At zero gate voltage, these devices are off. Enhanced mode MOSFETs are active when the gate voltage either exceeds the source voltage (NMOS) or subceeds the source voltage (PMOS).It’s worthwhile to take a brief digression to discuss one additional aspect of differentiating MOSFET operating modes: The manufacturing process was much simpler with single-step doping as opposed to four.ĭespite some initial drawbacks, such as the need for a large negative voltage on the gate in early PMOS manufacturing and slow switching speeds owing to large gate capacitances, the industry would quickly refine production methods to improve efficiency in both areas.

A MOSFET could occupy areas an order of magnitude smaller than that of BJTs.This meant less power consumed and less heat dissipated. Because MOSFETs are voltage-controlled instead of current-controlled, they required no quiescent current on the controlling terminal.PMOS fabrication was far kinder to early MOSFET manufacturing sophistication, and even at this point it offered significant benefits over BJT production: The early dominance of the BJT would give way to the MOSFET, but it was PMOS technology that would first make headway in the 1970s. MOSFETs Bring Producibility and Performance Compared to a p-n junction that is metallurgical (and thus a static material property during operation), the conduction of the MOSFET is field-induced, allowing for a greater level of control as well as reduced power demands. Above the substrate, a polysilicate or metal gate sits atop an oxide layer that provides the control mechanism for a MOSFET: by driving the gate voltage to a particularly high or low threshold, the particular FET style can conduct between the source and drain terminals using its majority charge carrier. Unlike the alternating doped regions of the BJT, the MOSFET uses n- or p-wells (for NMOS and PMOS, respectively) on top of a substrate that is doped to be rich in the charge carrier opposite of the wells. Unfortunately, this at-the-time ease of manufacturing has significant modern drawbacks relative to MOSFETs: difficulty in miniaturization, an inability to isolate minority and majority charge carriers, and greater power consumption.įar and away, the majority of active switching elements in a circuit today are MOSFETs (specifically CMOS, but more on that in a bit) due to their superior performance across a wide range of applications. In contrast to a FET, current flows in a BJT with both electrons and electron holes (the absence of an electron in a space that could be occupied by an electron).Īt the time of its invention, the BJT was a much simpler device to fabricate: it built on diode theory and could effectively be thought of as two back-to-back diodes sharing a p-silicon (positive or hole-rich) doped region for NPN BJTs or an n–silicon (negative or electron-rich) doped region for PNP BJTs. Until the 1970s, the primary active switching device in electronics was the bipolar junction transistor (BJT). NMOS FETs, it is worth describing the underlying shared MOSFET theory.

To better understand the intricacies of PMOS vs.

Yet it is their combined output that has vastly improved power performance in electronics over the past few decades. NMOS FET technology: which is better? At one time, both offered manufacturing advantages over the other. NMOS dichotomy after decades of refinement How CMOS leverages the advantages of PMOS and NMOS for a superior switching device.ĬMOS technology (shown above in an image sensor) marked the end of the PMOS vs. The early manufacturing difficulties of NMOS and its growth to prominence. The beginning of active switching elements and the transition to PMOS.

Vth nmos transistor