Microwave Power Transistors

Most microwave transistors are silicon, planar, epitaxially diffused n-p-n structures with emitter geometries designed to increase the ratio of active to physical area. The two most widely used emitter geometries are the interdigitated geometry and the overlay geometry. In an early interdigitated structure the emitters 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 with 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 in areas where diffusion is required in a base or emitter. With photoetching techniques, the emitter and base strip width and separation can be controlled to one micron.

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: (1) it distributes base current uniformly over all of the separate emitter sites, (2) it reduces distances between emitter and base, and (3) it reduces the base resistance and the contact resistance between the aluminium metallization material. The term "overlay" is derived from the fact that the emitter metallization lies over the base instead of adjacent to it, as in the interdigitated structure. The emitter current is carried in the metal conductors that cross over the base. The actual base and emitter areas beneath the pattern are insulated from one another by a silicon dioxide layer.

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.

The power-output capability of a transistor is determined by current- and voltage-handling capabilities of the device at the frequency range of interest. The current-handling capability of the transistor is limited by its emitter periphery and epitaxial-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.

In general, all RF power transistors have operating voltage restrictions, and only current-handling differentiates power transistors 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. Transistors 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 of a different-resistivity epitaxial layer for different operation voltages is becoming common.

Transistor efficiency is determined with device operating under signal-bias conditions at which the collector-to-base junction is reverse-biased and the emitter-to-base junction is forward-biased partially with the input drive signal. The collector efficiency of a transistor amplifier is defined as the ratio of the RF power output at the frequency of interest to the dc input power. Therefore, high efficiency implies that circuit loss be minimum and the ratio of the transistor output, the parallel equivalent resistance, and its collector load resistance be maximum. Thus, the transistor parameter which limits the collector efficiency is output admittance. The output admittance of a transistor pellet consists of two parts: an equivalent parallel output resistance which approaches l/ω₁C₀ at microwave frequency under small-signal conditions, and an output capacitance C₀. In a common-emitter circuit, C₀ is essentially the output capacitance because the impedance level at the base is low relative to the impedance level at the transistor output. The output capacitance represents effectively the transistor junction capacitance in series with a resistance. If the collector resistivity is increased, the effective output capacitance and the collector-base break-down voltage are both increased. In a power transistor, junction and epitaxial thickness variations cause variations in C₀ with V_cb. Thus, the dynamic output capacitance is a function or voltage swing and power level. It can be shown that the average l/ω₁C₀ under maximum voltage swing is equal to 2 C_cb where C_cb is measured at the voltage value of V_cb. For the first approximation, the large-signal output resistance can be assumed to be inversely proportional to C_cb. Because the ratio of the transistor output resistance to its collector load resistance determines the collector efficiency, a transistor with high output resistance and, hence, low C_cb is essential.

TEXT 11