Electronics - SS2 Physics Past Questions and Answers - page 4

A bipolar junction transistor (BJT) is a three-terminal device consisting of two pn junctions. It operates based on the principle of minority carrier injection. The BJT has three modes of operation: cutoff, active, and saturation. In the cutoff mode, both the base-emitter and base-collector junctions are reverse-biassed, and no current flows through the transistor. In the active mode, the base-emitter junction is forward biassed, allowing current to flow from the emitter to the base. The transistor acts as an amplifier in this mode. In the saturation mode, both junctions are forward biassed, and the transistor acts as a switch.

 

BJTs have various applications, including amplification, switching, and oscillation. In amplification, the BJT is used to amplify weak signals, such as audio or radio signals, to a higher power level. In switching applications, the BJT acts as an electronic switch to control the flow of current through a circuit. BJT-based oscillators generate continuous waveforms used in applications like radio frequency (RF) communication. BJT amplifiers and switches find widespread use in electronic devices, such as audio amplifiers, televisions, computers, and mobile phones.

A field-effect transistor (FET) is a three-terminal device that relies on the electric field control of the conductivity of a semiconductor material. It has three main types: the junction field-effect transistor (JFET), the metal-oxide-semiconductor field-effect transistor (MOSFET), and the insulated-gate bipolar transistor (IGBT). Here, we will focus on the MOSFET.

 

In a MOSFET, the voltage applied to the gate terminal controls the flow of current between the source and drain terminals. The gate voltage creates an electric field that modulates the conductivity of the channel region. The MOSFET operates in two modes: enhancement and depletion. In the enhancement mode, the channel is initially off, and a positive gate voltage is required to turn it on. In the depletion mode, the channel is initially on, and a negative gate voltage is required to turn it off.

 

The FET has several advantages over the BJT. It offers high input impedance, low input current, and low power consumption. The MOSFET, in particular, provides high switching speeds, making it suitable for applications requiring high-frequency operation. It is commonly used in integrated circuits (ICs), digital logic circuits, power amplifiers, switching power supplies, and computer processors.

 

Overall, the FET's ability to provide high input impedance, low power consumption, and high switching speeds makes it a versatile device for various electronic applications, ranging from small-scale integrated circuits to high-power devices.