Field-effect transistors (FETs) are broadly classified into two categories: junction field-effect transistors (JFETs) and insulated-gate field-effect transistors (MOS transistors).
Based on channel material and insulated-gate type, they are further divided into N-channel and P-channel types. Based on conduction mode, they are classified into depletion-mode and enhancement-mode. JFETs are all depletion-mode, while MOS transistors can be either depletion-mode or enhancement-mode.
FETs can be classified into junction field-effect transistors (JFETs) and MOS field-effect transistors (MOS transistors). MOS transistors are further divided into four main categories: N-channel depletion-mode and P-channel depletion-mode.
Junction Field-Effect Transistors (JFETs)
1. Classification of Junction Field-Effect Transistors: JFETs have two structural forms: N-channel JFETs and P-channel JFETs.
JFETs also have three electrodes: gate, drain, and source.
The arrow direction of the gate in the circuit symbol can be interpreted as the forward conduction direction of the two PN junctions.
2. Working principle of a junction field-effect transistor (JFET) (taking an N-channel JFET as an example): The structure and symbol of an N-channel JFET. Since the carriers in the PN junction are depleted, the PN junction is essentially non-conductive, forming the so-called depletion region. When the drain voltage ED is constant, the more negative the gate voltage, the thicker the depletion region formed at the PN junction interface, resulting in a narrower channel between the drain and source, and a smaller drain current ID. Conversely, if the gate voltage is less negative, the channel widens, and ID increases. Therefore, the gate voltage EG can control the change in drain current ID; that is, the JFET is a voltage-controlled element.
Insulated-Gate Field-Effect Transistor (MOSFET)
1. Classification of Insulated-Gate Field-Effect Transistors (MOSFETs): Insulated-gate field-effect transistors also have two structural forms: N-channel and P-channel. Regardless of the channel, they are further divided into enhancement-mode and depletion-mode types.
2. It is composed of metal, oxide, and semiconductor, hence it is also called a metal-oxide-semiconductor field-effect transistor, or simply a MOSFET.
3. Working Principle of Insulated Gate Field-Effect Transistors (using N-channel enhancement-mode MOSFETs as an example): It utilizes the gate voltage (UGS) to control the amount of induced charge, thereby altering the state of the conductive channel formed by these induced charges and ultimately controlling the drain current. During transistor manufacturing, a large number of positive ions are introduced into the insulating layer, inducing a significant amount of negative charge on the other side of the interface. These negative charges connect the highly doped N-region, forming a conductive channel, resulting in a large drain current ID even when VGS=0. When the gate voltage changes, the amount of induced charge in the channel also changes, and the width of the conductive channel changes accordingly. Therefore, the drain current ID changes with the gate voltage.
There are two operating modes for MOSFETs: those with a large drain current when the gate voltage is zero are called depletion-mode; those with zero drain current when the gate voltage is zero, requiring a certain gate voltage to generate drain current, are called enhancement-mode.
