APPLICATION

The name transistor comes from transfer-resistor because the resistance between the source and drain (emitter and collector) is controlled by the gate (base). The general applications can be digital and analog in nature. In digital applications transistor is used mainly as a three-terminal switch. In analog applications, it used as an amplifier for AC signals. Specific applications are discussed below.

Logic: a digital computer needs logic building blocks such as OR, AN1 NAND, NOT, etc., and a transistor is the primary component for these log blocks. Based on the logic building blocks, other digital circuits such as a flip-flop, shift resists and memory can be formed.

Control: as a switch, a transistor controls the passage of a signal. An example is the multiplexer/demultiplexer circuit which enables multiple channels to be carried via a single transmission line. A transistor also selects the components in an addressable array. As power device, a transistor controls the power delivered to the load from a power source.

Amplifier: a transistor provides gain for an AC signal. More complex circuits include differential amplifiers and operational amplifiers.

Oscillator: an amplifier with proper feedback can result in oscillation, such that a DC input can generate an AC output.

Variable resistor: as an analog switch, a transistor provides variable resistance. Simple examples are variable attenuator and variable phase- shifter.

Microwave applications: being a non-linear device, a transistor can be used in microwave mixing, detection, and modulation. These operations are similar to those on diodes.

Impedance transformation: the source follower and the emitter follower are special amplifiers where the voltage gain is approximately unity. The transistor transforms the output impedance of the signal to a much reduced value.

The performance of a JFET is not competitive compared to a MOSFET, and the JFET is, therefore, much less popular. It finds its applications for materials that do not grow the high-quality dielectric required for an IGFET. In comparison to a MOSFET, which has a metal Schottky gate, it offers some advantages. The built-in potential in the gate junction is higher so that higher input bias in the forward direction can be applied. This is important for enhancement devices. Using MOSFETs, there is difficulty in the formation of complementary circuits (n-channel and p-channel) because of the limitations of Schottky barriers on p-type materials. This can be avoided in a JFET. JFET technology is more compatible with the bipolar transistor and merged bipolar-JFET (BiFET) logic can be used. The high input impedance of the JFET compared to a bipolar transistor can be advantageous. Unlike a bipolar transistor, a JFET inherently has negative temperature coefficient so that thermal runaway is not a problem. The channel length of a JFET, however, is harder to control than that of an IGFET and MOSFET, due to the gate diffusion. This puts a lower limit on the channel length.

The JFET is often used as power transistor due to its robustness. It can also be used as a current limiter when the gate is shorted to the source. This two-terminal device has characteristics with Vq = 0, and is sometimes called a field-effect diode.