The circuit of Fig.(b) thus behaves like a two-input NOR gate in positive logic. For all other possible input combinations, the output is in logic ‘0’ state, because, with either Q1 or Q2 nonconducting, the output is nearly −VDD through the conducting Q3. This is possible only when both the inputs are in logic ‘0’ state. ground potential) only when both Q1 and Q2 are conducting. (b), the output goes to logic ‘1’ state (i.e. logic ‘1’).Figure (b) shows a PMOS logic based two-input NOR gate. When the input is at −VDD or near −VDD, Q2 conducts and the output goes to near-zero potential (i.e. logic ‘1’), Q2 remains in cut-off and −VDD appears at the output throughthe conducting Q1. For the circuit shown, GND and −VDD respectively represent a logic ‘1’ and a logic ‘0’ for a positive logic system. MOSFET Q1 acts as an active load for the MOSFET switch Q2. Figure (a) shows an inverter circuit using PMOS logic. The PMOS logic family uses P-channel MOSFETS. The first two are briefly discussed in this section. The competitors for LSI-class digital ICs are the PMOS, the NMOS and the integrated injection logic (I2L). The TTL, the CMOS and the ECL logic families are not suitable for implementing digital ICs that have a large-scale integration (LSI) level of inner circuit complexity and above. The MOS transistor can also be affected by breaks in the thin oxide layer of the gate that may destroy the device.įinally, note that the table above shows the differences in sign and direction of the currents and voltages between NMOS and PMOS transistors.Logic families discussed so far are the ones that are commonly used for implementing discrete logic functions such as logic gates, flip flops, counters, multiplexers, demultiplexers etc., in relatively less complex digital ICs belonging to the small-scale integration (SSI) and medium-scale integration (MSI) level of inner circuit complexities. NMOS FET Breakdown regionĪ MOS transistor can be affected by the avalanche phenomena in the drain and source terminals. In a similar way to the JFET transistors, it can be used to identify, by graphical methods, the bias point of the transistors. In this region, the quadratic relationship between VGS and ID is shown in the left part of the picture. The NMOSFET transistor behaves as a voltage controlled current source VGS. Is a characteristic parameter of the MOS transistor, which depends on the k constant and the size of the transistor gate (width W and length L). The transistor behaves as a nonlinear resistive element, controlled by voltage. We can verify that VGS < VT and the current ID is zero. These regions of operation are briefly described below. The image shows the curves of electrical characteristics of an NMOS transistor with the different regions of operation. In the MOSFET transistors, there are defined the same regions of operation: cutoff, linear, saturation and breakdown. JFET and MOSFET transistors have a very different physical structure, but their analytical equations are very similar. Typical values for this voltage are between 0.5 and 3 volts. If VGS < VT, the drain-source current is zero. This is a characteristic feature of the transistor. The minimum voltage needed to create the inversion layer is called threshold voltage (VT). If a positive voltage is applied to the gate, negative charges are induced (inversion layer) on the substrate surface and they create a conduction path between the Drain and Source terminals. The Gate with W and L dimensions is separated from the substrate by a dielectric (SiO 2), creating a similar structure of the capacitor plates. Normally the Source and the substrate are connected together. The next image shows the N channel MOSFET transistor physical structure with its four terminals: Gate, Drain, Source and Substrate. MOSFET transistors (NMOS) physical structure
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