped around channel. The Gate – Channel is reverse biased for normal operation.
JFETs are Depletion Mode devices. That means with the Gate grounded (with reference to the Source) the device will conduct through the channel. As you apply a reverse bias to the Gate you pinch off current through the channel, turning the transistor off. As we apply a voltage Gate-to-Source the channel area is Depleted of charge carriers.
In N-JFET devices as we apply a negative voltage to the Gate (negative compared to the voltage on the Source) the transistor turns off.
In P-JFET devices as we apply a positive voltage to the Gate (compared to the Source) the transistor turns off.
Schematic symbols have two popular variations. In some examples the Gate is shown offset from center. The lead closest to the Gate is the Source lead.
Depletion Mode MOSFETs
Metal Oxide Field Effect Transistors have a similar operation to JFETs in that they are Depletion Mode devices. With a ground in they are on. As we apply a voltage to the Gate they turn off. They differ from Junction FETs in the fact that there is no real junction between the Gate and Channel. Separating the Gate and Channel we have an insulating metal oxide barrier.
We have both N-Channel and P-Channel Depletion Mode devices. The Source is most often connected to the power rail and the output is taken from the Drain.
Schematic symbols are not too standard and this often leads to some confusion. The Gate is shown as off-center. The Channel is shown as two parallel lines indicating isolation between the Gate and Channel. The lead closest to the Offset Gate is the Source lead. We often show three leads on the Channel. Source and Drain on the ends and Substrate in the center. An inward pointing arrow on the Substrate indicates an N-MOSFET. An outward pointing arrow on the Substrate indicates a P-MOSFET. In a few cases the Substrate comes out as a separate lead. In most cases the Substrate is connected to the Source Lead.
In Depletion Mode devices the line connecting the Source, Substrate and Drain is an unbroken line.
Enhancement Mode MOSFETs
Enhancement Mode MOSFETs are normally off with a ground in (compared to the Source). As we apply a voltage to the Gate they turn on. The charge we apply to the Gate enhances the charge carriers through the Channel. These are the more popular types of MOSFETs we find in use by far.
In an N-MOSFET (Enhancement mode) the Source is grounded and we apply a positive voltage to the Gate to turn the Channel on. This is similar in operation to the NPN transistor circuits except that we have an input resistance around 1015 Ohms. A factor of around 1,000 times that of the Junction FETs.
In a P-MOSFET (Enhancement mode) the Source is usually connected to the positive rail voltage and as we move the voltage on the Gate toward ground the Channel turns on. This is much the same operation as PNP transistors.