InP HBT Devices

Despite the formidable progress in the field of integrated photonic circuits, transitors will remain a key technology for a large number of applications such as high data rate optical networks, mm-wave radio links, high-speed mixed-signal ICs or terahertz imaging. Ever since the invention of the bipolar junction transistor in 1947 [1] and the transition towards integrated circuit technologies [2] it has been a major goal to improve the performance of transistors. Silicon based transistor processes like CMOS have governed common IC applications for decades. However, high-frequency applications have been the domain of III/V heterostructure devices such as GaAs and InP based HBTs and HEMTs. Although recently Si/SiGe heterostructure bipolar transistors have shown impressive performances, InP based HBTs are the most promising contenders for +100Gb/s ICs.
In our research group we design and fabricate InP based double heterojunction bipolar transistors (DHBTs) and circuits. Compared to silicon devices InP DHBTs offer a wide range of advantages for high-frequency applications:

1. Bandgap Engineering
Heterostructures, i.e. combinations of semiconductor materials with variable energy gaps, can be used to control the flow of electrons and holes independently. Proper use of these heteroinerfaces can enhance electron transport which is critical for high-speed device operation.

2. Carrier Velocity and Mobility
Compound III/V materials generally offer carrier velocities and mobilities which are significantly higher compared to silicon. High carrier velocities and mobilities can drastically increase the device figures of merit f_t and f_max.

3. Wide Bandgap Collector
A wide bandgap collector offers superior device breakdown behavior for a given performance level. Collector-emitter breakdown levels of >2V are important for stacked CML or ECL circuit technologies.

[1] http://nobelprize.org/educational_games/physics/transistor/history/index.html
[2] http://nobelprize.org/educational_games/physics/integrated_circuit/history/index.html

InP-DHBTs for +100Gb/s Integrated Circuits

Materials & Characterization
The properties of the semiconductor materials involved in the DHBT layer structure have a major impact on the high-frequency operation of the devices. Epitaxial growth of compound semiconductors is carried out with special emphasis on good crystaline quality, material composition, interface apruptness and doping levels. Our activities in the field of semiconductor materials include:
- MOVPE growth of InP and related ternaries
- Growth of highly doped GaAsSb:C
- Hall measurements, Photoluminescence, X-ray diffraction

HBT Scaling
One of the most promising approaches to increase the high-frequency performance of transistors is the scaling of lateral and vertical device dimensions. Vertical layer thicknesses in the order of few nanometers can be accurately grown by metalorganic vapor phase epitaxy (MOVPE). Lateral device scaling, on the other hand, requires sophisticated lithographic processes such as electron beam lithography (EBL) to achieve sub-micron feature widths. Furthermore, etching processes must be optimized for accurate pattern transfer into the sub-micron domain. In this project we investigate and optimize the following approaches for device scaling:
- EBL definition of the HBT emitter and base contacts
- Hybrid dry/wet etching processes for emitter and base mesa formation
- Separated base contact structures

Measurement & Characterization
For the measurement and characterization of discrete HBT devices as well as complete integrated circuits our group utilizes a set of measurement equipment. We maintain a joint HF measurement park together with the Laboratory for Field Theory and Microwave Electronics (IfH). An assortment of our equipment is listed below:

- Wiltron 37269A and Agilent VNA for S-parameter characterization up to 110GHz
- Anritsu MS2668C and Agilent Spectrum Analyzer up to 50GHz
- HP4155 Parameter Analyzer for DC characterization
- Agilent Infiniium Digital Sampling Oscilloscope with 70GHz sampling head
- ELVA-1 BWO-D SGMW Series, 110-170GHz

Simulation & Circuit Design
Our group uses several CAD tools to design and simulate devices and integrated circuits. We apply Agilent's ADS package for the design of digital ICs such as static freqeuncy dividers. Futhermore, we make use of the Synopsis TCAD suite for hydrodynamic simulations of HBTs. Our simulation activities include:
- Digital IC simulation
- Physical device modeling
- Planar EM simulation of transmission lines

Contact

Urs Hammer, Phone +41 (0)44 632 76 13