High Mobility SiGe/Si n-MODFET Structures and Devices on Sapphire Substrates
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High Mobility SiGe/Si n-MODFET Structures and Devices on Sapphire Substrates Carl Mueller1, Samuel Alterovitz2, Edward Croke3, and George Ponchak2 1
Analex Corporation, Cleveland, OH, USA NASA Glenn Research Center; Cleveland, OH, USA 3 HRL Laboratories LLC, Malibu, CA, USA 2
ABSTRACT SiGe/Si n-type modulation doped field effect structures and transistors (n-MODFETs) have been fabricated on r-plane sapphire substrates. Mobilities as high as 1380 cm2/Vs were measured at room temperature. Excellent carrier confinement was shown by Shubnikov-de Haas measurements. Atomic force microscopy indicated smooth surfaces, with rms roughness less than 4 nm, similar to the quality of SiGe/Si n-MODFET structures made on Si substrates. Transistors with 2 µm gate lengths and 200 µm gate widths were fabricated and tested. An IDS of 9 mA was obtained by operating the transistor in an enhancement mode (positive VGS) and the maximum transconductance (gm) was 37 mS/mm at a VDS of 2.5 V. The transducer gain (Gt) measured with a loadpull system was 6.4 dB at 1 GHz for a VDS of 2.5 V and VGS=-0.4 V. INTRODUCTION System-on-a-chip (SOC) processes are under intense development for high-speed, high frequency transceiver circuitry. As frequencies, data rates, and circuit complexity increase, the need for substrates that simultaneously enable high-speed analog operation, low-power digital circuitry, and excellent isolation between devices becomes increasingly critical. SiGe/Si modulation doped field effect transistors (MODFETs) with high carrier mobilities are currently under development to meet the active RF device needs. However, since the substrate normally used is Si, the low-to-modest substrate resistivity causes large losses in the passive elements required for complete high frequency circuits [1, 2]. These losses become increasingly troublesome as device frequencies progress to the Ku-band (12 – 18 GHz) and beyond [3]. Relative to Si, the high electrical resistivity of sapphire enables superior performance passive devices, such as inductors [1, 2], and less crosstalk between devices. The use of silicon-on-sapphire circuits for low-power, radiation-hard applications is well known [4]. Thus, sapphire is an excellent substrate for high frequency SOC designs because it supports both active and passive RF devices, as well as low-power digital operations [5]. Both p- and n-type SiGe/Si MODFET devices on sapphire substrates have been reported. Koester et al. [6] reported a 0.1 µm by 50 µm gate, p-MODFET with transit (fT) and cutoff (fmax) frequencies of 50 and 116 GHz, respectively. While p-MODFET structures require a SiGe channel in compressive strain [7], n-MODFET structures require a conducting Si channel in tensile strain. However, the initial Si layer on sapphire is in compressive strain [8]. Thus, it is harder to grow n-MODFET structures, as the
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buffer must compensate for the original Si compressive strain and provide the required tensile strain for the channel. Recently, our group reported n-MODFET structures [9, 10], b
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