![]() The voltage levels of CMOS based logic are somewhat different from TTL, basically instead of the preset levels of Low(.4-.8v) and Hi(2-2.4v) the input logic levels of CMOS are mostly expressed as a ratio of the supple voltage. While this may be true in general, the legacy of TTL logic levels lives on in the form of TTL compatible families, usually denoted by a “T” in the family name. Many of the comments on the video on TTL properties mentioned that TTL is for the most part “mature”, old, and/or obsolete. The table below shows the curve between the newest families and obsolescence. ![]() The addition of protection diodes as shown is pretty standard across the board, though sometimes the diode function is really implemented with on board JFETs. The only way to relieve the shorted condition is to remove power from the device which allows all of the energized transistors to turn off. Another problem caused by excessive voltage is what is called “SCR Latchup”, basically an excessive voltage causes the PNPN junctions produced by layout to act as back to back transistors that cascade into full conduction resulting in a short circuit between power rails. A simple spark or otherwise invisible charge can ruin a MOS based device by punching holes in the gate insulation. the lack of a load resistance to a ground, means that a little bit of static charge on something like the human finger, can actually be disastrous for an unprotected CMOS circuit. Note the alternate way to draw the MOSFETs on the right that is a tad more intuitive as the bubble on the P-Channel indicates that a Low on its Gate will turn it on. Likewise when the Gate is Low, the P-Channel MOSFET is turned on pulling the output High. When the Gate is High the N-Channel MOSFET turns on pulling the output Low. The major modes are Enhancement and Depletion.Īn enhancement MOSFET needs a voltage applied to a gate for the device to turn on, it can be thought of as a normally closed switch as opposed to a depletion mode device which needs a gate voltage applied to turn off and can be thought of as a normally open switch.įET’s come in two different polarities based in part upon the polarity of the Gate signal and how it affects the device: An N-Channel device is activated when a positive voltage is applied to the Gate compared to the Source and a P-Channel activates with a negative voltage.īy combining an N-Channel device and a P-Channel MOSFETs an inverter is implemented. If a good insulator sounds like a dielectric, the makings of a capacitor, I would also say that well yes, it is. FET’s come in two major modes of which there are two different types based on polarity. If that sounds like glass, a really good insulator, I would say well yes it is. This is a real gap created by silicon dioxide, the “Oxide” in MOSFET. The other two pins are the Source and the Drain. Since the Gate is not insulated from the other terminals, known as the Source and Drain, there is a leakage current in JFETs that would not be present if the Gate was insulated from the Source and Drain.Įnter the Insulated Gate FET (IGFET) which is the basis for most of the transistor devices found on large scale integrated chips today. Looking at the diagram, the MOSFETs all show a distinct space between the Gate and the rest of the structure. The Junction Field Effect Transistor (JFET) utilizes voltage instead of current on its Gate input, somewhat like the Base on a Bipolar Transistor, to control the output voltage. As all of these current flows add up it means that at the end of the day there is a lot of current flowing which results in power being dissipated which ultimately results in heat. Regular transistors, known as Bipolar Junction Transistors (BJT) meaning that they are made from junctions that have a positive and a negative (PN) junction utilize current as the input and create gain by controlling output current. Likewise the Very Large Scale Integration (VLSI) designs, or Very Very Large Scale if you like counting the letter V when talking, are possible due to low power dissipation as well as other factors.ĬMOS, which means Complementary Metal Oxide Semiconductor, is based on combining two polarities of MOSFETS Metal Oxide Semiconductor Field Effect Transistors. the gates used in external logic, offer very fast speeds and high current drive capability as well as supporting the low voltages found in modern designs. Nowadays CMOS Small Scale Integration (SSI) logic families, I.E. This really couldn’t happen until the speeds and current drive capabilities of CMOS caught up to the other technologies, but catch up they did. CMOS opened the door for many if not most of the properties needed for today’s highly integrated circuits and low power portable and mobile devices.
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