In our previous article on Darlington Transistor, we have explained the classification of transistors. The operating principle of MOSFETs is about 20 years older than that of the BJT transistor.
An isolated gate field-effect transistor more commonly known as MOSFET (which stands for metal-oxide-semiconductor field-effect transistor). It is a type of field-effect transistor (FET). Like all transistors, the MOSFET modulates the current that passes through it using a signal applied to its electrode. It finds its applications in digital integrated circuits, especially with CMOS technology, as well as in power electronics.
The transistor is characterized by the charge of its majority carriers which determines whether it is of type P or N. MOSFET symbols are used to differentiate its type and category. MOSFETs are based on a metal-insulator-semiconductor structure, i.e. a layer-by-layer structure consisting of an insulated metallic gate electrode, a semiconductor, and the oxide dielectric (i.e., an insulating material) in between. In modern integrated circuits, in the course of technical development, the metallic gate has been replaced by highly doped polysilicon with metal-like electrical properties. Despite the different design, the designation MOSFET has also been largely retained for this variant.
Principle of Operation a MOSFET
A MOSFET is an active component with at least three terminals (electrodes): G (gate), D (drain), S (source). In some designs, an additional connector B (bulk, substrate) is routed to the outside, which is connected to the back of the chip. Since a voltage at the back of the chip generates additional electric fields that act on the channel, changing the voltage at the B terminal shifts the threshold voltage of the MOSFET. Most of the time, however, the substrate is internally connected to the source. Like other field-effect transistors, the MOSFET acts like a voltage-controlled resistor.
Unlike the bipolar transistor, the MOSFET transistor uses only one type of charge carrier, so it is unipolar. The operation is based on the effect of the electric field applied on the metal-oxide-semiconductor structure, i.e. the gate electrode, the insulator (silicon dioxide) and the semiconductor layer, also called substrate. Generally in microelectronics the metal layer is replaced by polycrystalline silicon.
A very useful analogy to easily understand how a FET works, without using electrostatic concepts, is to compare it to a water tap. The grid is the control analogous to the screw thread of the faucet that controls the flow of water (current). After a quarter turn, only a small trickle of water may flow. Then, the current increases rapidly with a small rotation. Finally, despite towers in the void, the current no longer increases, it saturates. Finally, if you want to increase the flow of the tap, you must increase the water pressure (grid-substrate potential difference).
Advantages and Disadvantages
A fundamental disadvantage of MOSFET technology is the low surface mobility of the charge carriers in the channel. However, by reducing the size of the component structures, this disadvantage can be compensated for and the switching speed increases. On the one hand, this makes it possible to produce faster individual transistors, and on the other hand, fine honeycomb structures can also be used to produce fast MOSFETs for large currents. By scaling to the submicron range, the MOSFET can be used for integrated digital applications with clock frequencies above 1 GHz. MOSFETs are particularly suitable for integrated circuits because of their simple manufacturing process (CMOS process) and lateral structure.
Unlike bipolar transistors, these are not controlled by a current flow but by a control voltage. In static operation, i.e. at a constant gate voltage, virtually no current flows through the gate. However, a considerable charge and discharge current is sometimes required to recharge the gate capacity. These currents, together with the gate leakage currents, which are no longer negligible in today’s microprocessors, cause the high power consumption of modern integrated circuits.
In power applications, the power MOSFET is superior to bipolar transistors in terms of short switching times and low switching losses. However, it does not reach their high reverse voltages. Compared to bipolar technology, the drain-source section of the MOSFET has a pure resistance characteristic that determines the static voltage drop and the static power dissipation during operation. This is the only way to achieve the high efficiencies of power electronic circuits, especially at low voltages and battery operation.