743

Low-voltage low-power K-band balanced RTD-based

MMIC VCO

Sunkyu Choi and Kyounghoon Yang

Department of EECS, Korea Advanced Institute of Science and Technology (KAIST),373-1, Guseong-dong, Yuseong-Gu, Daejeon, Republic of Korea (e-mail:

Abstract This paper presents a K-band balanced RTD-basedMMIC VCO with extremely low power consumption. In order toreduce the power consumption, an InP-based RTD is used formicrowave power generation. The K-band VCO generates an RFoutput power of -17.7 dBm at a DC power of 1.0 mW. A widetuning range of 2.1 GHz is obtained by using the base-collectorjunction of the integrated HBT as a varactor. In addition, a lowphase noise of -108 dBc/Hz at 1 MHz offset at 22.2 GHz has beenobtained.

Index Terms InP, monolithic microwave integrated circuit(MMIC), K-band, resonant tunneling diode (RTD), voltage-controlled oscillator (VCO).

I. INTRODUCTION

In recent years, interests for the K-band frequency range

have grown in the area of low-power MMICs for use inwireless communication and sensor systems. In these systems,MMIC VCOs are critical building blocks for the signal source.

Traditionally, the III-V 3-terminal devices, such as HBT andHEMT, have been used in K-band MMIC VCOs [1], [2]because of their excellent high frequency performance.However, the typical HBT or HEMT based VCOs suffer fromhigh DC power consumption since the conventional MMICVCO topology consumes considerable DC power, making itsapplication very limited for wireless communication andsensor systems requiring milli-Watt (mW) level DC power

consumption. As an alternative candidate for low DC power

consumption VCOs, the resonant tunneling diode (RTD) basedVCO has attracted a great deal of interests recently [3]-[5].The RTD shows a high device cutoff frequency and an

intrinsic negative differential resistance (NDR) at a lowapplied bias voltage, due to the quantum mechanical nature ofoperation [6]. The NDR characteristics at a low applied biasvoltage with the high device cutoff frequency make it possibleto implement high frequency low power MMIC VCOs.

In this paper, we report on fabrication and characterizationof a 1 mW RTD MMIC VCO, which has been designed for theK-band wireless communication/sensor systems. The balancedVCO topology is used to reduce the phase noise and generatethe differential outputs.

DC characteristics of a 2 x 1 pm2 RTDPeak voltage (Vp) 0.31 VPeak Current (IP) 2.3 mA

PVCR 10

AC characteristics of a 2 x 1 pm2 RTD

(Vs 0.34 V, IR =15mA)F (at 22.15 GHz) 6.3 dB

Rs 32 QRN -144 2CN 32 fF

64 GHz

R-R

-RNC CN

(a) (b)Fig. 1. (a) Equivalent small signal model and (b) parameter values ofthe fabricated RTD.

II. DEVICE AND MMIC TECHNOLOGY

The K-band balanced RTD MMIC VCO is fabricated usingthe stacked RTD/HBT layer structure. The RTD, HBT,varactor, inductor and thin film resistor are monolithicallyintegrated. The InP-based pseudomorphic subwell RTD layers[4] are used for the required device characteristics of low peakvoltage and high peak current. The fabricated 2 x 1 gm2 RTDshows a peak voltage (Vp) of 0.31 V, a peak current of 2.3 mA(Ip) and a peak-to-valley current ratio (PVCR) of 10 at roomtemperature. The equivalent circuit model of the fabricatedRTD at a bias voltage of 0.34 V is shown in Fig 1(a). RN is a

dynamic resistance which is varied with the applied biasvoltage and Rs is a parasitic series resistance which isindependent of the applied bias voltage. CN represents thecapacitance of the RTD. The cutoff frequency of the RTD isgiven by [7]

1 RN (1)2zRNCN RS

At the RTD bias voltage of 0.34 V, the values for RN Rs andCN are 144 Q, 32 Q and 32 fF, respectively. The calculated fcis 64 GHz which is sufficient for the K-band application. TheHBT with an emitter size of 0.4 x 4.2 gm2 was used in thebuffer. The HBT shows a maximum cutoff frequency of 90GHz and a maximum oscillation frequency of 65 GHz. Thevaractor was implemented by using the base-collector junction

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of the HBT. The capacitance of the varactor varies from 130to 82 fF with the corresponding quality factor varying from 24to 36 at 22.5 GHz under a tuning voltage (Vtune) range of 0 - -3 V. The spiral inductor was formed by using a goldinterconnection metal. The inductance of the spiral inductor is0.246 nH with the quality factor of 38 at 22.5 GHz.

III. RTD BALANCED VCO DESIGN

The circuit schematics of the single-mode and balanced K-band RTD VCOs are shown in Figs. 2(a) and (b). The single-mode VCO consists of a 2 x 1 gm2 RTD and a parallel L-Cresonator, and an HBT emitter-follower buffer. The RTD isused as the negative resistance cell based on the one-portnegative resistance oscillator topology, and the resonatordetermines the oscillation frequency. The HBT emitterfollower buffer is used to isolate the output 50 ) load formeasurement from the VCO core. Fig. 3 shows the simplifiedequivalent circuit model for the single-mode RTD VCO insteady state. The oscillation frequency is given as~~~~~ ~~~1

jLrCtune + CEq + CN ( 'R +Rp)(2)

In order to realize the proposed balanced VCO [5], the twosingle-mode VCOs are connected by the inductors andvaractors as shown in Fig. 2(b). The ac virtual ground formsat the node between varactors. Hence the differential signalscan be generated [8]. In this topology, the phase noise can bereduced due to its immunity to the common-mode noisegenerated from the power supply and substrate. The RTDgenerates the negative resistance at a very low voltage of 0.34V which reduces the DC power consumption significantlycompared to the conventional-type VCO. In addition, byusing the monolithically integrated base-collector junctionvaractor with a large capacitance ratio, the frequency tuningrange can be increased.

IV. EXPERIMENTAL RESULTS

The fabricated K-band balanced MMIC RTD VCO is shownin Fig. 4. The overall size of the VCO circuit excluding pads isabout 480 x 300 gim2. The MMIC VCO was measured on

wafer to verify the performance by using an HP8764Espectrum analyzer. Fig. 5 shows the measured output power

spectrum at a bias voltage of 0.34 V and a bias current of 2.97mA from the VCO core. The DC power dissipation of 1.0 mWis the lowest value among those of K-band MMIC VCOs tothe authors' knowledge. The output power of the MMIC VCOis -17.7 dBm after compensating the loss of the RF cable. Fig.6 shows the measured oscillation frequency and output powerwith respect to the tuning voltage from 0 V to -3 V. Thefrequency tuning range of 2.07 GHz (9%) from 20.91 to 22.98

(b)

Fig. 2. Circuit schematicbalanced RTD VCO.

of the (a) single-mode RTD VCO (b)

RTD Resonator Buffer

. 4 = 'neL REq

~~ ~ ~ ~ ~ t .CE Eq-R *

Fig. 3. Simplified equivalent circuit model for the single-mode VCO.

GHz is achieved. Fig. 7 shows the measured phase noiseversus the offset frequency. The obtained phase noise is -108dBc/Hz at 1 MHz offset at an oscillation frequency of 22.15GHz. The figure of merit (FOMT) including the frequencytuning range is defined as [9]

FOM =L(fff ) -20log fo FTR +10log( PDC~~foffI10 1mW

(3)

2

744

2x1 pm2RTD U

(a)

VBIAS

RTD2x1 ,jm2RTD

745

NIIO 22.0

," 21.53:r1

21.0U-

Fig. 4. Microphotograph of the fabricated K-band balanced RIDMMIC VCO.

TTELN 1'IRL OdiBm

MB1493GHz

IT---l y

CENTER 22. 1498GHzRBN I .MHz BWlN s ULU0MOm&

Fig. 5. Measured output spectrum of the K-band balanced RTDMMIC VCO at a bias current 2.97 mA and a bias voltage of 0.34 V.

where L(f0ff) is the measured phase noise at the frequencyoffset (f0ff) from the center frequency f0 and PDC is themeasured DC power consumption of the VCO in mW. FTR isthe frequency tuning range in percentage. The obtained FOMTof the MMIC VCO is -194 dBc/Hz which is better than thoseof previously reported K-band voltage controlled oscillators[10], [11]. The measurement results of the fabricated K-bandRTD MMIC VCO are summarized in Table I.

V. CONCLUSION

This paper presents the RTD based K-band balanced VCO.By using the InP RTD as a negative resistance cell, the DCpower dissipation is significantly decreased to 1 mW-level,which is the lowest value among those of K-band MMICVCOs to the authors' knowledge. The tuning range of 2.07GHz is achieved by using the base-collector junction of theHBT as a varactor. The phase noise is -108 dBc/Hz at 1 MHzoffset frequency at an oscillation frequency of 22.15 Hz. As aresult, the fabricated VCO shows an excellent FOMT value of -

194 dBc/Hz which is much better than those of previouslyreported K-band VCOs. This work demonstrates the potentialof RTD technology for a low power and low phase noise K-band

a,

00)0.

0

-3.0 -2.5 -2.0 -1 .5 -1 .0 -0.5 0.0

Vtune (V)

Fig. 6. Measured oscillation frequency and output power versus Vtune.

e dB/SPOT' FRO I 00MHRL -Se d8cBMZ.H tS 00 dBC:/Mz

F F FREUEC OFFSE

Fig. 7. Measured phase noise versus the offset frequency.

TABLE ISummary of performance characteristics for the fabricatedMMIC VCO

Performance K-band RTD based VCOOscillation Freq. 20.91 - 22.98 GHzTuning Freq. 2.07 GHz

Phase noise @ 1MHz -108 dBc/HzOutput power -17.7 dBm

DC power of VCO core 1.0 mWFOMT ofVCO core -194 dBc/Hz

VCO, which is a criticalcommunication and sensor systei

building block in wireless

ACKNOWLEDGEMENT

This work was supported by the National Program for Tera-level Nano-devices of the Ministry of Science and Technologyin the Republic of Korea as one of the 21- Century FrontierPrograms.

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