Transistors

Transistors can be devided into the categories: Bipolar, Unipolar (MOSFET) and Insulated-Gate Bipolar Transistors.

Bipolar Transistors

Bipolar transistors, so called Bipolar Junction Transistors (BJT) has three terminals connected to three doped sections. In an NPN-transistor, a thin P-doped conductor connects a heavily N-doped emitter and a heavily N-doped collector. In a PNP-transistor, a slightly N-doped conductor connects two heavily P-doped sections. Bipolar transistors are controlled by current. This means that the current entering the base terminal is amplified with a factor and allows a current to flow from the collector to the emitter. Bipolar transistors are cheap and are used in amplifiers and oscillators. They switch slower than MOSFET transistors.

MOSFET

Metal-Oxide Semiconductor-Field-Effect transistors, so called MOSFET’s, are very common in modern electronic equipment’s. Six MOSFET transistors are used to drive a brushless DC motor. To switch on and off a simple load, only one transistor is required. The MOSFET as a switch is typically used up to 300 V. If the voltage becomes higher, an IGBT can be considered. In vehicles with 12 V or 24 V batteries MOSFETs are used to turn lamps, motors and actuators on and off. ICs capable of delivering high current in the switching moment are used when the MOSFET switching needs to be fast. At slower switching, the MOSFETs can be driven in different ways if the voltage is correct.

In a MOSFET, the characteristics of semiconductor material is used to control a current with a voltage. The three terminals of the transistor are called gate, drain and source. The current channel goes from drain to source, while the gate controls the channel. The channel can be either N-doped or P-doped. Due to better performance, N-channel transistors are most common. A positive voltage on the oxide isolated gate terminal in an N-channel MOSFET injects electrons into the channel. A MOSFET has very high resistance on the gate terminal and it can be damaged by electrical discharge if it is not protected. The most common MOSFETs today are “enhancement mode”, which means that an electrical charge has to be added so that the transistor starts to conduct. In a depletion mode MOSFET, the transistor is normally conducting, but it stops to conduct when the charge is removed. Depletion mode MOSFETs are used in monitoring circuits and alarms.

IGBT

The Insulated Gate Bipolar Transistor (IGBT) is a power transistor with an isolated MOS-control that has been around since the 80’s. The IGBT is controlled with lower currents than the thyristor but cannot handle the same high current or power. An N-channel IGBT is basically an N-channel MOSFET on a P-doped substrate. A positive voltage over emitter and gate makes the electrons move to the gate. When the gate-emitter voltage reaches a voltage threshold, electrons are moving to the gate so that an channel opens and make electronics flow from collector to emitter. When the transistor conducts, there are electrical charges of electrons and holes in the channel. These charges need to be removed before transistor can be turn off. Therefor a tail current is created that makes the IGBT slower to switch than the MOSFET.

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