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

	MESFET	HEMT	HBT
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.