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 (L
_SD
) on the dc and high-frequency characteristics of indium-rich In
_0.8
Ga
_0.2
As/In
_0.52
Al
_0.48
As high-electron mobility transistors (HEMTs) on a 3-in InP substrate. L
_g
= 87 nm HEMTs with different values of LSD were fabricated ranging from 1.55 to 0.8 μm, 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
_{m_max}
) and on-resistance (R
_ON
) improved as L
_SD
scaled down to 0.8 μm. At the same time, the high-frequency figures of the merit, such as current-gain cutoff frequency (f
_T
) and maximum oscillation frequency (f
_max
), increased with the reduction of L
_SD
. These improvements are attributed to the reduction of series resistances. In particular, the L
_g
= 87 nm device with an L
_SD
= 0.8μm exhibited an excellent combination of dc and RF characteristics, such as g
_{m_max}
= 2.7 mS/μm, R
_ON
= 318 Ω · μm, f
_T
= 519 GHz, and f
_max
= 645 GHz, respectively. The results obtained in this letter indicate that the reduction of LSD down to 0.8 μm 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.