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Publication VITero Kiuru, Krista Dahlberg, Juha Mallat, Antti V. Räisänen, and Tapani Närhi.2011. Comparison of low-frequency and microwave frequency capacitancedetermination techniques for mm-wave Schottky diodes. In: Proceedings of the6th European Microwave Integrated Circuits Conference (EuMIC 2011).Manchester, UK. 10-11 October 2011. Pages 53-56. ISBN 978-2-87487-023-1. 2011 European Microwave Association (EuMA)Reprinted, with permission, from IEEE.This material is posted here with permission of the IEEE. Such permission ofthe IEEE does not in any way imply IEEE endorsement of any ofAalto University's products or services. Internal or personal use of thismaterial is permitted. However, permission to reprint/republish thismaterial for advertising or promotional purposes or for creating new collectiveworks for resale or redistribution must be obtained from the IEEE by writing topubs-permissions@ieee.org.By choosing to view this document, you agree to all provisions of the copyrightlaws protecting it.

Proceedings of the 6th European Microwave Integrated Circuits ConferenceComparison of Low-Frequency and MicrowaveFrequency Capacitance Determination Techniquesfor Mm-Wave Schottky DiodesTero Kiuru, Krista Dahlberg, Juha Mallat, and Antti V. RäisänenTapani NärhiMilliLab, SMARAD, Department of Radio Science and EngineeringAalto University School of Electrical Engineering, Espoo, Finlandtero.kiuru@aalto.fiRF Payload Systems DivisionEuropean Space AgencyNoordwijk, The Netherlands(C-V) parameters are then solved by fitting the results to anonlinear capacitance equation in a least-squares sense.Abstract—The differences of low-frequency (1.0 MHz) and highfrequency (3 - 10 GHz) capacitance determination techniques arecompared in this work. The low-frequency measurements aredirect capacitance measurements performed with an LCR meterand the capacitance determination at microwave frequencies isdone by extracting the capacitance from S-parametermeasurement results. Several discrete and monolithicallyintegrated Schottky diodes are measured with both techniquesand the differences of the techniques are discussed in the view ofthe obtained results. An evaluation is made on which technique isbetter suited for building a valid capacitance model for aSchottky diode operating at millimeter wave frequencies.In this work the low-frequency measurement techniqueusing an LCR Meter and the capacitance determination fromon-wafer S-parameter measurements are compared and thedifferences of the techniques are discussed based on themeasurement results. It is also shown that in the presence oftrap states of with monolithically integraged diode thetraditional low-frequency determination does not providereliable results. C-V parameters of several mixer diodes,discrete and monolithically integrated, are determined usingboth techniques. The monolithic diodes are fabricated usingUnited Monolithic Semiconductors (UMS) BES process. Thediscrete Schottky diodes are 1MSQ08 from AdvancedCompound Semiconductor Technologies (ACST), Darmstadt,Germany and SC2T6 from Virginia Diodes Inc. (VDI),Virginia, USA. To the authors’ knowledge this is the firstreport of a quantitative comparison of low-frequency andmicrowave frequency capacitance determination techniques formillimeter wave Schottky diodes.Keywords-component; Schottky diode, capacitance-voltage,parameter extractionI.INTRODUCTIONThe capacitance of a Schottky diode is a parameter ofimportance for the high frequency performance of the diode asthe core element of a mixer, detector, or a frequency multiplier.The Schottky diode capacitance can be divided into thejunction capacitance and parasitic capacitance. The junctioncapacitance and the series resistance create an RC circuit,which limits the highest operating frequency of the diode. Alarge nonlinear junction capacitance also makes the matchingmore difficult as the diode impedance becomes dependent onthe power of the input or LO signals [1]. A large parasiticcapacitance in turn impedes a wideband matching of the deviceas a large complex conjugate embedding impedance is difficultto realize over a wide bandwidth.The capacitance of a diode can be determined by a numberof ways. One can use theoretical calculations [2], physicalsimulations [3], 3D electromagnetic modelling [4], [5], lowfrequency measurements with an impedance analyzer [6] orwith an LCR meter [7], [8] or extraction from microwavemeasurements [9], [10]. A low-frequency measurement orextraction from microwave measurements yields a value for thetotal capacitance of the diode. The junction capacitance andparasitic capacitance along with other capacitance-voltage978-2-87487-023-1 2011 EuMAFigure 1. Photographs of a) the test mount for discrete diodes and b) diodeattached on the test mount.II.MEASUREMENTS TECHNIQUES AND C-V PARAMETEREXTRACTIONFor the tests, the discrete diodes are attached on coplanarwaveguide (CPW) test mounts fabricated on 254 µm thick5310-11 October 2011, Manchester, UK

estimate for the total capacitance can be extracted. The Sparameters are measured with an Agilent N5250 PNA. The RFpower level is kept under -25 dBm at all times. The contact tothe CPW pads is done with on-wafer probes. Before themeasurements, the system is calibrated using a CascadeMicrotech line-reflect-reflect-match (LRRM) calibration kit.fused quartz substrate with 3 µm gold-plating. The test mountand a close-up of a mounted discrete diode are shown inFigure 1. The monolithic diodes are embedded in a CPWtransmission line center conductor. The substrate thickness is100 µm. Figure 2. shows a test structure of a monolithicallyintegrated diode with the dimensions.A. Low-Frequency LCR Meter MeasurementsThe low-frequency measurements in this work have beenperformed using Agilent 4284A Precision LCR Meter. Themeasurement principle is a so-called auto-balancing method,which is usually the method of choice in today’s C-Vmeasurements [11]. The measurements are conductedaccording to [11]. The contact to the test structure is done usingphosphor bronze whiskers with a diameter of 25 µm. Themeasurement frequency is 1 MHz and the ac voltage 50 mV(a compromise between a good signal-to-noise ratio and thehighest possible bias voltage). Averaging factor of 16 is used inall measurements and four bias sweeps are conducted for oneconnection of the whiskers.Figure 3. Equivalent circuit for discrete Schottky diode on a test carrier ormonolithic Schottky diode. The equivalent circuit is valid when frequency isnot too high and the current throught the diode is negligible.The frequency range where the extraction is done must fulfilltwo conditions. First, the frequency should be high enough inorder to have the transmission coefficient value that issignificantly above that caused by the leakage resistance.Second, the frequency should be low enough for the reductionof the equivalent circuit of the diode to the circuit in Figure 3 tobe valid. The S-parameters were measured between 0.1 –110 GHz, but the frequency range of 3 – 10 GHz is used for thecapacitance extraction as it fulfills the above criteria.For the discrete diodes the removal of the test mount andwhisker parasitic capacitance is done by measuring an emptytest mount and reducing that capacitance from the capacitancemeasurement of the diode on the test mount. For the monolithicdiodes the calibration is done using three different methods:1. lifting one of the whiskers in air and measuring the openended capacitance, 2. lifting both whiskers and measuring theopen-ended capacitance, and 3. measuring the capacitance of agap which is fabricated on a monolithic test structure (as Fig. 2but without the tapered part). The last method is essentially thesame as used with the discrete diodes.C. C-V Parameter ExtractionIn order to get values for the junction and parasiticcapacitances, the total capacitance values are fitted to thenonlinear capacitance equation [1]CT C j C p C j0 V 1 Vbi 12 Cp ,(1)where Cj0 is the zero bias junction capacitance, V is the appliedvoltage, and Vbi is the built-in voltage. The measurements aredone in the bias voltage range of -3 - 0.5 V but in theextraction, all the measurement points cannot be used formonolithic diodes because above 0.4 V the LCR Meter resultsare unstable as the monolithic diodes start conducting at lowerbias voltages than the discrete diodes.Figure 2. Photograph of the CPW test structure with a monolithicallyintegrated diode (UMS).III.B. Total Capacitance Determination from S-parameterMeasurement ResultsAt lower microwave frequencies and in the bias voltagerange, where the diode is not yet conducting, the equivalentcircuit of the diode can be reduced to pi-network of a seriescapacitor, CT, and small capacitances from pad edges to theground, Cg1 and Cg2. This is shown in Figure 3. The totalcapacitance, CT, is the sum of the junction capacitance, Cj, andthe parasitic capacitance, Cp. The magnitude of thetransmission coefficient is dependent only on the value of thetotal capacitance. By fitting the measured value of thetransmission coefficient to a calculated value, an accurateMEASUREMENT RESULTSThe measured C-V parameters are summarized in Table Ifor four monolithically integrated diodes with different anodewidths and for two discrete diodes. The total capacitancevalues as a function of the bias voltage for one discrete and onemonolithic diode are shown in Figure 4. and Figure 5. It can beseen in Figure 4. that the capacitance curve of a discrete diodecan be similarly determined using either one of the techniques.Extraction gives for the total capacitance of the SC2T6 diode9.9 fF and 10 fF, extracted from the S-parameters andmeasured with LCR Meter, respectively. For comparison, thetotal capacitance given by the manufacturer is 8 – 10 fF. Onlyone type of discrete diode can be measured using the LCR54