Bipolar devices collector-base junction

Bipolar transistors are realized using either an npn- or pnp-junction sequence. The different segments of the device are named as collector, base, and emitter electrode, respectively. In order to operate the transistor, one of the junctions is forward biased, while the other is biased in reverse. Using a small control current over the base electrode, a significant current between the collector and emitter electrodes is enabled. 

To achieve the lowest possible delay a bipolar switching transistor developed by IBM minimizes parasitic resistances and capacitances. It consists of self-aligned emitter and base contacts, a thin intrinsic base with an optimized collector doping profile, and deep-trench isolation (36). Devices must be isolated from each other to prevent unwanted interactions in integrated circuits. While p—n junctions can be used for isolation, IBM s approach etches deep trenches in the siUcon wafer which are filled with Si02 to provide electrical insulation. 

Both bipolar junction transistors (BJTs) and field-effect transistors (FETs) are charge-control devices [15]. The functions of the emitter, base, and collector electrodes of the BJT are replaced by the source, drain, and gate... 

In Figure 5-la is shown a schematic representation of a silicon MOSFET (metal-oxide-semiconductor field effect transistor). The MOSFET is the basic component of silicon-CMOS (complimentary metal-oxide-semiconductor) circuits which, in turn, form the basis for logic circuits, such as those used in the CPU (central processing unit) of a modern personal computer [5]. It can be seen that the MOSFET is isolated from adjacent devices by a reverse-biased junction (p -channel stop) and a thick oxide layer. The gate, source and drain contact are electrically isolated from each other by a thin insulating oxide. A similar scheme is used for the isolation of the collector from both the base and the emitter in bipolar transistor devices [6],... 

CMOS devices can develop a serious problem called latchup, in which junctions in different devices connect and form a forward-biased diode structure, leading to a catastrophic current which destroys the circuit. As illustrated in Fig. 14.5a, the latchnp is caused by the formation of a pnpn device between the terminal of VSS and VDD (see Chap. 9, Sect. 1.3). In a latchup condition, the pnpn device is biased snch that the collector current of the pnp bipolar transistor supplies a base current to the npn bipolar transistor in a positive feedback situation. The latchup can cause device function failure or even self-bumout. Figure 14.5b shows the bipolar components and resistive components of a latchup configuration. The conduction state of a pnp device requires Vq, and the conduction state... 

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