The amount of current varies with the applied voltage so the device works as an amplifier as well as a switch.įor most general purpose transistor applications we need devices which are non-conducting with zero bias on the control input (base or gate). A positive potential on this conducting gate causes the P-type material just under the insulator to become N-type, joining the drain and source diffusions and allowing a current to flow. Over the strip between the diffusions is a very thin layer of silicon dioxide (or some other insulator) covered with a conducting film (usually aluminum or polycrystalline silicon). The N-channel version consists of a strip of P-type silicon with two N-type diffusions. FETs have very high input resistance (but their input capacitance may be quite large - tens or even hundreds of pF) and are therefore transconductance (I out/ V in) devices. There are two types of FET, junction FETs (JFETs) and Insulated Gate FETs (IGFETs), more commonly, but less accurately, called Metal Oxide Silicon FETs (MOSFETs) which is the name I shall use here, and both come in either polarity (N-channel for positive supply, P-channel for negative). (“Super-beta” transistors with ß up to several thousand are possible, but they have a very narrow base region and low breakdown voltages and are so fragile that they are rarely used except within analog integrated circuits.) For most applications a minimum value of 80-100 is adequate but higher values to a few hundred are not uncommon. It is clear that the greater the value of ß the better the current amplifier. (The ß or h fe is the current gain of the transistor.) The input impedance is neither low nor linear so we can also view a BJT as an I out/ V in (transconductance) amplifier with a silicon diode as its input device. If a current flows from the base to the emitter and a positive bias is present on the collector, a larger current, proportional to the base current, flows in the collector.įigure 2 An NPN Bipolar Junction Transistor (BJT)įrom figure 2 we see that a BJT is a current amplifier - the output current is ß times the input current, and ß may vary slightly with the base current so that the amplifier is not quite linear.

An NPN transistor consists of a thin base of P-type semiconductor sandwiched between two N-type regions, the emitter and the collector. For the rest of the article, except when specifically addressing this issue, we shall use the positive cases (NPN & N-channel) for all our examples.Īlthough FETs had been demonstrated and patented almost twenty years earlier than BJTs 1 the first practical transistors were bipolar 2. This is critically important, but so obvious that little further discussion is needed on the topic. The most basic question of all when choosing a transistor, though, is not whether it's a BJT or an FET but its polarity - in use is its output terminal positive or negative with respect to its common terminal? If the answer is positive we need an NPN BJT or an N-channel FET, otherwise we need a PNP or a P-channel. There are two basic types of transistor - bipolar junction transistors and field-effect transistors, known respectively as BJTs and FETs. But we do need to know what they do and it may be helpful to know a little about why they behave as they do - so we'll talk, just a little, about transistor structures. There are plenty of textbooks which give a good summary of the basics and there are innumerable other books and articles on both basic principles and detailed studies of particular issues. We shall not discuss the physics of transistors here. How do we make a good choice of a transistor without wasting time on unnecessary detail? Which do we choose - and why?įor many applications there is no need to choose a particular transistor - we should just use the first reasonably suitable one that comes to hand. There are tens of thousands, possibly hundreds of thousands, of different types of discrete transistor and there are almost always a few places in a system where a discrete transistor is necessary. The correct question is not “Where do I get that specific device?” but “What other, easily obtained, devices will work in this application?”

One of the common questions the author and his colleagues in the Applications Department are asked is “The application note for XXXX calls for a 3N14159 transistor - where can I get one?” Research reveals that the 3N14159 has been obsolete for years - or is only obtainable (in minimum orders of 1,000,000 pieces) with a lead time of 21 months from a factory in Timbuktu.

Vceo transistor definition