High Electron Mobility Transistors
High Electron Mobility Transistors(HEMTs) [340] are an advanced modification of the simple bulk FET, such as the MEtal Semiconductors Field Effect Transistor (MESFET). Typically, a semiconductor material (barrier) with a comparably wider bandgap is grown
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6.1 General Considerations High Electron Mobility Transistors (HEMTs) [340] are an advanced modification of the simple bulk FET, such as the MEtal Semiconductors Field Effect Transistor (MESFET). Typically, a semiconductor material (barrier) with a comparably wider bandgap is grown on top of a semiconductor material with a higher mobility and comparably lower bandgap. If the bandgap alignment of the two materials is appropriately chosen, a channel forms due solely to the alignment of the band edges and not, as for silicon MOSFETs or III-V MESFETs [460], due to an oxide/semiconductor interface or doping profiles. The channel material is not intentionally doped. Growing only two materials on top of each other results in a single heterostructure device. When a material similar or identical to the barrier material below the channel is introduced, a second heterostructure forms , resulting in a double heterostructure HEMT. As is typical for FETs, the channel region is controlled by a gate Schottky contact positioned above the barrier layer to control the flow of carriers in the channel. Two ohmic contacts, which are positioned on top of highly conductive and highly doped cap layers, lead to the control of the current flow from the source ohmic contact to the drain ohmic contact. Ideally the control of the channel region is purely electrostatic without current flow through the gate. In reality a low gate current is visible; the current amplification of HEMT, however, amounts to at least four orders of magnitude.
6.2 High-Speed and High-Power AIGaAs/InGaAs PHEMTs AIGaAs/InGaAs HEMTs on GaAs substrates are standard devices in modern microwave and millimeter-wave industrial applications. HEMTs provide a combination of high-speed performance and high breakdown voltages enabling operation up to 100 GHz. This allows extremely low-noise performance and very good highpower capabilities. This section presents simulation examples of state-of-the-art industrial HEMT technology development using MINIMOS-NT.
V. Palankovski et al., Analysis and Simulation of Heterostructure Devices © Springer-Verlag Wien 2004
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6.2 High-Speed and High-Power A1GaAs/lnGaAs PHEMTs
passivation )
I) -doping
Fig. 6.1. Layer structure of a double-recess double-heterojunction AIGaAs/lnGaAs pseudoIllorphic HEMT.
6.2.1 Device Fabrication A typical modern HEMT structure is given in Fig. 6.1. An InGaAs channel, usually with 25% In content, is sandwiched between Alo.2Gao.sAs layers using pseudomorphic growth on semi-insulating (s.i.) GaAs substrate. For the ohmic contact formation, highly n-doped cap layers are grown on top of the AlGaAs layers. HEMTs are unipolar devices and n-doping is achieved by so-called 6doping or bulk layer-doping in the two AlGaAs layers by introducing silicon with a thickness of 1-5 nm and concentrations of up to 6x 10 12 cm- 2. High-performance devices contain additional layers between the barriers and the cap layers. These layers vary in doping and material composition. The design of the gate-drain region, with single or doub
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