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The device landscape has seen huge developments in the last twenty years. Bipolar transistors have been replaced by MOSFETs, where big gains in RDSON and robustness have been made, not to forget the combination of a bipolar transistor with a smaller MOS to drive it monolithically, the so-called IGBT.

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The picture above shows a cross section of a vertical planar MOSFET, as a schematic representation and a cross-section.

A insulated gate bipolar transistor, although containing two elements instead of one, is not much more complex (as shown below):

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The symbol on the right indicates how a bipolar transistor is being driven by a MOSFET, and the left picture shows the vertical structure of the device as it is implemented in silicon.

Since MOSFETs are capable of higher switching frequencies (hence, smaller inductors), the applications that require relatively low current and fast switching operation or linear characteristics of I-V are usually built with MOSFET, whereas higher-power applications that require higher gain as well as high current and moderate switching operation are usually built with IGBTs. These are also easier to scale up in breakdown voltage, with the most common values of 1200V, 1700V and 3500V for higher-power systems - values that are next to impossible for MOSFETs, let alone commercially.

So where are the big differences between these and the ideal switch? First of all, to drive the real switch some power is required. This power has to be provided by the gate driver. As both the MOSFET and the IGBT represent a capacitive load, the power required can be calculated with the gate capacitance, the required drive voltage, and the switching frequency. A bipolar transistor requires a base current that, in the case of an IGBT, is elegantly taken from the high power rail and sunk into the load. Since this power can be quite high, bipolar transistors are being used in switch-mode power supplies in only very few cases these days.

As the gate represents a capacitive load, high peak currents can occur when the gate driver switches. These peak currents are in direct correlation to the switching speed of the main switch, and that can be a good or a bad thing. Usually, fast switching is desired since the device will spend less and less time in the "linear" region (between fully on and fully off), but faster current change dI/dt in the circuit can lead to unwanted side effects, like high peak voltages that can destroy the switches or other components. And, fast switching inevitably creates electromagnetic emission that needs to be filtered away to comply with the regulations.

Another difference between the ideal switch and the MOSFET or IGBT is that the on-resistance of both devices is non-zero, leading to conduction losses. In the case of the IGBT, it is even worse - a more or less constant voltage drop across the device will lead to on-state losses that can be quite high especially at low loads.

The third difference is that parasitic capacitances in the devices store energy and release it exactly when the device is changing state from on to off or vice versa. These losses can be significant, and they cause power dissipation even when there is no load attached.