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2.1.2 Why and Where III-V HBTs?

The AlGaAs/GaAs and InGaP/GaAs HBTs benefit from a single heterojunction formed between the AlGaAs wide bandgap emitter and the GaAs p-type base. InP/InGaAs and InAlAs/InGaAs grown on InP substrate gives double heterojunction devices as both emitter and collector regions include wide bandgap materials.

In terms of speed the III-V HBTs are among the fastest devices. Transit frequencies $f_{\mathrm{T}}$ of about 150 GHz and maximum oscillation frequencies $f_{\mathrm{max}}$ of more than 250 GHz [14,15] were reported for HBTs on GaAs. Transfer substrate InAlAs/InGaAs HBTs on GaAs with $f_{\mathrm{T}}>$250 GHz [16] and record InP-based HBTs with $f_{\mathrm{max}}>$800 GHz were demonstrated [17] but they are still lacking level of integration ($<$1000 transistors per chip) compared to the GaAs-based HBTs. Table 2.1 summarizes state-of-the-art HBTs from different technologies with their impressive cutoff frequencies.

Table 2.1: High-frequency properties of state-of-the-art HBTs
Substrate Emitter/Base $f_{\mathrm{T}}$ [GHz] $f_{\mathrm{max}}$ [GHz] References
GaAs AlGaAs/GaAs 83 253 Matsushita, 1995 [18]
GaAs AlGaAs/InGaAs 140 250 NEC, 1998 [14]
GaAs InGaP/GaAs 156 256 Hitachi, 1998 [15]
(GaAs) InAlAs/InGaAs 251 233 UC Santa Barbara, 1998 [16]
InP InAlAs/InGaAs 162 820 UC Santa Barbara, 1999 [17]
InP InP/GaAsSb 216 240 SFU Burnaby, 2000 [19]
Si Si/SiGe 154 48 Hitachi, 2000[20]
    122 163 Hitachi, 2000[21]

Heterostructure field-effect transistors HFETs, and especially HEMTs, cover higher frequencies (see Table 2.2), have higher PAE than III-V HBTs and show comparable breakdown voltages. However, their low level of integration ($<$100 transistors per chip) and $>$10% larger chip size lead to higher cost of production. In addition, the breakdown voltages cannot be so easily controlled as in HBTs, due to the influence of surface effects. The III-V market tendency in the last two years shows the increasing importance of HBTs (see Table 2.3).

Table 2.2: Shares of HBTs, HEMTs, and MESFETs on the III-V market
Substrate Channel $f_{\mathrm{T}}$ [GHz] $f_{\mathrm{max}}$ [GHz] $l_\mathrm {g}$ [nm] References
InP lattice-matched 350 350 30 NTT, 1998 [22]
InP pseudomorphic 340 250 50 Hughes, 1992 [23]
InP graded 305 340 100 TRW, 1994 [24]
GaAs metamorphic 204 188 180 UI Urbana, 1999 [25]
GaAs metamorphic 188 312 150 DaimlerChrysler, 2000 [26]

Table 2.3: High-frequency properties of state-of-the-art HFETs
1998 75% 8% 17%
2000 60% 10% 30%

GaAs MESFETs and MESFET-based monolithic microwave integrated circuits (MMICs) are still key parts of the existing cellular phones, as they offer acceptable performance at a reasonable cost [27]. However, drawbacks are the need of double voltage supply and the large chip size. High PAE is needed to increase the battery lifetimes. HBTs are devices which at higher material cost offer high performance.

The III-V HBTs are considered essential for high-power amplifiers at 3 V power supply, as they offer high current amplification and PAE at 0.9/1.8 GHz [28]. A small chip-size 2 W MMIC based on AlGaAs/GaAs HBTs with record performance for wireless applications (62% PAE at 1.8 GHz) was demonstrated in [27]. Considering higher frequencies for future wireless applications InP-based and even SiGe MMICs with excellent performance, 48% and 24 % PAE respectively, at 25 GHz were recently reported [29,30] (see Table 2.4).

Table 2.4: HBT IC applications
Substrate Emitter/Base $f_{\mathrm{T}}$/ $f_{\mathrm{max}}$ [GHz] Advantage References
GaAs AlGaAs/GaAs - 62% PAE at 2 W Siemens, 1998 [27]
InP InGaAs/InAlAs 70/120 48% PAE at 25 GHz TRW, 1999 [29]
InP InP/InGaAs 116/169 40 Gb/s at 72 GHz NTT, 1999 [25]
Si Si/SiGe -/60 ECL gate delay 5.5 ps Hitachi, 2000 [21]
Si Si/SiGe 50/50 24% PAE at 25 GHz Daimler, 2000 [30]

A further advantage of III-V HBTs is the low phase noise figure making them attractive for digital applications. Digital ICs with AlGaAs/GaAs and InP/InGaAs HBTs are used for fiber-optic transmission of 40 Gb/s and 60 Gb/s, respectively.

next up previous contents
Next: 2.1.3 Future Up: 2.1 State-of-the-art Heterostructure Devices Previous: 2.1.1 Why and Where
Vassil Palankovski