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Semiconductor technologies for power management (Part 1)

Posted: 13 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.

Power management is generally accomplished by a combination of small signal transistors acting as the brain, power transistors acting as solid state switches that control the power flow from the source to the load, and passive components like resistors, capacitors and inductors, acting as sensing and energy storing elements. A semiconductor IC can incorporate on a single die a large number of small signal transistors as well as limited values of passive components (resistors capacitors and lately even inductors) and power transistors carrying a few Amperes. For larger levels of power, external discrete transistors built with specialized processes are utilized in conjunction with the IC. In this chapter, we will see how ICs and discrete transistors require very different methods of fabrication. We will first discuss the ICs typically incorporating the desired power management control algorithm and the process and package technologies utilized for their construction. Subsequently, we will discuss the discrete power transistors, called to duty when the power levels cannot be handled monolithically by the integrated circuit, and the process and package technologies utilized for their construction.

ICs power technology: processing and packaging
The power of the IC process consists in its ability to etch a high number of electrical components on a small silicon die and interconnect them to perform the desired actuation function. The main electrical components on board an IC are:

? Bipolar NPN transistors
? Bipolar PNP transistors
? Diodes
? CMOS transistors
? DMOS transistors
? Resistors
? Capacitors

The electrical properties of some of these components are discussed in other chapters. In this section we will illustrate the physical structure of these components as they are generated on the surface of a silicon die.

Diodes and Bipolar transistors
Semiconductor crystals derive their amplification properties from bringing together materials of opposite electrical properties, namely N-type and P-type materials. N-type materials are materials that, even if neutrally charged, have an excess of free electrons, or negative charges. In other words these electrons are very weakly tied to their nucleus and hence easy to move around in the form of an electric current. In homogeneous materials atoms bond together by sharing their outer shell electrons: a kind of holding hands by sharing one electron with a neighbor atom. In the case of silicon (column IV of the Periodic Table of Elements) each atom shares its four outer shell electrons with four neighbor atoms.

If we now introduce inside silicon one atom from column V of the Periodic Table of Elements, namely one having five outer shell electrons, this atom will bond with four neighboring silicon atoms but will have an excess of one electron un-bonded or free to move around. As this electron moves around, the foreign atom is left with a positively charged nucleus. Notice that the entire compound is still electrically balanced but the only difference now is that we have an electron that is much easier to move around. Column V elements like phosporus (P), arsenic (As) and antimony (Sb) are called donor materials because they produce an excess of electrons inside column IV materials like silicon. Similarly if we introduce inside silicon one atom from column III of the Periodic Table of Elements, namely one having three outer shell electrons, this atom will bond with three neighboring silicon atoms but the fourth silicon neighbor will not get an electron. A positively charged 'hole' is created, namely an incomplete bond between two atoms made of one single electron instead of two. Eventually due to thermal agitation this hole will get filled by an electron. This means that the foreign atom has now an extra electron and is left negatively charged, while somewhere out there a silicon atom is missing an electron and is hence positively charged. In other words, the hole is moving freely around the silicon lattice. Column III elements like boron (B), gallium (Ga), indium (In), and aluminum (Al) are called acceptor materials because they readily accept an electron from a nearby silicon-silicon bond creating an excess of holes inside silicon.

A material doped with donors, meaning that it has an excess of negatively charged free electrons, is referred to as an N-type material, while one doped with acceptors, meaning that it has an excess of positively charged holes, is referred to as a P-type material. An N- and a P-type material brought together will form a junction. The simplest semiconductor element, the rectifying diode (Fig. 1), is formed by such a junction between a P- and an N-type material. A positive potential applied to the P side will push the excess of holes toward the junction where they will recombine with excess electrons in the N-type material, sustaining a current flow in this "forward" direction.

Most of the current in the P region is made by the movement of holes, while most of the current in the N region is created by moving electrons. This device is called bipolar, referring to a conduction mechanism based both on electrons and holes. If a negative potential is applied to the P material, and a positive one is applied to the N material, the charges are pushed away from the junction, resulting in zero conduction. The property of desing current only in one direction is the rectifying effect of a diode.

Diode in conduction mode

Figure 1: Diode in conduction mode

Figure 1 illustrates the diode conduction mode, in which a forward bias voltage V pushed a current I through the diode. Notice that the physical current in the wire is made of electrons (represented by negative circles) moving in the opposite direction of the conventionally positive current. Inside the diode the current is made of electrons in the N material and holes (positive circles) inside the P materials. The P-to-metal contact (anode) provides a mechanism for exchanging holes in the semiconductor for electrons which can travel in the external circuit.

A diode is a two terminal device, which, in conduction mode, yields from the cathode (N-side) the same amount of current injected from the anode (P-side). A diode is a passive device lacking the ability to amplify, or, modulate such flow of current.

Amplification requires a third terminal with the ability to modulate the current flow.

If we add a P to the N side of our PN junction, we create a PNP structure. The PNP structure is a three terminal device with two junctions, the PN junction, or emitter-base junction, normally positively biased, and the NP junction, or base collector junction, normally negatively biased. If the intermediate N layer (base) is thin enough and the base-emitter junction is forward biased, a positive charge injected from the emitter can reach the collector without significant recombination in the base. While the charge moves from one side (emitter) to the other (collector), its amount is determined by the magnitude of the positive potential Vbe applied to the forward-biased base-emitter junction (Fig. 2).

A small voltage variation in this junction produces a large current variation in the collector. On the other hand, the thin base assures little charge recombination in the base, namely a small current flow in the base need be supplied in order to sustain a large current flow from the emitter to the collector. Typically a 1uA current in the base can sustain a 100uA current flow from emitter to collector, resulting in a gain of 100 from input (base) to output. This is the amplifying effect in a PNP transistor. A PNP transistor moves charges from a positive potential to a grounded (zero potential) load; this is referred to as current sourcing. If the load is at a positive potential then the dual of the PNP, the NPN transistor (Fig. 3), will be able to move charges from the positively biased load to ground. As for the diode, the PNP transistor (or its dual, the NPN transistor) is a bipolar device because its conduction mechanism is based on both electrons and holes. For example in the PNP transistor, the bulk of the current flow is made of holes, the majority carriers in emitter and collector but minority carriers in the base.

In the base a small percent (let's say 0.5 percent) of holes recombines with electrons, which are continuously supplied as base current. The base current also sustains a small electrons current flowing from the base to the emitter (let's say another 0.5 percent of the collector current). As explained earlier, a total base current typically of 1 percent of the collector current is necessary to sustain the transistor conduction state.

PNP transistor in conduction mode

Figure 2: PNP transistor in conduction mode

NPN transistor in conduction mode

Figure 3: NPN transistor in conduction mode

Fig.3 shows the NPN transistor in principle. In reality semiconductor integrated circuits are built in a planar fashion, meaning all the components and their terminal mast will be etched via a lithographic process on the surface of a wafer. Fig. 4 shows a realistic construction of an NPN transistor in a modern BCD (Bipolar-CMOS-DMOS) integrated circuit process. Starting with a substrate P+ material offering mechanical support, a layer of lightly doped silicon material is grown (P- EPI for epitaxial or superficial growth). This layer is then doped with donor and acceptor materials, according to the rules explained above, to produce a device that is both electrically viable and topologically accessible. The Emitter (Emitter) and Base (PWELL) diffusions are clearly marked in Fig. 4.

The current flow descends vertically from the emitter through the base into the collector N material. The collector material is a composite of lightly doped N material (HVNWELL) that determines the voltage breakdown characteristics of the device, followed by a heavily doped N material (NBL for N buried layer) which offers a low resistance horizontal path to the collector current. Finally, the stack of N materials SINK (for sinker), NWELL and N+ complete the path in the vertical direction, allowing the current to resurface at the collector (Collector) contact. Finally a protection layer (top layer) is deposed on top of the entire die to prevent contamination.

NPN Transistor construction

Figure 4: NPN Transistor construction (Click to view full image)

The PNP transistor is illustrated in Fig. 5. The bulk of the current flow is horizontal from Emitter to Collector and the buried sequence of N materials here is utilized to provide a path for the base current to resurface back to Base contact.

PNP transistor construction

Figure 5: PNP transistor construction (Click to view full image)

This tutorial is based on Chapter 1 of "Managing Power Electronics: VLSI and DSP-Driven Computer Systems," by Reno Rossetti, to be published in 2006 by John Wiley and Sons Inc.

Read Part 2




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