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GaAs heterojunction bipolar transistors
Solid-State Electronics 81 (2013) 5–7
Contents lists available at SciVerse ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Design of emitter ledge for thermal stability of AlGaAs/GaAs heterojunction bipolar transistors H.W. Lim, C.H. Baek 1, B.K. Kang ⇑ Department of Electrical Engineering, Pohang University of Science and Technology, San 31, Hyoja-Dong, Pohang, Kyungpook 790-784, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 September 2012 Received in revised form 11 December 2012 Accepted 25 December 2012 Available online 4 February 2013 The review of this paper was arranged by Prof. E. Calleja Keywords: Heterojunction bipolar transistor (HBT) Base ballast resistor AlGaAs ledge Bypass capacitor Thermal stability Power density
a b s t r a c t This paper presents a design of emitter ledge that achieves thermal stability of the AlGaAs/GaAs heterojunction bipolar transistors (HBTs). The HBTs use a p+-base layer to implement base ballast resistors, a fully-depleted AlGaAs ledge to implement input bypass capacitors, and boron ion implantation to reduce the base–collector parasitic capacitance. The minimum emitter ledge length for the experimental HBTs is estimated theoretically as 8.27 lm, at which power density is 2.77 mW/lm2. Experimental results show that the HBTs were thermally stable at an emitter ledge length of 10 lm, and their RF properties were degraded little from those of the HBTs without the emitter ledge when the n collector underneath the emitter ledge was implanted with boron ions. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The operation of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) at a high power level is limited by the thermal instability [1]. The thermal instability of HBT can be prevented mechanically using either thermal shunts [2] or flip-chip bonding [3], but fabricating these components requires additional process steps. The thermal instability can also be prevented electrically by using either emitter ballast resistors [4,5] or base ballast resistors [6]. The emitter ballast resistors degrade both the RF characteristic and collector power efficiency of HBTs [5]. The base ballast resistors prevent the thermal instability by decreasing the dc current gain of HBT when the junction temperature increases [6]. Bypass capacitors connected in parallel with the base ballast resistors are required to prevent degradation of the HBTs’ RF characteristic. An HBT that can implement both base ballast resistors and bypass capacitors without any additional process steps, was proposed by Oh et al. [7]. They used a p+-base layer to implement the base ballast resistors, a fully-depleted AlGaAs ledge to implement the bypass capacitors, and boron (B) ion-implantation to reduce the base–collector capacitance which degrades the RF char⇑ Corresponding author. Tel.: +82 54 279 2226; fax: +82 54 279 2903. E-mail address: [email protected] (B.K. Kang). System LSI Division, Samsumg Electronics Co. Ltd., San 24, Nongseo-dong, Giheung-gu, Yongin-City, Gyeonggi-Do 449–771, Republic of Korea. 1
0038-1101/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.sse.2012.12.010
acteristic of HBT. However, they did not analyze the thermal and RF properties of the resulting HBT. This paper uses the HBT structure proposed in [7], analyzes thermal properties of HBT, and proposes the minimum emitter ledge length required to secure thermal stability without sacrificing frequency characteristics. Minimum length of emitter ledge for thermal stability is given in Section 2, experimental results are given in Section 3, and a conclusion is given in Section 4. 2. Minimum length of emitter ledge for thermal stability The AlGaAs/GaAs HBTs for thermal analysis is assumed to have an emitter of length Le and width We, a base of thickness Xb and metal width Wb, and an AlGaAs ledge length of Seb (Fig. 1). The effective specific contact resistance of the emitter is assumed as qe,eff. The collector current Ic is given by [8]:
Ic ¼ Ic0 exp
qðV be;j þ /ðT T 0 ÞÞ ; gc kT
ð1Þ
where Vbe,j is base-emitter junction voltage, Ic0 is collector saturation current, gc is ideality factor of collector current, k is Boltzman’s constant, q is the unit charge of electron, T is junction temperature, T0 is junction temperature at a low Ic, and / oVbe,j/oT is the thermal-electrical feedback coefficient which reflects a decrease of Vbe,j due to an increase of T. Because q/(T T0) = Eg(T) and the bandgap energy at T is given as Eg(T) = Eg0bT [4], / satisfies [9]:
6
H.W. Lim et al. / Solid-State Electronics 81 (2013) 5–7
Fig. 1. Structure of experimental AlGaAs/GaAs HBTs.
/¼
b g k Ic c ln ; T q Ic0
where Eg0 is the bandgap energy at 0 K and b is a constant. When HBT operates at a base current Ib and a base-emitter terminal voltage of Vbe, (1) can be rewritten as
Ic ¼ Ic0 exp
Fig. 2. Measured (symbols) and calculated (lines) regression loci at Vce = 8 V for HBTs with Seb = 0, 2.5, 5, and 10 lm.
qðV be Ib ðRb þ Re Þ Ic Re þ /ðT T 0 ÞÞ ; gc kT
because Vbe,j = Vbe Ib(Rb + Re) IcRe, where Rb and Re are base and emitter resistances, respectively. This equation is solved for Vbe using the base-to-collector current gain b Ic/Ib as
V be ¼
gc kT q
ln
Ic Ic0
þ
Rb þ Re Ic þ Ic Re /ðT T 0 Þ: b
ð2Þ
Defining a thermal resistance as Rth oT/op = (Ae/Vce)oT/oIc, where p is the power dissipation per unit area of the emitter and Ae is the emitter area of the HBT, the Ic at which the thermal regression occurs is obtained by differentiating (2) with respect to Ic and setting oIc/oVbe ? 1 as
Ic ¼
gc kT q
b : ðb/Rth V ce =Ae Þ Rb ð1 þ bÞRe
Fig. 3. Vbe versus p measured at T = 300 K for HBTs with Seb = 0 lm, and T versus Vbe measured at p = 2.77 mW/lm2.
The condition for an onset of thermal regression is obtained from the above equation as (b/RthVce/Ae) (1 + b)Re Rb 0, because Ic > 0 and RbIc/b kT/q. So, the minimum Rb for thermal stability is
Rb;min ¼
b/Rth V ce ð1 þ bÞRe : Ae
ð3Þ
The emitter resistance Re is given by
Re ¼
qe;eff W e Le
ð4Þ
:
When the sheet resistance of the base layer is Rb,s (X/h) and the specific base contact resistivity is qb,c, the intrinsic base resistance Rbi, the extrinsic base resistance Rbx, and the base contact resistance Rb,c are given as [10]:
Rbi ¼
Rb;s W e ; 12Le
Rbx ¼
Rb;s Seb ; 2Le
and Rb;c ¼
qb;c 2W b Le
:
ð5Þ
Using (3)–(5) and Rb = Rbi + Rbx + Rb,c, the minimum length of Seb,min of the AlGaAs ledge for thermal stability is obtained as
Seb;min ¼
qb;c 2 We : ðb/Rth V ce qe;eff ð1 þ bÞÞ þ W e Rb;s 6 Rb;s W b
ð6Þ
3. Experiment The experimental AlGaAs/GaAs HBTs with different Seb s were fabricated using the HBT fabrication process in [7]. Each emitter
finger had a width We of 2.5 lm and a length Le of 30 lm. Three emitter fingers were used; the spacing between adjacent fingers was 40 lm. The base contact of Wb = 2 lm was on both sides of the emitter. The base collector capacitance was reduced by implanting B ions at an energy of 300 keV. The projected range and straggle of B ions at 300 keV were 6000 Å and 1500 Å, respectively. The dose of ion implantation was 1 1012 cm2. The thermal characteristics of the fabricated HBTs were calculated using (2) and measured using a semiconductor parameter analyzer (4156C, Agilent Technologies, Inc.). The calculated regression loci at Vce = 8 V (solid lines, Fig. 2), which are useful to identify onset of thermal stability, agree quite well with the measured ones (marks, Fig. 2); thermal regression was observed at Seb = 0, 2.5 and 5 lm, but not observed at Seb = 10 lm. To obtain the thermal resistance Rth = (oT/oVbe)(oVbe/op), the variation of Vbe for the HBT of Seb = 0 lm was measured at Vce = 8 V and T = 300 K while varying the power density p (Fig. 3). Vbe changed from 1.559 to 1.461 V when p changed from 2.08 to 3.47 mW/ lm2, resulting in oVbe/@p = 83.25 mV lm2/mW. When T of substrate changed from 300 to 360 K, Vbe at p = 2.77 mW/lm2 (Vce = 8 V, Ic = 26 mA) changed from 1.514 to 1.373 V, resulting in oT/oVbe = 423.7 K/V. These results give the Rth for experimental HBT of 35.27 K lm2/mW at p = 2.77 mW/lm2. To obtain thermal–electric feedback coefficient / for the HBT of Seb = 0 lm, Vbe at Vbc = 0 V and Ic = 26 mA were measured while varying T; the condition Vbc = 0 V minimizes the heat generation at the b–c junction so that the substrate temperature is close to
H.W. Lim et al. / Solid-State Electronics 81 (2013) 5–7
7
be used to achieve thermal stability of HBTs with a little degradation of their RF properties. 4. Conclusion Thermal properties of the experimental AlGaAs/GaAs HBTs are analyzed theoretically to find the minimum emitter ledge length for thermal stability of HBTs. The minimum emitter ledge length for the experimental HBTs is estimated as 8.27 lm at which power density is 2.77 mW/lm2. Experimental results agree well with the theoretically estimated ones; the experimental HBTs of which the n collector underneath the emitter ledge was implanted with B ions, were thermally stable at an emitter ledge length of 10 lm, and had 2% lower fmax and 4.3% lower froll-off than the HBTs without the emitter ledge. These results demonstrate that the emitter ledge can be used to achieve thermal stability of HBTs with little degradation of their RF characteristics. Fig. 4. Measured power gains versus frequency for experimental HBTs.
Acknowledgment Table 1 Measured values of fmax s and froll-off s for the experimental HBTs. Seb (lm)
fmax (GHz)
froll-off (GHz)
Remark
0 2.5 5.0 10.0
38.18 26.57 22.04 17.22
18.7 15.5 13.7 12.5
Without B+ implantation
10.0
37.42
19.5
With B+ implantation
the junction temperature. Vbe changed from 1.651 to 1.518 V when T changed from 300 to 360 K, which gives / = 2.22 mV/K. The product of minimum ledge length Seb,mim and emitter width We, which is required for thermal stability, was calculated using (6) and the intrinsic HBT parameters of b = 15.9 at the regression point (Vce = 8 V and Ic = 26 mA), gc = 1.1026, Rth = 35.27 K lm2/ mW, qe.eff = 420.3 X lm2, qb,c = 399.6 X lm2, Rb,s = 240 X/h, and Wb = 2 lm. Seb,minWe at p = 2.77 mW/lm2 was 8.27 2.5 lm2. The maximum available power gains of the experimental HBTs were measured at Vce = 3 V and Ic = 20 mA using a network analyzer (37397C, Anritsu Co). Without B ion implantation, the HBT of Seb = 0 lm had the highest power gain, as expected. An increase of Seb increased Rbx and Cbc, and therefore decreased the power gain (Fig. 4). The B implantation reduced the power gain by 2 dB, but decreased neither fmax nor froll-off from the values for the HBT with Seb = 0 lm; the measured fmax and froll-off were 38.18 and 18.7 GHz for Seb = 0 lm, and were 37.42 and 19.5 GHz for Seb = 10 lm (Table 1 ). These results demonstrate that the emitter ledge can
This work was supported by the Korean Ministry of Education under the BK21 program. References [1] Liou LL, Bayraktaroglu B. Thermal stability analysis of AlGaAs/GaAs heterojunction bipolar transistors with multiple emitter fingers. IEEE Trans Electron Dev 1994;41:629–36. [2] Sewell J, Liou LL, Barlage D, Barrette J, Bozada C, Dettmer R, et al. Thermal characterization of thermally-shunted heterojunction bipolar transistors. IEEE Electron Device Lett 1996;17:19–21. [3] Jenkins T, Bozada C, Dettmer R, Sewell J, Via D, Barrette J, et al. Comparison of thermal-shunt and flip-chip HBT thermal impedance: comment on novel HBT with reduced thermal impedance. IEEE Microw Guided Wave Lett 1993;6(7):268–9. [4] Arnold Robert P, Zoroglu Demir S. A quantitative study of emitter ballasting. IEEE Trans Electron Dev 1974;21(7):385–91. [5] Gao Guang-bo, Selim Ünlü M, Morkoç Hadis, Blackburn David L. Emitter ballasting resistor design for, and current handling capability of AlGaAs/GaAs power heterojunction bipolar transistors. IEEE Trans Electron Dev 1991;38:185–96. [6] Liu William, Khatibzadeh Ali, Sweder Jim, Chau Hin-Fai. The use of base ballasting to prevent the collapse of current gain in AlGaAs/GaAs heterojunction bipolar transistor. IEEE Trans Electron Dev 1996;43(2):245–51. [7] Oh TK, Lee JG, Yi KH, Baek CH, Ihn BU, Kang BK. Properties of AlGaAs/GaAs heterojunction bipolar transistor with AlGaAs ledge bypass capacitor. Solid State Electron 2002;46:35–48. [8] Liu William, Khatibzadeh Ali. The collapse of current gain in multi-finger heterojunction bipolar transistors: its substrate temperature dependence, instability criteria, and modeling. IEEE Trans Electron Dev 1994;41(10):1698–707. [9] Liu William, Yuksel Ayca. A survey of thermal-electric feedback coefficients in GaAs-based heterojunction bipolar transistors. Solid-State Electron 1995;38(2):407–11. [10] Liu William. Handbook of III–V heterojunction bipolar transistors. New York: Wiley; 1998.
Contents lists available at SciVerse ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Design of emitter ledge for thermal stability of AlGaAs/GaAs heterojunction bipolar transistors H.W. Lim, C.H. Baek 1, B.K. Kang ⇑ Department of Electrical Engineering, Pohang University of Science and Technology, San 31, Hyoja-Dong, Pohang, Kyungpook 790-784, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 September 2012 Received in revised form 11 December 2012 Accepted 25 December 2012 Available online 4 February 2013 The review of this paper was arranged by Prof. E. Calleja Keywords: Heterojunction bipolar transistor (HBT) Base ballast resistor AlGaAs ledge Bypass capacitor Thermal stability Power density
a b s t r a c t This paper presents a design of emitter ledge that achieves thermal stability of the AlGaAs/GaAs heterojunction bipolar transistors (HBTs). The HBTs use a p+-base layer to implement base ballast resistors, a fully-depleted AlGaAs ledge to implement input bypass capacitors, and boron ion implantation to reduce the base–collector parasitic capacitance. The minimum emitter ledge length for the experimental HBTs is estimated theoretically as 8.27 lm, at which power density is 2.77 mW/lm2. Experimental results show that the HBTs were thermally stable at an emitter ledge length of 10 lm, and their RF properties were degraded little from those of the HBTs without the emitter ledge when the n collector underneath the emitter ledge was implanted with boron ions. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The operation of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) at a high power level is limited by the thermal instability [1]. The thermal instability of HBT can be prevented mechanically using either thermal shunts [2] or flip-chip bonding [3], but fabricating these components requires additional process steps. The thermal instability can also be prevented electrically by using either emitter ballast resistors [4,5] or base ballast resistors [6]. The emitter ballast resistors degrade both the RF characteristic and collector power efficiency of HBTs [5]. The base ballast resistors prevent the thermal instability by decreasing the dc current gain of HBT when the junction temperature increases [6]. Bypass capacitors connected in parallel with the base ballast resistors are required to prevent degradation of the HBTs’ RF characteristic. An HBT that can implement both base ballast resistors and bypass capacitors without any additional process steps, was proposed by Oh et al. [7]. They used a p+-base layer to implement the base ballast resistors, a fully-depleted AlGaAs ledge to implement the bypass capacitors, and boron (B) ion-implantation to reduce the base–collector capacitance which degrades the RF char⇑ Corresponding author. Tel.: +82 54 279 2226; fax: +82 54 279 2903. E-mail address: [email protected] (B.K. Kang). System LSI Division, Samsumg Electronics Co. Ltd., San 24, Nongseo-dong, Giheung-gu, Yongin-City, Gyeonggi-Do 449–771, Republic of Korea. 1
0038-1101/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.sse.2012.12.010
acteristic of HBT. However, they did not analyze the thermal and RF properties of the resulting HBT. This paper uses the HBT structure proposed in [7], analyzes thermal properties of HBT, and proposes the minimum emitter ledge length required to secure thermal stability without sacrificing frequency characteristics. Minimum length of emitter ledge for thermal stability is given in Section 2, experimental results are given in Section 3, and a conclusion is given in Section 4. 2. Minimum length of emitter ledge for thermal stability The AlGaAs/GaAs HBTs for thermal analysis is assumed to have an emitter of length Le and width We, a base of thickness Xb and metal width Wb, and an AlGaAs ledge length of Seb (Fig. 1). The effective specific contact resistance of the emitter is assumed as qe,eff. The collector current Ic is given by [8]:
Ic ¼ Ic0 exp
qðV be;j þ /ðT T 0 ÞÞ ; gc kT
ð1Þ
where Vbe,j is base-emitter junction voltage, Ic0 is collector saturation current, gc is ideality factor of collector current, k is Boltzman’s constant, q is the unit charge of electron, T is junction temperature, T0 is junction temperature at a low Ic, and / oVbe,j/oT is the thermal-electrical feedback coefficient which reflects a decrease of Vbe,j due to an increase of T. Because q/(T T0) = Eg(T) and the bandgap energy at T is given as Eg(T) = Eg0bT [4], / satisfies [9]:
6
H.W. Lim et al. / Solid-State Electronics 81 (2013) 5–7
Fig. 1. Structure of experimental AlGaAs/GaAs HBTs.
/¼
b g k Ic c ln ; T q Ic0
where Eg0 is the bandgap energy at 0 K and b is a constant. When HBT operates at a base current Ib and a base-emitter terminal voltage of Vbe, (1) can be rewritten as
Ic ¼ Ic0 exp
Fig. 2. Measured (symbols) and calculated (lines) regression loci at Vce = 8 V for HBTs with Seb = 0, 2.5, 5, and 10 lm.
qðV be Ib ðRb þ Re Þ Ic Re þ /ðT T 0 ÞÞ ; gc kT
because Vbe,j = Vbe Ib(Rb + Re) IcRe, where Rb and Re are base and emitter resistances, respectively. This equation is solved for Vbe using the base-to-collector current gain b Ic/Ib as
V be ¼
gc kT q
ln
Ic Ic0
þ
Rb þ Re Ic þ Ic Re /ðT T 0 Þ: b
ð2Þ
Defining a thermal resistance as Rth oT/op = (Ae/Vce)oT/oIc, where p is the power dissipation per unit area of the emitter and Ae is the emitter area of the HBT, the Ic at which the thermal regression occurs is obtained by differentiating (2) with respect to Ic and setting oIc/oVbe ? 1 as
Ic ¼
gc kT q
b : ðb/Rth V ce =Ae Þ Rb ð1 þ bÞRe
Fig. 3. Vbe versus p measured at T = 300 K for HBTs with Seb = 0 lm, and T versus Vbe measured at p = 2.77 mW/lm2.
The condition for an onset of thermal regression is obtained from the above equation as (b/RthVce/Ae) (1 + b)Re Rb 0, because Ic > 0 and RbIc/b kT/q. So, the minimum Rb for thermal stability is
Rb;min ¼
b/Rth V ce ð1 þ bÞRe : Ae
ð3Þ
The emitter resistance Re is given by
Re ¼
qe;eff W e Le
ð4Þ
:
When the sheet resistance of the base layer is Rb,s (X/h) and the specific base contact resistivity is qb,c, the intrinsic base resistance Rbi, the extrinsic base resistance Rbx, and the base contact resistance Rb,c are given as [10]:
Rbi ¼
Rb;s W e ; 12Le
Rbx ¼
Rb;s Seb ; 2Le
and Rb;c ¼
qb;c 2W b Le
:
ð5Þ
Using (3)–(5) and Rb = Rbi + Rbx + Rb,c, the minimum length of Seb,min of the AlGaAs ledge for thermal stability is obtained as
Seb;min ¼
qb;c 2 We : ðb/Rth V ce qe;eff ð1 þ bÞÞ þ W e Rb;s 6 Rb;s W b
ð6Þ
3. Experiment The experimental AlGaAs/GaAs HBTs with different Seb s were fabricated using the HBT fabrication process in [7]. Each emitter
finger had a width We of 2.5 lm and a length Le of 30 lm. Three emitter fingers were used; the spacing between adjacent fingers was 40 lm. The base contact of Wb = 2 lm was on both sides of the emitter. The base collector capacitance was reduced by implanting B ions at an energy of 300 keV. The projected range and straggle of B ions at 300 keV were 6000 Å and 1500 Å, respectively. The dose of ion implantation was 1 1012 cm2. The thermal characteristics of the fabricated HBTs were calculated using (2) and measured using a semiconductor parameter analyzer (4156C, Agilent Technologies, Inc.). The calculated regression loci at Vce = 8 V (solid lines, Fig. 2), which are useful to identify onset of thermal stability, agree quite well with the measured ones (marks, Fig. 2); thermal regression was observed at Seb = 0, 2.5 and 5 lm, but not observed at Seb = 10 lm. To obtain the thermal resistance Rth = (oT/oVbe)(oVbe/op), the variation of Vbe for the HBT of Seb = 0 lm was measured at Vce = 8 V and T = 300 K while varying the power density p (Fig. 3). Vbe changed from 1.559 to 1.461 V when p changed from 2.08 to 3.47 mW/ lm2, resulting in oVbe/@p = 83.25 mV lm2/mW. When T of substrate changed from 300 to 360 K, Vbe at p = 2.77 mW/lm2 (Vce = 8 V, Ic = 26 mA) changed from 1.514 to 1.373 V, resulting in oT/oVbe = 423.7 K/V. These results give the Rth for experimental HBT of 35.27 K lm2/mW at p = 2.77 mW/lm2. To obtain thermal–electric feedback coefficient / for the HBT of Seb = 0 lm, Vbe at Vbc = 0 V and Ic = 26 mA were measured while varying T; the condition Vbc = 0 V minimizes the heat generation at the b–c junction so that the substrate temperature is close to
H.W. Lim et al. / Solid-State Electronics 81 (2013) 5–7
7
be used to achieve thermal stability of HBTs with a little degradation of their RF properties. 4. Conclusion Thermal properties of the experimental AlGaAs/GaAs HBTs are analyzed theoretically to find the minimum emitter ledge length for thermal stability of HBTs. The minimum emitter ledge length for the experimental HBTs is estimated as 8.27 lm at which power density is 2.77 mW/lm2. Experimental results agree well with the theoretically estimated ones; the experimental HBTs of which the n collector underneath the emitter ledge was implanted with B ions, were thermally stable at an emitter ledge length of 10 lm, and had 2% lower fmax and 4.3% lower froll-off than the HBTs without the emitter ledge. These results demonstrate that the emitter ledge can be used to achieve thermal stability of HBTs with little degradation of their RF characteristics. Fig. 4. Measured power gains versus frequency for experimental HBTs.
Acknowledgment Table 1 Measured values of fmax s and froll-off s for the experimental HBTs. Seb (lm)
fmax (GHz)
froll-off (GHz)
Remark
0 2.5 5.0 10.0
38.18 26.57 22.04 17.22
18.7 15.5 13.7 12.5
Without B+ implantation
10.0
37.42
19.5
With B+ implantation
the junction temperature. Vbe changed from 1.651 to 1.518 V when T changed from 300 to 360 K, which gives / = 2.22 mV/K. The product of minimum ledge length Seb,mim and emitter width We, which is required for thermal stability, was calculated using (6) and the intrinsic HBT parameters of b = 15.9 at the regression point (Vce = 8 V and Ic = 26 mA), gc = 1.1026, Rth = 35.27 K lm2/ mW, qe.eff = 420.3 X lm2, qb,c = 399.6 X lm2, Rb,s = 240 X/h, and Wb = 2 lm. Seb,minWe at p = 2.77 mW/lm2 was 8.27 2.5 lm2. The maximum available power gains of the experimental HBTs were measured at Vce = 3 V and Ic = 20 mA using a network analyzer (37397C, Anritsu Co). Without B ion implantation, the HBT of Seb = 0 lm had the highest power gain, as expected. An increase of Seb increased Rbx and Cbc, and therefore decreased the power gain (Fig. 4). The B implantation reduced the power gain by 2 dB, but decreased neither fmax nor froll-off from the values for the HBT with Seb = 0 lm; the measured fmax and froll-off were 38.18 and 18.7 GHz for Seb = 0 lm, and were 37.42 and 19.5 GHz for Seb = 10 lm (Table 1 ). These results demonstrate that the emitter ledge can
This work was supported by the Korean Ministry of Education under the BK21 program. References [1] Liou LL, Bayraktaroglu B. Thermal stability analysis of AlGaAs/GaAs heterojunction bipolar transistors with multiple emitter fingers. IEEE Trans Electron Dev 1994;41:629–36. [2] Sewell J, Liou LL, Barlage D, Barrette J, Bozada C, Dettmer R, et al. Thermal characterization of thermally-shunted heterojunction bipolar transistors. IEEE Electron Device Lett 1996;17:19–21. [3] Jenkins T, Bozada C, Dettmer R, Sewell J, Via D, Barrette J, et al. Comparison of thermal-shunt and flip-chip HBT thermal impedance: comment on novel HBT with reduced thermal impedance. IEEE Microw Guided Wave Lett 1993;6(7):268–9. [4] Arnold Robert P, Zoroglu Demir S. A quantitative study of emitter ballasting. IEEE Trans Electron Dev 1974;21(7):385–91. [5] Gao Guang-bo, Selim Ünlü M, Morkoç Hadis, Blackburn David L. Emitter ballasting resistor design for, and current handling capability of AlGaAs/GaAs power heterojunction bipolar transistors. IEEE Trans Electron Dev 1991;38:185–96. [6] Liu William, Khatibzadeh Ali, Sweder Jim, Chau Hin-Fai. The use of base ballasting to prevent the collapse of current gain in AlGaAs/GaAs heterojunction bipolar transistor. IEEE Trans Electron Dev 1996;43(2):245–51. [7] Oh TK, Lee JG, Yi KH, Baek CH, Ihn BU, Kang BK. Properties of AlGaAs/GaAs heterojunction bipolar transistor with AlGaAs ledge bypass capacitor. Solid State Electron 2002;46:35–48. [8] Liu William, Khatibzadeh Ali. The collapse of current gain in multi-finger heterojunction bipolar transistors: its substrate temperature dependence, instability criteria, and modeling. IEEE Trans Electron Dev 1994;41(10):1698–707. [9] Liu William, Yuksel Ayca. A survey of thermal-electric feedback coefficients in GaAs-based heterojunction bipolar transistors. Solid-State Electron 1995;38(2):407–11. [10] Liu William. Handbook of III–V heterojunction bipolar transistors. New York: Wiley; 1998.