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تفاوت ترانزیستور bjt و fet در چیست؟
http://www.felezjoo.com/viewtopic.php?f=29&t=75		 صفحه 1 از 1
نویسنده:  reza vir [ چهارشنبه 23 اسفند1391 - 10:15am ]
عنوان پست:  تفاوت ترانزیستور bjt و fet در چیست؟
تفاوت ترانزیستور bjt و fet در چیست؟
اگر تفاوت در ساختارشو بخواین كه خیلی چیزا میشه گفت كه تو كتاب های فیزیك الكترونیك باید دنبالش بگردین ولی در مدار عملی چه فرقی دارن من چند مورد رو میگم :
1- برای فركانس های بالاتر bjt بهتر هست
2- برای سرعت بیشتر fet بهتر هست. سرعت سویچینگ بالا داره
3- برای مقاومت ورودی زیاد fet بهتر هست. تقریبا گیتش جریان نمیكشه
4- ولتاژ DS اشباع كمتر هست پس تو كلید ها بهتر جواب میده (استفاده به عنوان كلید)
....
از لحاظ فیزیک الکترونیک :

ترانزیستورهای بی جی تی ، ترانزیستورهای دو قطبی هستند یعنی هم حاملین اقلیت و هم حاملین اکثریت ( الکترون و حفره ) در برقراری جریان نقش دارند. اما در ترانزیستورهای فت (ترانزیستورهای اثر میدان ) عامل ایجاد جریان کانال است .

در دو قطبی ها ما برای کنترل جریان ، به ترانزیستور بار می دهیم ولی در فت ها برای کنترل جریان ولتاز گیت را کنترل می کنیم

BJT vs MOSFET
The transistors BJT and MOSFET are both useful for amplification and switching applications. Yet, they have significantly different characteristics.
BJT, as in Bipolar Junction Transistor, is a semiconductor device that replaced the vacuum tubes of the old days. The contraption is a current-controlled device where the collector or emitter output is a function of the current in the base. Basically, the mode of operation of a BJT transistor is driven by the current at the base. The three terminals of a BJT transistor are called the Emitter, Collector and Base.
A BJT is actually a piece of silicon with three regions. There are two junctions in them where each region is named differently ‘“ the P and N. There two type of BJTs, the NPN transistor and the PNP transistor. The types differ in their charge carriers, wherein, NPN has holes as its primary carrier, while PNP has electrons.
The operation principles of the two BJT transistors, PNP and NPN, are practically identical; the only difference is in biasing, and the polarity of the power supply for each type. Many prefer BJTs for low current applications, like for switching purposes for instance, simply because they’re cheaper.
Metal Oxide Semiconductor Field-Effect Transistor, or simply MOSFET, and sometimes MOS transistor, is a voltage-controlled device. Unlike the BJT, there is no base current present. However, there’s a field produced by a voltage on the gate. This allows a flow of current between the source and the drain. This current flow may be pinched-off, or opened, by the voltage on the gate.
In this transistor, a voltage on an oxide-insulated gate electrode can generate a channel for conduction between the other contacts ‘“ the source and drain. What’s great about MOSFETs is that they handle power more efficiently. MOSFETs, nowadays, are the most common transistor used in digital and analog circuits, replacing the then very popular BJTs.
Summary:
1. BJT is a Bipolar Junction Transistor, while MOSFET is a Metal Oxide Semiconductor Field-Effect Transistor.
2. A BJT has an emitter, collector and base, while a MOSFET has a gate, source and drain.
3. BJTs are preferred for low current applications, while MOSFETs are for high power functions.
4. In digital and analog circuits, MOSFETs are considered to be more commonly used than BJTs these days.
5. The operation of MOSFET depends on the voltage at the oxide-insulated gate electrode, while the operation of BJT is dependent on the current at the base.
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