In many low power microwave bipolar transistors the high frequency performance is largely controlled by the current-dependent time constant of the emitter base junction. This invalidates the use of Nielsen's expression which was derived for the noise figure of transistors controlled by the base transit time, which is substantially independent of emitter current. Values of noise figure obtained from this equation by substituting a current dependent cut-off frequency ƒα, derived from the transition frequency ƒT, are considerably higher than the measured values. The more recent approximation due to Malaviya and van der Ziel gives better agreement, but is still not entirely consistent with the measurements. A new expression is developed to take into account the emitter time constant, and this gives good agreement with measurements of minimum noise figure up to 3 GHz, and over a wide range of emitter current.

Limitations of Nielsen's and related noise equations applied to microwave bipolar transistors, and a new expression for the frequency and current dependent noise figure

Most previously published methods of measuring transistor base resistance are surveyed and compared. The input impedance circle diagram method is examined in detail and correction factors due to parasitic capacitances are derived. Emitter series resistance is also estimated from this data. A new method of measuring base resistance requiring much less measurement effort is introduced and shown to give good agreement with the circle diagram method. This method is called the phase cancellation method and gives an estimate of base resistance from the common base input impedance at the collector current where its imaginary part is zero. Also an estimate of series emitter resistance is obtained from this measurement and shown to agree well with other methods.

Characterization and measurement of the base and emitter resistances of bipolar transistors

The standard equivalent II network noise treatment of microwave transistors with constant parameters suffers from two defects: 1) There is a considerable input conductance at high frequencies that varies as ω2and has full thermal noise associated with it; it increases the noise figure. 2) There is a considerable correlation between input noise and output noise at high frequencies, it reduces the noise figure, and its effect predominates. Both effects are taken into account in our treatment, and a final expression for F_{\min} - 1 is derived that is easily evaluated. Due to the fact that the two errors partly compensate each other, the older equivalent II network with constant elements is accurate within 20 percent up to the cutoff frequency f T .

Accurate expression for the noise temperature of common emitter microwave transistors

An improved small-signal bipolar microwave transistor utilizing ion-implantation has been developed. The structure comprises oxide isolation, arsenic-doped emitters and an ion-implanted base region. A noise figure of 3·9 dB at 8 GHz has been achieved. The variation of the device parameters with the implanted dose has been investigated and the results are interpreted in terms of a simplified model. Implanted emitters and diffused emitters have been evaluated and found to give similar results.

Low-noise implanted-base microwave transistors

Recent advances in microwave transistor technology, including 1 μm mask resolution, arsenic diffused emitters, and low-resistance contacts compatible with shallow junctions, have resulted in significant improvements in the high-frequency performance of small-signal npn bipolar silicon transistors. These improvements are described and, in particular, the high-frequency performance of arsenic diffused emitter and phosphorus diffused emitter devices are compared in detail in terms of ƒ T , ƒ max , noise figure and available power gain. The effect of collector thickness and doping on ƒ T and ƒ max is discussed and experimental results are compared with computed values. By suitable choice of collector thickness and doping either ƒ T or ƒ max may be optimized: ƒ T = 15 GHz and ƒ max = 26 GHz have been observed. The minimum noise figure is found to be independent of collector thickness and a typical noise figure of 4 dB at 4 GHz is obtained.

Design and performance of small-signal microwave transistors

The theory of noise in transistors is applied to microwave transistors at microwave frequencies and an expression for the noise figure is obtained in terms of easily measurable parameters of the transistor. By making some simplifying assumptions, the expression for the noise figure can be reduced to a simple form containing only one parameter, which is useful for engineering applications. The effects of feedback and parasitics introduced by the encapsulation are taken into account. Friiss' formula is modified in order to take the effects of image frequency response on the measurements into account. A method of measuring gain and noise figure simultaneously is also described. Experimental verification of the theory up to 4 GHz is presented.

S.D. Malaviya

Papers

A simplified approach to noise in microwave transistors