A direct consequence of MOSFET working leads to their usage as a switch. A n-channel MOSFET shown by Figure 1 can act as a switching circuit when it operates in cut-off and saturation regions. This is because the MOSFET in the figure will be ON when the VGS voltage is positive, which causes the MOSFET to behave like a short circuit. Thus one gets the output voltage almost equal to zero. Further if the VGS is zero, then the MOSFET will be OFF and thus the output voltage will be equal to VDD.
Such MOSFETs are used to perform switching actions in case of basic buck converters used in DC-DC power supplies (Figure 2). Here one MOSFET switch stores the energy into the inductor while, the other releases it into the load, in alternate cycles.
Further the MOSFETs are used even in the case of motor control applications, where the switching action is controlled by either a half-bridge control circuit which uses two MOSFETs or by a full-bridge control circuit using four MOSFETs. Here the movement of DC motors or brushless stepper motors can be controlled by using Power MOSFETs by employing the techniques like Pulse Width Modulation (PWM).
Enhancement n-channel MOSFETs are in their OFF state when no gate-to-source voltage is applied. However when biased with a suitable positive voltage, it starts to conduct allowing the flow of drain current through it. This current is seen to increase in magnitude as the bias voltage increases which in turn leads to the increase in output voltage. Thus the MOSFETs serve as amplifiers as shown by Figure 3. MOSFET amplifiers are used in radio-frequency applications and in sound systems.
The switching action of MOSFETs can be exploited to design chopper circuits as shown by Figure 4. Here the DC voltage, VDC is converted into AC voltage with the same amplitude level, VAC by biasing the MOSFET using a square voltage waveform between its gate and source terminals. This causes the MOSFETs to operate in cut-off and saturation regions in alternate cycles.
Linear Voltage Regulators
Depletion type MOSFETs in source-follower configuration are used in linear voltage regulator circuits as pass transistors (Figure 5). Here the source voltage, VL follows the gate voltage, VG minus the gate-to-source voltage, VGS. Further VGS increases with an increase in the drain current, ID. Thus if the gate voltage is fixed, then the source voltage will reduce as the load current, IL increases.
Apart from these, Depletion mode MOSFETs are used in the design of off-line switch-mode power supplies which operate under the scenario of wide voltage variations. Further they are used in the design of variable frequency drives, voltage-ramp generator circuits and serve as voltage-controlled potentiometers due to their property to behave as voltage-controlled resistors. MOSFETs can also be used as constant current sources and can be used to design current monitoring circuits in conjunction with op-amps. They are used as mixers or oscillators in radio systems and can be used to drive high-current/high voltage networks without drawing current or power from the driving circuit, due to their high impedance which isolates the two parts of the network. Lastly one has to note that the MOSFETs also find their applications in the fields of digital electronics and microprocessors, due to their high input impedance and fast switching speeds.