A Field-Effect Transistor (FET) does not have input characteristics similar to Bipolar Junction Transistors (BJTs) because it is voltage-driven rather than current-driven. In FETs, the input is controlled by an electric field created by the voltage applied to the gate terminal, which modulates the conductivity of the channel between the source and drain terminals. This lack of input current results in different behavior compared to BJTs, where input characteristics are defined by the base current and its relationship with the base-emitter voltage.
FETs have high input impedance because the gate terminal is insulated from the channel by a thin oxide layer in MOSFETs or by the reverse-biased p-n junction in JFETs. This insulation means that virtually no current flows into the gate, resulting in very high resistance to the input signal. High input impedance makes FETs ideal for use in amplifiers and other applications where it is important to avoid loading the preceding stage in the circuit.
The characteristics of a Field-Effect Transistor (FET) include high input impedance, which minimizes the loading effect on previous stages of a circuit, and low output impedance, making them effective as buffers. FETs are voltage-controlled devices where the current through the channel between the source and drain is controlled by the voltage applied to the gate. They exhibit a linear region where they can act as amplifiers and a saturation region where they function as switches. FETs also typically have low noise levels and are more thermally stable compared to BJTs.
The input current to a Junction Field-Effect Transistor (JFET) is effectively zero because the gate-source junction is reverse-biased. In this reverse-bias condition, only a very small leakage current, typically in the nanoampere range, flows through the gate. This negligible current results in almost no power being drawn from the input signal, which contributes to the high input impedance of JFETs.
The FET is called a Field-Effect Transistor because its operation is based on the control of the electric field. The voltage applied to the gate terminal creates an electric field that influences the conductivity of the semiconductor channel between the source and drain terminals. This field effect modulates the current flow through the channel, allowing the FET to function as an electronic switch or amplifier. The name highlights the mechanism of action that distinguishes FETs from other types of transistors, such as BJTs, which rely on current control.