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Semiconductor technologies for power management (Part 2)

Posted: 20 Jun 2005 ?? ?Print Version ?Bookmark and Share

Keywords:power management? small signal transistor? transistor? semiconductor ic? bipolar npn transistor?

By Reno Rossetti
Fairchild Semiconductor Inc.

Metal-Oxide-Semiconductor (MOS) transistors
N-MOS and P-MOS transistors are analogous respectively to NPN and PNP transistors but their conduction mechanism is based completely on one type of carrier: holes for the PMOS and electrons for the NMOS. For the PMOS (Figure 6) two P-type regions are separated by one N-type region. Such an N-type region is exposed to a "gate" or plate that can be polarized negatively, attracting the positive charges inside the N-material to the point of forming a conduction channel (enhancement of the channel). Hence the material is enhanced into a P-type for the duration of the applied gate voltage polarization and current can flow between what has become a simple sequence of three P-type materials from the source to the region under the gate to the drain.

The gate plate?originally made of metal in older processes, now typically made of poly-silicon?is isolated from the semiconductor by a thin layer of oxide material, which explains the name MOS (meta-oxide-semiconductor) transistor. In this structure the gate voltage plays the role of the base current in the bipolar transistor, namely sustaining the transistor current flow. However since enhancement in the PMOS is produced electro-statically, meaning, in absence of charge movement, this device has a perfect transfer of current from source (the dual of the emitter in the bipolar transistor) to drain (the dual of the collector in the bipolar transistor). The lack of base current, a net loss in the bipolar transistor, makes these devices valuable in many competing applications.

Figure 7 shows the construction of an N-Channel MOS transistor with the two N diffusions (N) separated by a P material (PWELL) and the gate (Gate) separate from the PWELL by a thin oxide layer. Such a P-type region is exposed to a "gate" or plate that can be polarized positively, attracting the negative charges inside the P-material to the point of forming a conduction channel (enhancement of the channel). Hence the material is enhanced into a N-type for the duration of the applied gate voltage polarization and current can flow between what has become a simple sequence of three N-type materials from the source to the region under the gate to the drain.

PMOS transistor

Figure 6: PMOS transistor

NMOS transistor

Figure 7: NMOS transistor

DMOS transistor
In MOS transistors conductivity rises as the gate length, or separation of Source and Drain, decreases. Making small openings in the oxide to depose a tiny gate requires expensive machinery and sophisticated lithographic processes. In some instances the problem can be circumvented by producing a small effective gate length by subsequent diffusion of two opposite materials in a wide opening. In the DMOS of Figure 8, (DMOS is short for double-diffused NMOS transistor) a small gate length is obtained by following a deep P diffusion (Source PDIFF) with a shallower N diffusion (N). The two materials penetrate with different lengths under the gate area, with the denser P+ traveling further than the lighter N material. Proper dosage and conditions will create an effective gate (the residual PDIFF material not eaten up by the N diffusion) that is much smaller than the drawn gate length. In this structure the current flow proceeds from source, under the gate and horizontally to the Drain contact. Sinker (SINK) and Buried Layer diffusions here have a protection function (anti- latch action).

DMOS N-Channel transistor

Figure 8: DMOS N-Channel transistor

CMOS transistor

When we connect the source of a PMOS transistor to a positive supply, the source of an NMOS transistor to ground, and we short together the respective gates, we obtain the CMOS or complementary MOS transistor, an inverting element that is at the foundation of logic design.

Passive components
In addition to active components (components that can amplify a signal) like transistors, integrated circuits processes also provide a slew of passive components, like resistors, capacitors and lately even inductors. Figure 9 is an example of a resistor, a two terminal device simply obtained by a long and narrow deposition of an N diffusion material.

N+ resistor

Figure 9: N+ resistor

Read Part 1

Read Part 3




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