A simple npn or pnp transistor has not much use in circuits. The hybridization of npn/pnp transistor has offered us with BJT- Bipolar junction field effect transistor, Field Effect Transistors (FET), Junction Field Effect Transistors (JFET) , Metal oxide semiconductor field effect transistor (MOSFET). Thus we can have the application of transistor with much greater power handling capacity with reduced loss.
Bipolar Junction Transistors
The base-emitter junction acts like a forwardbiased diode, meaning it must first be given a forward-bias voltage (about 0.7 V for silicon transistors, 0.3 V for germanium) before any appreciable base current
flows at all. Once the base has been elevated to the bias voltage, any further increase in base voltage will start the base current flowing.
Field Effect Transistors (FET)
The field effect transistor (FET) is another three-terminal solid-state amplifying device. FETs perform the same job as the BJTs of the previous section, but because of some advantages are becoming more common in power applications. These advantages include high input impedance, high switching speeds, and less temperature sensitivity. There are two types of FETs, the junction FET (JFET) and the metal-oxide semiconductor FET (MOSFET). Both types have three terminals; the drain (D), the source (S), and the gate (G). Also, FETs are made to be either N-channel or P-channel; the type determines the direction of current through the device.
Junction Field Effect Transistors (JFET)
The simple JFET circuit is shown in Figure. The load current IDS passes through the drain (D) to the source (S). The amount of drain current is controlled by the voltage between the gate and the source (VGS) as contrasted with the BJT, where the collector current is controlled by the base current.
Metal oxide semiconductor field effect transistor (MOSFET)
MOSFETs are particularly important in industry because power MOSFETs, which can handle many amperes, are becoming more popular. In Figure, notice that in the FET symbol the gate is shown as not even touching the rest of the device. This means that the gate is capacitively coupled, causing a very high DC input resistance and allowing for what is called the enhancement mode of operation. In the enhancement mode, all gate voltages are positive.