What are the differences between BJTs and FETs?
Answer:
*bjt is current controlled device.
fet is voltage controlled device.we can control output of fet by adjusting gate voltage.
*fets have low trans conductance than bjts,
bjts r bipolar,fets r unipolar .
*The field-effect transistor (FET) is a three-terminal device used for a variety of applications that match, to a large extent, those of the BJT transistor described in Chapters 3 and 4. Although there are important differences between the two types of devices, there are also many similarities that will be pointed out in the sections to follow.
*The primary difference between the two types of transistors is the fact that the BJT transistor is a current- controlled device, while the JFET transistor is a voltage-controlled device. In other words, the current IC is a direct function of the level of IB. For the FET the current ID will be a function of the voltage VGS applied to the input circuit .In each case the current of the output circuit is being controlled by a parameter of the input circuit- in one case a current level and in the other an applied voltage.
http://cwx.prenhall.com/bookbind/pubbook...
*Just as there are npn and pnp bipolar transistors, there are n-channel and p-channel field-effect transistors. However, it is important to keep in mind that the BJT transistor is a bipolar device-the prefix bi- revealing that the conduction level is a function of two charge carriers, electrons and holes. The FET is a unipolar device depending solely on either electron (n-channel) or hole (p-channel) conduction.
The term field-effect in the chosen name deserves some explanation. We are all familiar with the ability of a permanent magnet to draw metal filings to the magnet without the need for actual contact. The magnetic field of the magnet has enveloped the filings and attracted them to the magnet through an effort on the part of the magnetic flux lines to be as short as possible. For the FET an electric field is established by the charges present that will control the conduction path of the output circuit without the need for direct contact between the controlling and the controlled quantities.
http://www.cjseymour.plus.com/elec/intrb...
The main difference is that with a BJT there is a (small) base current, where FETs have essentially zero base current (picoamperes are typical, I think). A FET works by applying an electric field to a chunk of doped silicon through a layer of SiO2; this causes a small part of the base to change from P-type (for a P-channel MOSFET) which allows that region to conduct electricity from the N-doped drain to the N-doped source. An NPN-type BJT works by adding electrons to the central P-doped layer, allowing the N-doped ends to connect electrically.
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