there is no need to take into account frequency moments, transients, etc. By the way, they are called static because a constant voltage is applied to the gate. We turn immediately to the weekend or stock. Since, in the operating mode, the gate current is usually low or even zero, we will not consider the graphs of the input characteristics of the field-effect transistors. Static characteristics of a field-effect transistor with a control pn junction The arrow here indicates the direction from the p-layer to the n-layer. The essence of his work will not change.Ĭonditional graphic images of field-effect transistors are shown in the figure ( a - with a p-type channel, b - with an n-type channel). It goes without saying that you can make a transistor with an n-type channel and a p-type gate. That is why the pn junction has such an uneven shape, expanding towards the flow area. Įven at zero voltage at the gate, between the gate and the drain, there is a reverse voltage equal to the source-drain voltage. If the magnitude of the reverse voltage becomes so large that the blocking layer closes the channel, the transistor will go into cutoff mode. So, in the operating mode of a field-effect transistor with a control pn junction, the voltage at the gate must be either zero (the channel is fully open) or reverse. By increasing or decreasing the reverse voltage on it, we open / close gateways on it, adjusting the "water supply" (output current). The following analogy can be drawn: the pn junction is a dam blocking the flow of charge carriers from the source to the drain. Thus, the output current of the transistor is regulated by the voltage (electric field) of the gate. Accordingly, if we apply a reverse bias voltage to the transition, then closing it will significantly increase the channel resistance and decrease the current between the source and drain. And since the n-layer is significantly at the same channel, most of the transition region depleted of mobile charge carriers will be in the p-layer. Naturally, a pn junction appears between the gate and the p-region below it (the channel ). On top of this plate there is an area with the opposite type of conductivity, to which the third electrode is connected - the gate. At the opposite ends, it has electrodes, by applying a voltage to which we receive current from the source to the drain. So, how does the first type of field effect transistors work? The device is based on a semiconductor plate with conductivity (for example) of p-type. įield effect transistor with pn junction control And, like their bipolar "brothers", field-effect transistors still play a huge role in electronics.īefore proceeding to the story about the physics of the work of unipolar transistors, I want to recall the links that can refresh your knowledge of the pn junction: one and two. In the 50s, it was first theoretically described, and then the field effect transistor with a control pn junction was implemented. But the level of technology allowed to implement it only in 1960. In general, the idea of the latter appeared in the 20s of the 20th century, long before the invention of bipolar transistors. Therefore, field-effect transistors with a control pn junction and with an isolated gate are distinguished. But it can be implemented in at least two ways. The structure seems simple and very similar to a bipolar transistor device. The three contacts of field-effect transistors are called the source (source of current carriers), the gate (control electrode), and drain (the electrode where the carriers flow). This means that only one type of charge carrier (or electrons, or holes) is involved in the flow of current. But in bipolar transistors, as we remember, the output current is controlled by the input base current.Īnother fact about field effect transistors can be found by drawing attention to their other name, unipolar. The definition not only confirmed our assumptions, but also demonstrated the feature of field-effect transistors - the output current is controlled by changing the applied electric field, i.e. What will official sources tell us?įield-effect transistors are active semiconductor devices, usually with three pins, in which the output current is controlled by an electric field. And thirdly, at the heart of their work lies the pn junction. Secondly, they are supposed to have three contacts. What can be assumed already by their one name? First, since they are transistors, they can somehow control the output current. And now let's talk about field effect transistors.
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