![]() ![]() Once this voltage exceeds a specific threshold value, electrons will begin flowing easily between source and drain. If the gate voltage is instead positive, an electric field will penetrate through the oxide layer and attract electrons into the silicon layer (often called the inversion layer) directly beneath the gate. Therefore, no current will flow in this configuration-other than unavoidable leakage currents. If there is no voltage (or a negative voltage) upon the gate, the semiconductor material beneath it will contain excess holes, and very few electrons will be able to cross the gap, because one of the two p- n junctions will block their path. The metal gate, usually aluminum, is deposited atop the oxide layer just above the gap between source and drain. Except for the two points at which metal leads contact these regions, the entire semiconductor surface is covered by an insulating oxide layer. In an n-channel MOS (NMOS) transistor, for example, the source and the drain are two n-type regions that have been established in a piece of p-type semiconductor, usually silicon. MOS-type transistorsĪ similar principle applies to metal-oxide- semiconductor (MOS) transistors, but here it is the distance between source and drain that largely determines the operating frequency. Their microns-thick bases permitted transistors to operate above 100 megahertz (100 million cycles per second) for the first time. That is a major reason why there was so much interest in developing diffused-base transistors during the 1950s, as described in the section Silicon transistors. Narrow base layers also promote high-frequency operation of junction transistors: the narrower the base, the higher the operating frequency. ![]() To achieve bipolar operation, it also helps that the base layer be narrow, so that electrons (in n- p- n transistors) and holes (in p- n- p) do not have to travel very far in the presence of their opposite numbers. ![]() Experiments with increasingly pure crystals of silicon and germanium showed that it indeed occurred, making bipolar junction transistors possible. Such bipolar operation was not at all obvious when Shockley first conceived his junction transistor. For this current to flow, some of the electrons would have to survive briefly in the presence of holes in order to reach the second n-layer, they could not all combine with holes in the p-layer. Thus, a weak electrical signal applied to the inner, base layer would modulate the current flowing through the entire device. In the n- p- n junction transistor, for example, electrons would flow from one n-layer through the inner p-layer to the other n-layer. Electric current would flow from one end to the other, with the voltage applied to the inner layer governing how much current rushed by at any given moment. In a 1949 paper Shockley explained the physical principles behind the operation of these junctions and showed how to use them in a three-layer- n- p- n or p- n- p-device that could act as a solid-state amplifier or switch. Shockley recognized that these rectifying characteristics might also be used in making a junction transistor. Brattain invented their point-contact device, Bell Labs physicist William B. Shortly after his colleagues John Bardeen and Walter H. SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!. ![]() Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.Britannica Beyond We’ve created a new place where questions are at the center of learning.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.COVID-19 Portal While this global health crisis continues to evolve, it can be useful to look to past pandemics to better understand how to respond today.Student Portal Britannica is the ultimate student resource for key school subjects like history, government, literature, and more.Demystified Videos In Demystified, Britannica has all the answers to your burning questions.This Time in History In these videos, find out what happened this month (or any month!) in history.#WTFact Videos In #WTFact Britannica shares some of the most bizarre facts we can find.Britannica Classics Check out these retro videos from Encyclopedia Britannica’s archives.Britannica Explains In these videos, Britannica explains a variety of topics and answers frequently asked questions. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |