Impact of the Source-to-Drain Spacing on the DC and RF Characteristics of InGaAs/InAlAs High-Electron Mobility Transistors

In this letter, we investigated the impact of the source-to-drain spacing (<inline-formula> <tex-math notation="LaTeX">${L}_{\textsf {SD}}$ </tex-math></inline-formula>) on the dc and high-frequency characteristics of indium-rich In<sub>0.8</sub>Ga<sub>0.2</sub>As/In<sub>0.52</sub>Al<sub>0.48</sub>As high-electron mobility transistors (HEMTs) on a 3-in InP substrate. <inline-formula> <tex-math notation="LaTeX">${L}_{g} = \textsf {87}$ </tex-math></inline-formula> nm HEMTs with different values of <inline-formula> <tex-math notation="LaTeX">${L}_{\textsf {SD}}$ </tex-math></inline-formula> were fabricated ranging from 1.55 to <inline-formula> <tex-math notation="LaTeX">$0.8~\mu \text{m}$ </tex-math></inline-formula>, and their dc and RF responses were measured and analyzed in detail. In order to suppress the increase of the parasitic gate capacitance, we maintained the gate stem height as high as 200 nm in our device fabrication. Both the maximum transconductance (g<sub>m_max</sub>) and on-resistance (<inline-formula> <tex-math notation="LaTeX">${R}_{\textsf {ON}}$ </tex-math></inline-formula>) improved as <inline-formula> <tex-math notation="LaTeX">${L}_{\textsf {SD}}$ </tex-math></inline-formula> scaled down to <inline-formula> <tex-math notation="LaTeX">$0.8~\mu \text{m}$ </tex-math></inline-formula>. At the same time, the high-frequency figures of the merit, such as current-gain cutoff frequency (<inline-formula> <tex-math notation="LaTeX">${f}_{T}$ </tex-math></inline-formula>) and maximum oscillation frequency (<inline-formula> <tex-math notation="LaTeX">${f}_{\textsf {max}}$ </tex-math></inline-formula>), increased with the reduction of <inline-formula> <tex-math notation="LaTeX">${L}_{\textsf {SD}}$ </tex-math></inline-formula>. These improvements are attributed to the reduction of series resistances. In particular, the <inline-formula> <tex-math notation="LaTeX">${L}_{g} =87$ </tex-math></inline-formula> nm device with an <inline-formula> <tex-math notation="LaTeX">${L}_{\textsf {SD}} = \textsf {0.8} \mu \text{m}$ </tex-math></inline-formula> exhibited an excellent combination of dc and RF characteristics, such as g<sub>m_max</sub> = 2.7 mS/<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">${R}_{\textsf {ON}} = \textsf {318}\,\,\Omega \cdot \mu \text{m}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">${f}_{T} = \textsf {519}$ </tex-math></inline-formula> GHz, and <inline-formula> <tex-math notation="LaTeX">${f}_{\textsf {max}} = \textsf {645}$ </tex-math></inline-formula> GHz, respectively. The results obtained in this letter indicate that the reduction of <inline-formula> <tex-math notation="LaTeX">${L}_{\textsf {SD}}$ </tex-math></inline-formula> down to <inline-formula> <tex-math notation="LaTeX">$0.8~\mu \text{m}$ </tex-math></inline-formula> continues to improve both the dc and RF characteristics of the InGaAs/InAlAs HEMTs, and a possible increase in parasitic capacitance components, associated with a T-shaped, is still negligible in our device architecture.