Monolithic Integrated Circuits

In a monolithicIC all the circuit components are manufactured into or on top of a substrate of silicon 200-300 um thick. Chips for monolithic ICs usually measure from 1.5 x 1.5 to 10 x 10 mm. As compared with film and hybrid ICs, monolithic chips have the highest packaging density and the best reliability.

Fig. 8.3. Element isolation by a p-n junction

Bipolar transistors can be manufac­tured by planar-epitaxial technology. The collector, base and emitter regions are formed in a silicon substrate by diffusion as shown in Fig. 8.4.

Fig. 8.4. Bipolar transistor of a monolithic IC

Typical bipolar transistors for monolithic ICs have a common-base (beta) current gain of 200, a cutoff frequency of up to 500 MHz, a collector capacitance of not over 0.5 pF, a collector breakdown voltage of 50 V, and an emitter breakdown voltage of 8 V.

Multiemitter transistors employ in digital ICs (Fig. 8.5). It can be turned on by applying a forward voltage pulse to any of the four emitters.

Fig. 8.5. Multiemitter transistor

The superbeta transistor have the base a mere 0.2-0.3 um wide so that the beta current gain may be increased by a factor of several thousand. However, the breakdown voltage of a supergain transistor is brought down to as low as 1.5-2 V.

Compound transistors used in ICs are each a pair of transistors so connected that a high-beta device results. Most often, use is made of what is known as the Darlington pair(Fig. 8.6). The resultant beta current gain of the compound transistor:

. (8.1)

When = 100, we have = 10⁴. In prac­tical compound transistors, may be as high as several thousand.

Fig. 8.6. Circuit of a compound transistor

The Schottky transistor is essentially a bipolar transistor in which a Schottky diode is used as a clamp between the base and collector. The device is advantageous in that it is free from diffusion capacitance and can theretore operate ai irequencies as high as 3-15 GHz. Figure 8.7 shows the structure of the Schottky transistor in an IC and its circuit connection.

Fig. 8.7. Schottky barrier transistor

Integrated MOSFETs are ousting bipolar transistors in ICs on an ever increasing scale. Enhancement-type MOSFETs are especially simple to manufacture.

Fig. 8.8. Enhancement-type MOS-transistor of a monolithic IC: with n-channel

Figure 8.9 shows a partial sectional view of a monolithic IC corresponding to the circuit diagram of Fig. 8.2, that is, consisting of a diffused capacitor, an integrated transistor, and an integrated resistor.

Fig. 8.9. Monolithic IC

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