2108 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 52, NO. 9, SEPTEMBER 2005 Small-Signal Characterization of SiGe-HBT -Doubler up to 120 GHz Mojtaba Joodaki Abstract—In this brief, small-signal characterizations of selectively im- planted collector (SIC) and non-SIC SiGe-heterojunction bipolar transis- tors (HBT) -doublers up to 120 GHz are measured, analyzed, and com- pared with those of the corresponding single devices. The measured results confirm a great improvement in transit frequency with almost no consid- erable decrease in the maximum stable gain/maximum available gain, and maximumoscillationfrequency.Sincethe -doublercanbeusedeasilyin- stead of a single transistor, a much higher transit frequency and RF power can be achieved while using a technology with a moderate . Index Terms— -doubler, S-parameter measurements, SiGe-HBT, transit frequency. I. INTRODUCTION Although the RF SiGe-HBTs are mature technologies resulting in many opportunities in milli- and submillimeter-wave frequencies, there are still many microwave and optoelectronics applications re- quiring better electrical parameters such as higher breakdown voltages, higher available power, higher early voltage and a higher (transit frequency). To improve the , an -multiplier can be used [1]. Fig. 1 illustrates a single bipolar transistor and an -multiplier con- figuration. Although the -multiplier concept is already introduced and implemented for frequencies lower than a few gigahertz [1], still they have not been extensively analyzed for higher frequencies. To our knowledge, the -multiplier using SiGe-HBTs yet has not been investigated and the influence of this circuit configuration, which is used instead of a single device, on different device parameters and characteristics is not available. In this brief, for the first time, dc and small-signal characteristics of an -doubler are presented, discussed, and compared with a single device for a selectively implanted collector (SIC) and non-SIC SiGe-HBTs. The main advantage of the -multi- plier is that it can be used as a single device for RF circuit design with a higher RF power and transit-frequency, which are required in many applications such as radar and optical receiver. These achievements result from reduction of the collector-base and input capacitances per collector current density and the thermal resistance. Higher collector voltage in the -multiplier is responsible for reduction of the col- lector-base capacitance and the current-mirror configuration of the diode and output transistors keeps the current density equal to that of the single transistor for each transistor in the -multiplier. From the Fig. 1(b) it can be seen that the total input capacitance of the -multiplier is equal to serial combination of the input capacitance of the input transistor and input capacitance of all output transistors. The input capacitance of a single transistor is known as [2] (1) in which is the depletion capacitance of the emitter–base junction, is the collector bias current, is the emitter junction nonideal factor, Manuscript received May 20, 2005. The review of this brief was arranged by Editor J. N. Burghartz. The author is with ATMEL Germany GmbH, Heilbronn 74025, Germany (e-mail: Mojtaba.Joodaki@infineon.com). Digital Object Identifier 10.1109/TED.2005.854289 Fig. 1. Schematic diagrams. (a) Single transistor. (b) -doubler. and is the forward transit times. The equivalent input capacitor of the -doubler can be approximated by (2) where the and are the input capacitors of the input and output transistors, is the diode capacitor, is the collector junction capacitor of the output transistor, is the current gain of the input transistor, transconductance of the output transistor, and is the load which is 50 . Since in our transistors , the can be approximated by . From our measurements it is understood that the input capacitor of the input transistor in -doubler configuration is larger than that when is used as a single device, which the could be responsible for it. In -multiplier the power dissipated (heat source) is divided among input and output transistors, which results in a lower thermal resistance and higher dc and RF powers. DC and small-signal characterizations for the -doublers for both and non-SIC transistors are presented and compared with those of the corresponding single transistors in Sections II and III, respectively. All transistors are fabricated using the SiGe-2RF technology developed at ATMEL Germany GmbH and they have the same emitter area of 10 0.5 m. The only difference between SIC and non-SIC transistor is the SIC implantation. II. DC CHARACTERISTICS The dc measurements are performed for single transistor and -doubler with and without SIC. Fig. 2 illustrates the Gummel-plots for the single transistors and the -doublers. Collector voltage was 1.5 V for the single transistors and 2.5 V for the -doublers. Output characteristics of the single transistors and the -doublers are presented in Fig. 3. Although there is a collector voltage shift of 0.75 V for -doublers, the variation domain of the col- lector voltage remains the same while the collector current is doubled for a similar base current. III. SMALL-SIGNAL CHARACTERISTICS S-parameter measurements are performed for single and -dou- blers up to 120 GHz. First of all the network analyzer is calibrated using the LMR standard de-embedding structures and then the effects of the parasitic elements are subtracted according the enhanced three-step de-embedding approach in [3]. Fig. 4 depicts small-signal common–emitter current gain against frequency for all variants. It can be seen from the measured results, in spite of the single devices, 0018-9383/$20.00 © 2005 IEEE