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1) Most microwave transistors are silicon, planar, epitaxially diffused n-p-n structures with emitter geometrics designed to increase the ratio of active to physical and the overlay geometry. In an early interdigitated structure the emeitters and bases are built like a set of interlocking combs. The emitter and base areas are controlled by masking and diffusion. The oxide deposit, formed wit silicon heated to a high temperature, masks the transistor against either an n- or p- type impurity. This oxide is removed by the usual photoetching techniques I areas where diffusion is required in a base or emitter. With photoetching techniques, the emitter and base atrip width and separation can be controlled to one micron.

2) Overlay structure differs from interdigitated structure in three ways: pattern, composition and metallization. In a modern overlay transistor structure many small, separate emitter sites are used instead of the continuous emitter strip. This arrangement provides a substantial increase in overall emitter periphery without requiring an increase in physical area of the device, and thus raises the device power-frequency capability. As for composition, in addition to the standard base and emitter diffusions, an extra diffused region is made in the base to serve as a conductor grid. This p+ re­gion offers three advantages: it distributes base current uniformly over ail of the separate emitter sites, it reduces distances between emitter and base, and it reduces the base resistance and the contact resistance between the aluminium metallization material. The term «overlay» is derived from the feet that the emitter metallization lies over the base instead of adjacent to it, as in the interdigitated structure. The emit- tercurrent is carried in the metai conductors that cross overthe base. The actual base and emitter areas beneath the pattern are insulated from one another by a silicon diox­ide layer.

3) The design of microwave power transistors has diverged from that of small- signal transistors. The important performance criteria in microwave power-amplifier circuits are power output, power gain, and efficiency. Transistors suitable for power amplification must deliver power efficiently with sufficient gain at the frequency range of interest.

4) The power-output capability of a transistor is determined by current- and voltage-handling capabilities of the device at the frequency range of interest. Die cur­rent-handling capability of the transistor is limited by its emitter periphery and epi- taxial-layer resistivity. The voltage-handling capability of the device is limited by the break-down voltages which are, in turn, limited by the resistivity of the epitaxial layer and by the penetration of the junction.

5) In general, all RF power transistors have operating voltage restrictions, and only current-handling differentiates power trans tstots from small-signal units. At high current level the emitter current of transistor is concentrated at the emitter-base edge; therefore, transistor current-handling can be increased by the use of emitter geome­tries which have high emitter-periphery-to-emitter-area ratios and by the use of im­proved techniques in the growth of collector substrate material. Trans «store for large- signal applications should be designed so that the peak currents do not cause base widening which would limit the current handling of the device. Basewidth widening is severe in transistors in which the collector side of the collector-base junction has a lower carrier concentration and higher resistivity than the base side of the junction. However, the need for low-resistivity material in the collector to handle high currents without base widening severely limits the break-down voltages as discussed previ­ously. As a result, the use ofa different-resistivity epitaxial layer for different opera­tion voltages is becoming common.

4. Выпишите из текста 5 слов, оформленных окончанием "S",и определи­те по грамматическим признакам, какой частью речи являются слова и какую функцию это окончание выполняет.

5. Выпишите из текста 2 предложения, содержащие сказуемое в активном залоге. Подчеркните сказуемое и определите его видовременную форму. Измените предложения таким образом, чтобы сказуемое имело все воз­можные грамматические формы.