Bipolar transistors amplify small changes in current at the input and output a large change in current at the output. The gain of a bipolar transistor is defined as the ratio of the output to input current (beta). Another type of transistor, called a field-effect transistor (FET), converts a change in input voltage into a change in output current. The gain of a FET is equal to its transconductance, defined as the ratio of the change in output current to the change in input voltage. Commonly on the market for the general N channel and P channel, depletion type MOS tube (N channel). And P channel common for low voltage MOS tube. 

The mos transistor is a metal-oxide-semiconductor field effect transistor, or metal-insulator-semiconductor. MOS tube source and drain can be switched, they are formed in the P type backgate N type region. In most cases, the two regions are the same, and even if the two ends are reversed, the performance of the device will not be affected. such a device is considered to be symmetrical.