Modulation Doped FETs
Authors: Ding Kai , Zhu Congyong , Ferreyra Romualdo A. , Morkoç Hadis 
Published: December 2024

Conventional modulation‐doped field‐effect transistors (MODFETs) with unprecedented performance, for example, a power gain of 15 dB at 190–235 GHz and a noise level of 1.2 dB with 7.2‐dB gain in the 90‐GHz range, have been demonstrated. Passivation process is of fundamental importance in the stability, good performance, and extension of device operative lifetime. We discuss strategies used to passivate the surface of GaAs and related compounds and GaN in the context of FETs. Recent research on the enhancement‐mode PMODFET (E‐PMODFET) variety for applications in high‐speed and low‐power digital circuits and power amplifiers with single power supply is described. Reliability of MOSFET based on GaAs is reviewed to some extent. Scalability issues as well as progress in FinFET‐based on InGaAs channel are summarized. Also to be noted is that III–V compound semiconductors as an alternative to Si as the channel material to improve the performance of metal‐oxide–semiconductor field‐effect transistors (MOSFETs) on Si platforms are a very attractive option for the next‐generation high‐speed integrated circuits but face serious challenges because of the lack of a high‐quality and natural insulator.

III‐Nitride‐based HFETs showed tremendous performance in both high‐power RF and power‐switching applications. AlGaN/GaN‐based high‐power HFETs on SiC substrate with 60‐nm gate lengths have achieved maximum oscillation frequency of 300 GHz. On‐resistance of 1.1–1.2 Ω mm as well as drain current of ∼0.9 A/mm was also achieved. For HFET devices operated in class AB mode on GaN semiinsulating substrates, a continuous‐wave power density of 9.4 W/mm was obtained with an associated gain of 11.6 dB and a power‐added efficiency of 40% at 10 GHz. III‐Nitride devices for power‐switching application have achieved near‐theoretical limit for vertical devices‐based GaN native substrates and breakdown voltage as high as 1200 V and on‐resistance as low as 9 mΩ‐cm
2
for lateral HFET devices on low‐cost silicon substrates. Because of the much larger 2DEG density in lattice‐matched InAlN/GaN HFETs, drain current as high as 2 A/mm was demonstrated, and the highest current gain cutoff frequency of 370 GHz was also reported on 7.5‐nm‐thick In
0.17
Al
0.83
N barrier HFETs. The very low on‐resistance allows high drain current, but it is subject to the junction temperature the devices can tolerate and is also restricted by the thermal expansion mismatch of the GaN‐on‐Si structures. Normally‐on and Normally‐off GaN HFETs with breakdown voltages in the range of 20–900 V are already commercially available. However, their competitivity against Si‐based IGBT and super junction MOSFETs and SiC‐FETs would depend on several factors such as voltage derating (used voltage versus the breakdown voltage), long‐term reliability, and cost.

The advent of high‐quality SiGe layers on Si substrates has paved the way for the exploration and exploitation of heterostructure devices in an Si environment. MODFETs based on the Si/SiGe have been achieved with extraordinary
p
‐channel performance. With 0.25‐μm gate lengths, the current gain cutoff frequency is about 40 GHz. When the gate length was reduced to 0.1 μm, the current gain cutoff frequency increased to about 70  GHz. MODFETs based on Ga
2
O
3
, especially β‐Ga
2
O
3
, have attracted a good deal of interests by the potential high breakdown voltage of Ga
2
O
3
but suffer from limitations imposed by both low electron mobility (affects efficiency and loss) and low thermal conductivity, hindering heat dissipation.