There are a lot of MOSFET drivers around these days. MOSFET drivers often contain MOSFETs themselves. There are several reasons for needing MOSFET drivers:
Drive current – MOSFETs can have very high gate capacitance. For example, the IRF530NS from International Rectifier is a 90mW device which can withstand 17A continuous drain current at 100V and has 920pF of input capacitance. For lowest ON resistance you would want to drive the gate as high as possible, say 15V and to minimize power dissipation you want to switch quickly between ON and OFF and vice versa otherwise the transistor will spend a relatively long time in the saturation region rather than the linear or ohmic region. The linear/ohmic region can also be called the triode region. Anyone from a bipolar transistor background may find these terms confusing as the saturation region of a bipolar transistor is not synonymous with the saturation region of a MOSFET.
A simple simulation of this schematic:
Gives the following results:
So you can see that to get a fast switching time you need a peak gate current of 1A just to get 17A from the MOSFET. If you limit the gate current to say 100mA by adding 100W in series with the gate, there will be a significant rise and fall time as well as a switch on/off delay. The image below shows a comparison of the drain current with no series gate resistor (yellow trace) and with 100W (blue trace), limiting the gate current to 100mA. The magenta signal is the drive signal seen before the series resistor. You can see the effect of the gate current limitation on the rise and fall times as well as the delay. If you look at the MOSFET power dissipation you will also see that it increases from 28W to 105W with the gate resistor. So, a high gate drive current such as the current from a MOSFET drive IC can save a lot of power and hence heat by speeding up the MOSFET switching.
One of the reasons for the delay in the switch on/off is the charge storage in the channel rather than simply the capacitance of the gate. ON Semiconductor application note AND9083-D.pdf explains gate charge in some detail.
If you want to drive a MOSFET from some logic such as a CPLD or microcontroller then you clearly need something to boost the current as your CPLD won’t drive 1A or even 100mA. Also, while there are a lot of “logic level” MOSFETs now which can be driven from 3.3V or 2.5V logic levels for example, there are still many occasions when you will need to use more voltage. So, you also need the MOSFET driver to increase the drive voltage as well as the current. In the simulation examples given the gate drive used is 15V. For an example of a MOSFET driver, look at something like the Microchip TC1426. That is a dual MOSFET driver that will drive up to 1.2A and level shift up to 16V while interfacing to 5V logic.
High side drivers – Rather than using an NMOS to sink current and PMOS to source current, a common way of driving high current, high voltage loads is with two NMOS transistors. Higher carrier mobility means that NMOS transistors are lower resistance for a given size and gate capacitance than PMOS so are preferred. However, to use them to source a current you need to drive the gate above your supply voltage and operate it as a “source follower”. The MOSFET drain will be connected to the positive supply and the load to the source. An example would be something like the IRS2001 from International Rectifier.
The key trick with these devices is the “bootstrap”. Using the capacitor shown between Vb and Vs (the source of the high side NMOS) and some circuity to the IC it creates a voltage higher than Vs by 10V to 20V and uses this to drive HO – the high side gate pin. This is in addition to providing level shifting from low level logic signals to drive the MOSFET gates. In this particular IC there are two drivers, a high side and low side driver for a “push-pull” type of drive. Also, it has individual input pins so the timing of each transistor can be individually controlled so you can introduce a dead-band if necessary when neither transistor is turned on.