- MOSFET Definition: A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is defined as a key component in electronic circuits, essential for designing integrated circuits.
- Large and Small Signal Models: MOSFET circuits use both large signal (nonlinear) and small signal (linearized) models for analysis.
- Driver Circuits: MOSFET driver circuits are crucial for optimizing the turn-on and turn-off times, preventing overheating and ensuring efficient operation.
- Switching Circuits: MOSFETs are used in switching circuits, with n-channel MOSFETs preferred for their efficiency over p-channel MOSFETs.
- Inverter Circuits: MOSFET inverters, including resistive load n-MOS, active load n-MOS, and CMOS inverters, are fundamental in digital circuit design, each type offering distinct advantages.
A MOSFET, the most commonly used three-terminal device, has revolutionized electronic circuits. Today, designing integrated circuits without MOSFET seems impossible.
These are quite small and their process of manufacturing is very simple. The implementation of both analog and digital circuits integrated circuits is successfully done because of the characteristics of MOSFET, MOSFET circuits can be analyzed in two ways-large signal model small signal model.
The large signal model is nonlinear and used to determine the device currents and voltage values. The small signal model is a linearized version of the large signal model. MOSFET operates in three regions: cut-off, triode, and saturation. When the gate-to-source voltage (VGS) is less than the threshold voltage (Vtn), the device is in the cut-off region. As an amplifier, it operates in the saturation region. As a switch, it operates in the triode or cut-off region.
MOSFET Driver Circuits
For helping MOSFET’s to maximize the turn on and turn off time, the driver circuits are needed. If the MOSFET takes relatively long time going in and out of conduction, then we cannot use the advantage of using MOSFETs. This will cause MOSFET to heat up and device will not function properly. MOSFET drivers can often use bootstrap’s circuit to create voltages to drive the gate to a higher voltage than the MOSFETs supply voltage.
Practically the gate of MOSFET acts like a capacitor to the driver, or the driver can turn on or off MOSFET very rapidly, by charging or discharging the gate respectively.
MOSFET Switching Circuits
MOSFETs operate in three regions: cut-off, triode, and saturation. When in the cut-off or triode region, MOSFETs can function as switches.
MOSFET switching circuits have two main parts: the MOSFET (acting as a transistor) and the on/off control block. When the transistor is on, it passes voltage to a specific load. N-channel MOSFETs are often preferred over p-channel MOSFETs due to their advantages.
In a MOSFETs switching circuit the drain is connected directly to the input voltage and the source is connected to the load. For turning on n-channel MOSFET, the gate to source voltage must be greater than the threshold voltage must be greater than the threshold voltage of the device. For p channel MOSFET the source to gate voltage must be greater than the threshold voltage of the device. MOSFET behaves as a better switch than BJT because the offset voltage does not exist in MOS switches.
MOSFET Inverter Circuits
Inverter circuit is one of the fundamental building blocks in digital circuit design (not to be confused with a power inverter). The inverters can be applied directly to the design of logic gates and other more complex digital circuits. The transfer characteristics of an ideal inverter is shown below.
Early MOS digital circuits were made using p-MOSFET. But with the advancements of microelectronics technology the threshold voltage of MOS can be controlled and an MOS technology becomes dominant, as the majority carries of n-MOS, i.e electrons are twice faster than the holes, the majority carriers of p-MOS, so the inverter circuits also using n-MOS technology until CMOS technology arrived. Here we discuss three types of MOS inverter circuits.
Resistive load n-MOS inverters :
It is the simplest MOSFET inverter circuits, it has a load resistance R and n-MOS transistor connected in series between supply voltage and ground as shown below.
If Vin is less than the threshold voltage of the n- MOS the transistor is off. The capacitor can be changed to supply voltage and the output voltage equals to the supply voltage. When the input is greater than the threshold voltage of the transistor and we get zero voltage at output it’s disadvantages is that it occupies large area IC fabrication.
Active load n MOS inverter:
Here we use n MOS transistors as active load instead of resistor. There are two kinds of transistors in the circuit pull down transistor to pull the output voltage to the lower supply voltage (usually OV) and pull up transistor to pull the output voltage to the upper supply voltage.
In the following circuit, we can see a pull up and pull down n MOSFET. The gate of the pull up is shorted to supply voltage to make it always on.
CMOS inverter:
The CMOS inverter is built using an n MOS – p MOS pair sharing a common gate. P channel transistor is used as pull up transistor and v channel transistor is used as pull down transistor.
When, Vin is less than the threshold of n MOS the n MOS turns off but p MOS turns on. The capacitor thus will be charged to supply voltage and we obtain equals to supply at output.
When, Vin is greater than the threshold of n MOS the n MOS turns on but p MOS turns off. The capacitor thus will be discharged to supply voltage and we obtain voltage equals to zero at output.
The advantages are CMOS inverters circuit dissipates power only during switching event and in the voltage transfer curve we observe sharp transition. But in fabrication extra process steps are required.
