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Solid-State Electronics 51 (2007) 900–904

Effect of base–emitter breakdown on class-C power performanceof common-base InGaP/GaAs HBTs

Subrata Halder a,*, James C.M. Hwang a,1, Jiyoun Lim b,2, Sanghoon Cheon b,2

a Lehigh University, Bethlehem, PA 18015, USAb Knowledge*on Inc., Iksan 570210, Republic of Korea

Received 11 September 2006; received in revised form 25 January 2007; accepted 21 March 2007Available online 23 May 2007

The review of this paper was arranged by Prof. E. Calleja

Abstract

Under class-C common-base configuration, unit-finger InGaP/GaAs HBTs exhibit record power density of 15 mW/lm2 emitter area.However, the power density decreases rapidly with increasing number of emitter fingers. Based on measured dynamic drive and load linesunder nearly isothermal pulse operations, it was concluded that the power density of multi-finger HBTs is limited by base–emitter reversebreakdown. In addition, the base–emitter breakdown voltage is much lower under RF operation than under DC operation. The RFbase–emitter breakdown effect was added to a commercially available large-signal HBT model, which more accurately predicted theclass-C power performance of common-base multi-finger HBTs.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Breakdown; Class-C; RF power; Common-base; Hetero-junction bipolar transistor; Pulsed measurement; Load–pull measurement; Wave-form; Dynamic drive line; Dynamic load line

1. Introduction

Under class-C common-base configuration, we havefound [1] that unit-finger InGaP/GaAs HBTs exhibitrecord power density of 14 mW/lm2 emitter area [1]. Weattributed this record power density to (i) much higheropen-emitter collector–base breakdown voltage (BVCBO)than open-base collector–emitter breakdown voltage(BVCEO), and (ii) much higher Kirk threshold currentunder isothermal operation than under CW operation. Itwould be very desirable to extend such a record power den-sity to multi-finger HBTs with much larger emitter areas.In this paper, we report that, under similar configurations,

0038-1101/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.sse.2007.03.010

* Corresponding author. Tel.: +1 610 758 5103; fax: +1 610 758 4244.E-mail addresses: suh2@lehigh.edu (S. Halder), cheon.sanghoon@

knowledge-on.com (S. Cheon).1 Tel.: +1 610 758 5103; fax: +1 610 758 4244.2 Tel.: +82 42 936 6695; fax: +82 17 410 8049.

isothermal output power of multi-finger HBTs do not scalelinearly with the total emitter area. We attribute the lowerpower density of multi-finger HBTs to base–emitter reversebreakdown. The detailed modeling and characterization ofthe effect of base–emitter breakdown are described in thefollowing.

2. Experimental

Commercially available [2] high-voltage InGaP/GaAsHBTs were used in the present study. Single- and four-fin-ger HBTs were used to illustrate how their performancescale with the number of emitter fingers, with each fingerbeing 2 lm wide and 20 lm long. The GaAs substratethickness is 100 lm. The GaAs dice are attached to Al car-riers by Ag epoxy, resulting in thermal resistances of 910and 470 �C/W for the single- and four-finger HBTs, res-pectively. Otherwise, the HBTs exhibit similar DC and

0

10

20

30

40

0 10 20 30

INPUT POWER (dBm)

OU

TP

UT

PO

WE

R (

dB

m)

Fig. 2. Pulsed output power of (m) single- and (j) four-finger HBTs.

S. Halder et al. / Solid-State Electronics 51 (2007) 900–904 901

small-signal RF characteristics as in the following: Thethermal time constant is of the order of 2 ls. Under DCconditions, BVCEO � 23 V, BVCBO � 35 V, and BVBEO ��7 V. The forward current–gain cutoff frequency fT is35 GHz. The maximum frequency of oscillation fMAX is60 GHz.

Class-C common-base power performance was mea-sured on the HBTs using a pulsed load–pull power andwaveform measurement setup as shown in Fig. 1. Duringthe measurement, the base is grounded through a 50 Xresistor, while the collector bias voltage is pulsed from 0to 14 V. To approach isothermal operation, a pulse widthof 300 ns and a pulse repetition frequency of 2.75 kHzare used. The average collector current IC is monitoredacross a 33 X resistor in series with the collector. Thesource and load impedances are first tuned automaticallyunder CW conditions, and then manually fine-tuned to givethe best pulsed performance. The input power PIN at6 GHz is swept from 9 to 21 dBm. An HP70820A micro-wave transition analyzer (MTA) is used to sample the cur-rent and voltage waveforms at both the input and output.The raw waveforms measured at the MTA are de-embed-ded to the probe tip, after accounting for the tuner loss.The de-embedded waveforms of base current IB, base–emitter voltage VBE, collector current IC, and collector–emitter voltage VCE are then used to construct dynamicdrive lines of instantaneous IB vs. instantaneousVBE anddynamic load lines of instantaneous IC vs. instanta-neousVCE. Four harmonics are usually included in thewaveform measurement. The output power POUT is calcu-lated from the measured fundamental VCE waveform,assuming the MTA’s input impedance is 50 X. Detailedmeasurement procedure can be found in Refs. [3–5].

INCIDENCE COUPLER

SOURCETUNER

50 ΩPROBE

REFLECTION COUPLER

HP 8340B SYNTHESIZED

SWEEPER

HP 8116A PULSE/FUNCTION

GENERATOR

HP 4142B POWERSUPPLY

50 Ω

PULSE TRIG

10MHz REFERE

BIAS T

Fig. 1. Pulsed load–pull power and

3. Output power and waveforms

Fig. 2 shows that, under the above-described test condi-tions, the single-finger HBT gives a maximum POUT of28 dBm (15 mW/lm2), while the four-finger HBT gives30 dBm (6.3 mW/lm2). In neither case the POUT appearsto be saturated. However, further increase in PIN wouldresult in instantaneous device failure. Although the thermalresistance does not scale linearly with the number of fin-gers, the reason that power output does not scale linearlywith the number of fingers cannot be due to thermallydependent effects such as current-collapse [6] and Kirk [7]effects, because the present test conditions approach iso-thermal operation. The reason is apparent only after thefollowing waveform analysis.

50 ΩPROBE

LOADTUNER BIAS T

HP 71500A TRANSITIONANALYZER

SWITCH

GER

NCE

HBT

COLLECTORSWITCHING

CIRCUITOSCILLO-

SCOPE

33 Ω

waveform measurement setup.

-20

20

60

100

140

-2 0 2 4 6 8

EMITTER-BASE VOLTAGE (V)

EM

ITT

ER

CU

RR

EN

T (u

A)

Fig. 4. Inverse-mode (base–emitter junction reverse biased while base–collector junction forward biased) DC I–V characteristics of a four-fingerHBT showing both on-state and off-state base–emitter breakdown. IB =0,160,320, . . .,800 lA bottom up.

0

25a

902 S. Halder et al. / Solid-State Electronics 51 (2007) 900–904

Fig. 3 shows dynamic drive and load lines constructedfrom the measured waveforms on a four-finger HBT. Itcan be seen that, while the peak VCE continues to increasewith PIN, the peak VBE is clamped at �3 V – which is sig-nificantly lower than BVBEO. This indicates that POUT islimited by input breakdown instead of output breakdown.This indicates also the base–emitter breakdown voltage ismuch lower under RF than under DC, probably due tonon-quasi-static effects such as excess mobile charge carri-ers in the base–emitter space-charge region during the neg-ative RF cycle [8–10]. Fig. 4 confirms that the inverse-mode(with the base–emitter junction reversed and the base–col-lector junction forward biased) on-state base–emitterbreakdown voltage is approximately �3 V, while the off-state base–emitter breakdown voltage remains at approxi-mately �7 V. Such an input breakdown is well known forclass-C amplifiers [11], but has not been the focus ofHBT developers. This is because most HBTs are developedfor common-emitter class-AB amplifiers with a forward-biased base–emitter junction.

As mentioned above, the reason input breakdown is amore severe limitation for four-finger HBTs than for sin-

-3 -2 -1 0 1-4 2

-0.1

0.0

0.1

-0.2

0.2

BASE-EMITTER VOLTAGE (V)

RF

BA

SE

CU

RR

EN

T (

A)

10 200 30

0.1

0.2

0.3

0.0

0.4

COLLECTOR EMITTER VOLTAGE (V)

CO

LL

EC

TO

R C

UR

RE

NT

(A

)

Fig. 3. Dynamic (a) drive and (b) load lines of a four-finger HBT under aninput power of 16,17, . . ., 23 dBm at 6 GHz.

-125

-100

-75

-50

-25

-8 -6 -4 -2 0

BASE-EMITTER VOLTAGE (V)

BA

SE

CU

RR

EN

T (

nA

)

-500

-400

-300

-200

-100

0

100

-10 -8 -6 -4 -2 0

BASE-EMITTER VOLTAGE (V)

BA

SE

CU

RR

EN

T (

uA

)

b

Fig. 5. (a) DC base–emitter breakdown characteristics of a single-fingerHBT at (r) 20, (j) 60, and (m) 100 �C. (b) (r) DC vs. (j) pulsed base–emitter breakdown characteristics of a four-finger HBT at 25 �C. Thepulsed characteristics were measured at 0.8 ls after the base–emittervoltage was applied.

gle-finger HBTs is not thermal, because BVBEO is not astrong function of temperature as shown in Fig. 5a. It is

S. Halder et al. / Solid-State Electronics 51 (2007) 900–904 903

also not due to pulsed operation because, without any RFsignal, the magnitude of pulsed BVBEO is higher than thatof DC BVBEO (Fig. 5b). The reason is simply that, for class-C operation, the base–emitter junction is reverse biased sothat the input resistances for both single- and four-fingerHBTs appear to be open circuit to the RF signal source.Since approximately four times higher PIN is required todrive the four-finger HBT to the same level of gain com-pression as that of the single-finger HBT, approximatelytwice of the base–emitter voltage swing is required for thefour-finger HBT than that for the single-finger HBT. Thiscauses the four-finger HBT to breakdown at a lower com-pression level than the single-finger HBT. This explainswhy the maximum PIN and POUT of the four-finger HBTare only approximately 2 dB higher than that of the sin-gle-finger HBT.

4. Large-signal model

To account for the base–emitter breakdown effect, acommercially available [12] large-signal HBT model wasmodified by adding a voltage-dependent current sourceIBR between the base and emitter. The voltage dependenceof IBR is similar to that of [13]:

IBR ¼ IBRSfexp½�ðV BE þ V BRÞ=kT � � 1g; ð1Þ

0

20

40

60

80

0 1 2 3

COLLECTOR-EMITTER VOLTAGE (V)

CO

LL

EC

TO

R C

UR

RE

NT

(m

A)

0

10

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30

40

50

0 20 40 60 80 100 120 140

COLLECTOR CURRENT (mA)

CU

T O

FF

FR

EQ

UE

NC

Y f

T (G

Hz)

Fig. 6. (Symbol) Measured vs. (curve) simulated (a) I–V characteristicsand (b) forward current–gain cutoff frequency fT of a four-finger HBT.IB = 160,320, . . .,800 lA bottom up in (a). VCE = 1, 2, 3, and 4 V top tobottom in (b).

where IBRS is the saturation current, VBR is the kneevoltage, k is the Boltzmann constant, and T is the junctiontemperature. Notice that VBR = 3 V according to RFbreakdown characteristics instead of DC breakdown char-acteristics. Such a modified model remains valid for DCand small-signal RF characteristics over a wide bias rangeas shown in Fig. 6.

The HBT model, with and without modification forbase–emitter breakdown, was used to simulate thesource–pull contours of a four-finger HBT. Fig. 7a showsthat, for the same maximum POUT, the optimum sourceimpedance simulated with base–emitter breakdown ismuch closer to the measured value than that simulatedwithout base–emitter breakdown. Also, as expected, themodel with base–emitter breakdown predicts a much stee-per roll off when the input is mismatched. Therefore, themodel without base–emitter breakdown is not only off tar-get, but also may imply a more robust amplifier than itactually is (see Fig. 7b).

Fig. 7. (Symbol) Measured optimum source impedance vs. (curve)simulated source–pull contours in 0.2 dB steps of a four-finger HBTunder an input power of 22 dBm. The simulation was performed (a) withand (b) without base–emitter breakdown.

904 S. Halder et al. / Solid-State Electronics 51 (2007) 900–904

5. Conclusion

Under class-C common-base configuration, althoughunit-finger InGaP/GaAs HBTs exhibit very high powerdensity, due to base–emitter reverse breakdown, the powerdensity does not scale linearly with the number of fingers.In addition, the base–emitter breakdown voltage is muchlower under RF operation than under DC operation. TheRF base–emitter breakdown effect was added to a commer-cially available large-signal HBT model, which more accu-rately predicted the class-C power performance ofcommon-base multi-finger HBTs. To fully realize thepower capacity of multi-finger HBTs, it will be necessaryto improve the device design for higher base–emitter break-down voltage or to improve the circuit design to preventbase–emitter breakdown.

Acknowledgement

This work was supported in part by the Commonwealthof Pennsylvania, Department of Community and Eco-nomic Development.

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