The bipolar transistor amplifies the small change of the input current and outputs a large change of the output current. The gain of bipolar transistors is defined as the ratio of output to input current (beta). Another kind of transistor, called field effect transistor (FET), converts the change of input voltage into the change of output current. The gain of FET is equal to its transconductance, which is defined as the ratio of the change of output current to the change of input voltage. N channel and P channel are common in the market. The common P channel is low voltage MOS tube.
_FETs affect the current flowing through the transistor by projecting an electric field on an insulating layer. In fact, no current flows through the insulator, so the GATE current of the FET tube is very small. The most common FET uses a thin layer of silicon dioxide as an insulator under the GATE pole. This kind of transistor is called metal oxide semiconductor (MOS) transistor, or metal oxide semiconductor field effect transistor (MOSFET). Because MOS transistors are smaller and more energy-efficient, they have replaced bipolar transistors in many applications.