Transistors – Field Effect and Bipolar Transistors: MOSFETS and BJTs
The red spheres represent positive current, which is in the opposite direction of electron current. In a “Bipolar Junction Transistor”, a small current flowing into the middle terminal controls a much larger current through the transistor’s two other terminals. Bipolar Transistors are one of the two main types of transistors. In this other type of transistor, the middle terminal does not have any current passing through it during steady state conditions. Here, it is the “voltage” of the middle terminal that controls the current through the other two terminals. This is called a “Field Effect Transistor.” There are two main types of “Field Effect Transistors” and two main types of “Bipolar Transistors.” We can tell them apart by whether the arrow is pointing toward or away from the middle terminal. In bipolar transistors, the positive current flows in the direction of the arrow. The red spheres symbolize the positive current. This is a “NPN” bipolar transistor. This is a “PNP” bipolar transistor. Here, it is the positive current flowing “out” of the middle terminal that controls the transistor. Let’s now focus on Field Effect Transistors. The two main types of “Field Effect transistors” are two different types of “Metal Oxide Semiconductor Field Effect Transistors”, abbreviated “MOSFETs.” This is an “N Channel” MOSFET. As always, the red spheres represent the direction of positive current. Here, a decrease in the voltage on the middle terminal will cause a decrease in current. An increase in the voltage on the middle terminal will cause an increase in current. This relationship is reversed for a “P Channel” MOSFET. In both cases, the voltage that controls the transistor is the voltage between the terminals that we call the “Gate” and the “Source.” In the N channel example, the “Source” is shown at the bottom. In the P channel example, the “Source” is shown at the top. In both cases, the voltage difference between the “Gate” and the “Source” has to exceed a certain “threshold voltage” before it starts having any effect. Since it is the voltage between the “Gate” and the “Source” that controls the transistor, the current will not increase much if we increase the voltage at the “Drain.” But, if we reduce the “Drain” voltage, we will eventually reach a point where the current will significantly decrease. The behavior of Bipolar Transistors is different than Field Effect Transistors, and the terminals have different names. Here, when the current is flowing, the difference between the “base” voltage and the “emitter” voltage stays at about 0.7 volts, and the “collector” current is the “base” current multiplied by a large constant number. Suppose we add another resistor as shown. A change in the current through this resistor causes a change in the voltage drop across it. Here, the collector voltage is 0.3 volts above the emitter voltage. If the base current is increased further, the collector voltage is unable to go any lower. Here, the all the currents are staying constant. Here, the bipolar transistor is behaving like a switch in the on position, but it is not an ideal switch because there is a 0.3 volt drop across it. On the other hand, when a “MOSFET” behaves like a switch in the on position, it is not an ideal switch because it behaves like a resistor with a small resistance value. A MOSFET is unable to block in the reverse direction because there is a diode inherently built into its body. Also inherently built into each MOSFET are capacitors between each of the terminals. Although it is not shown in this video, these capacitors need to charge and discharge, and there is therefore a brief momentary current through the gate terminal when the gate voltage changes.