domingo, 27 de junio de 2010

Submicron InP–InGaAs Single Heterojunction Bipolar Transistors With fT of 377 GHz

The high-frequency performance of InP-based heterojunction bipolar transistors (HBTs) has steadily increased over the last few years. While double heterojunction transistors (DHBTs) have received much attention recently, InGaAs-based DHBT devices require complicated grading schemes to overcome current blocking at the base-collector junction [1]–[4], and high-quality GaAsSb material [5] is still difficult to obtain. In this paper, we report the fastest bipolar transistor to date using a simplistic SHBT layer structure. The submicron emitter dimensions allow for low-power operation while maintaining excellent dc characteristics. Such devices are critical to support high-speed low-power applications, such as 40-Gb/s OEIC receivers [6], [7] and analog-to-digital converters.

The epitaxial structure used in this work was directed toward achieving high current cutoff frequencies by scaling the layer thicknesses and high-power cutoff frequencies by a submicron lateral scaling process. The wafers were grown on Fe-doped semi-insulating (100) InP substrates by MBE. The layer structure is scaled from the previously reported University of Illinois at Urbana–Champaign (UIUC) structure in [6], [8]. The emitter doping level is increased to reduce emitter parasitic resistances and enhance current injection efficiency. The structure also employs a 300- compositionally graded base with an Indium mole fraction of 0.5 to 0.53 and C-doped (cm , sq) grown on a 1500- InGaAs collector. The design of the material structure has been specialized for UIUC submicron processing.

The high-frequency devices were fabricated using a standard mesa process, utilizing both electron-beam and optical contact lithography. The process features an airbridge (referred to as a
bridge in this work) to isolate the base terminal from the active device, thereby drastically reducing extrinsic parasitic capacitances. The fabrication relies exclusively on wet etching to achieve the undercutting desired to allow both the self-aligned base metal and the -bridge release. Hexagonal emitters were defined using a Leica/Cambridge EBMF 10.5 e-beam system, resulting in a minimum emitter footprint of 0.35 m. The undercut during the emitter-base etch was precisely controlled to within 60 nm. The self-aligned base metal pattern was also e-beam defined, and a 650- Ti–Pt–Au base metal stack was then deposited by e-beam evaporation.
The use of such thin base metal did not adversely affect the mechanical robustness of the bridge. The devices are electrically isolated while simultaneously releasing the bridge and then planarized and encapsulated with bizbenzocyclobutene (BCB). The cured BCB provides structural support to the bridge during subsequent high-temperature processing steps. An etchback using a reactive ion etch (RIE) is then performed to expose the base, emitter, and collector terminals. NiCr resistors for on-wafer calibration are thermally deposited, followed by e-beam deposition of the overlay RF pads. An SEM image of a fabricated 0.35 8 m device before planarization is shown in Fig. 1.

Typical values of dc gain vary from 25 to 40 between 1 A and 1 mA, with remaining constant at 40 above 1 mA. Base and collector ideality factors are 1.35 and 1.18, respectively. A common emitter family of curves is shown in Fig. 2, where the collector–emitter offset voltage is approximately 0.17 V and the knee voltage is less than 0.7 V. The commonemitter breakdown voltage is approximately 3.8 V for
the 0.35 8 m device, and the avalanche breakdown at the peak collector current is 2 V.

The HBTs were characterized with an HP8510C network analyzer from 0.5 to 50 GHz. The calibration was performed with on-wafer short-open-load-thru (SOLT) standards. The current gain, Mason's unilateral gain, and MSG/MAG for a 0.35 16 m HBT are shown in Fig. 3. The cutoff values
were obtained by averaging the 20 dB/decade extrapolations from 35 to 50 GHz. The dependence of and versus extrapolation frequency is shown in the inset of Fig. 3. The peak RF performance yields an of 377 GHz and occurs at an Ic of 31 mA, corresponding to a Jc of 650 kA/cm when device undercutting is factored into the emitter area calculation. An of 230 GHz was achieved simultaneously at a of 0 V. Fig. 4 shows the cutoff frequencies versus collector current for various voltages. At of 0.1 V, the peak values for and are 368 and 238 GHz respectively, showing a weak dependence of the collector-base voltage on RF performance. We have also measured several HBTs with different emitter length as shown in Table I. For a 0.35 8 mHBT, an of 370 GHz with associated of 280 GHzwas achieved. An alternative layout for the 0.35 8 m HBT, featuring a narrower base metal finger, yielded an of 363 GHz with associated of 310 GHz.

A summary of the characteristics of the most recent high-speed bipolar transistor is shown in Table I. The product for the UIUC devices exceeds 1550 GHz V, well above the Johnson limit of 200 GHz V [9]. In comparison, the UIUC InP–InGaAs SHBT surpass the best reported SiGe HBT (490 GHz V) [10], InAlAs–InGaAs SHBT (360 GHz V) [11] and InP–InGaAs DHBT(682 GHz V) [1] and approach the latest InP–GaAsSb DHBT (1800 GHz V) [5].
Transistor model parameter extraction was performed to better understand the dominant delay terms limiting the device speed. The major delay terms are as follows: ps, ps, ps, ps and ps. These equivalent circuit parameters confirm the dominant delays are due to the forward transit time, , and the base-collector charging capacitance.
This paper has demonstrated superior dc and RF characteristics for SHBTs. The aggressive scaling of the epitaxial structure coupled with submicron emitter dimensions has produced record current gain cutoff frequencies. The RF performance along with the high-breakdown voltages exceeding 3.7 V suggest that SHBT devices will be important for low-voltage low-power mixed signal circuit applications.
Adriana Gabriela Trujillo
SECC. 02

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