Herbert Kroemer, who passed away on 8 March 2024 at the age of 95, was a towering figure within the electron device research and development community for more than five decades. His work has had a massive influence on both electronic and optoelectronic device development. Every cell phone, every fiber optic link bears his stamp. Many foundational concepts of our electron devices world were first introduced by Herb. He was the earliest and most accomplished advocate of heterojunctions between different semiconductor materials for the improvement of devices, and heterojunctions have become one of the central themes of semiconductor research.

We report on GaN-based field effect transistors with laterally-gated multiple 2DEG channels, called BRIDGE (buried dual gate) HEMT. A unique epi/device structure and operation of the BRIDGE HEMT demonstrate device characteristics suitable for efficient and linear millimeter-wave power amplifier MMIC applications. The BRIDGE HEMT reported in this work consists of 6 Si-modulation-doped AlGaN/AIN/GaN/AlGaN quantum well channels with a net 2DEG density of 4.8 × 10 13 cm -2 and electron mobility of 1800 cm 2 /V·s. Co-optimization of the multi-channel epitaxial design and device process enabled to use Schottky-gates with lateral field plates in the BRIDGE HEMTs. The transistor exhibited good channel pinch-off at a threshold voltage of −5 V, flat $\boldsymbol{I}_{\mathbf{d}}-\boldsymbol{V}_{\mathbf{d}\mathbf{s}}$ curves, an off-state breakdown voltage of $> 30\mathbf{V}$ ) a knee current density of 1.6 A/mm and a transconductance of 0.5 S/mm at a gate voltage of 1.0 V. Load-pull measurement performed at 30 GHz exhibited a maximum output power density $(\boldsymbol{p}_{\mathbf{o}\mathbf{u}\mathbf{t}})$ of 7.1 W/mm at a quiescent drain voltage of 14 V, corresponding to a dynamic knee current density of 2 A/mm, with a power-added efficiency (PAE) of 39% under class-AB operation.

A unique combination of high mobility, high velocity and high sheet density of the 2DEG formed in GaN-based heterostructures has enabled GaN-based HEMTs to be used in a wide range of applications from RF power amplifiers to efficient power converters. Today’s complex communication systems require transceivers to process RF signals efficiently with large bandwidth and high fidelity. While GaN-based HEMT technology has advanced to reach higher power densities, it has not fundamentally changed the power requirements for the linearity performance. To address fundamental limitations of HEMT’s power/linearity/efficiency/frequency tradeoff, we proposed a transistor structure called BRIDGE FET (buried dual gate FET) where gate electrodes are buried into AlGaN/GaN heterostructures and contact laterally with multiple 2DEG channels [1]. A deliberate elimination of a conventional top-contact gate leads to a unique device operation principle and performance advantages for improved linearity and efficiency at large signal operations; (1) The drain-source current is controlled solely by modulating the width of the 2DEG channels by the lateral gate electric field while maintaining the 2DEG density. (2) The MESFET-like device operation enables gradual pinch-off, greatly reducing g m derivatives near pinch-off. (3) Lack of density modulation with V gs leads to a constant electron velocity at high electric field, eliminating a typical g m roll-off at high V gs. This results in a constant gain along a resistive load line. (4) The buried gates forms Schottky contacts to the GaN channels below the 2DEG layers. This enhances electron confinement and improves electrostatic isolation between the source and drain, significantly reducing g d at high V ds. (5) Elimination of the top-contact gate prevents electrons from being trapped on the surface, suppressing current collapse at high voltage operations. (6) An absence of inverse piezoelectric effect due to the reduced vertical electric field at the drain-side of the gate improves device reliability under high voltage stress. In this presentation, current status of our development of multi-2DEG channel BRIDGE FETs will be discussed. This work was sponsored by DARPA-MTO DREaM program under DARPA/CMO Contract No. FA8650-18-C-7807. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the Defense Advanced Research Projects Agency or the U.S. Government. [1] K. Shinohara et al., IEEE EDL, vol. 39, no. 3, p. 417, March 2018.

In this letter, we report on GaN-based field-effect transistors with laterally gated two-dimensional electron gas (2DEG). The drain current of the transistor is controlled solely by modulating the width of the 2DEG between buried gates. The lateral Schottky gate contact to the GaN channel layer enhances electron confinement by raising electrostatic potential below the 2DEG, improving isolation between the source and drain. Complete elimination of a top-contact gate reduces the density of trapped electrons near the surface and alleviates capacitive coupling between the trapped electrons and the 2DEG. Owing to the unique device structure and operation principle, the 150-nm-gate transistors with a channel width of 250 nm demonstrated: extremely small output conductance, drain-induced barrier lowering, knee voltage, and knee current collapse, greatly reduced gm derivatives near threshold, and nearly constant RF gain along the resistive load line. Furthermore, a preliminary accelerated life test indicated enhanced device reliability due to an absence of the inverse piezoelectric effect. The proposed transistors hold great promise for realizing reliable and efficient power amplifiers with improved transistor linearity.

A millimeter (mm) wave amplifier that combines both high-power output and a wide bandwidth would enable dramatic improvements in areas where high-power radio signals with significant information content are required. However, to date, the only available devices are either high-power vacuum electronic devices with limited bandwidths, or larger bandwidth semiconductor amplifiers with low-power output. Here, we show a new design of a vacuum electronic traveling-wave tube amplifier that has a measured output power of 107 W with a 3-dB bandwidth of 6 GHz centered on 200 GHz. The power-bandwidth product of this amplifier is 642 W-GHz. These results provide a demonstration of a high-power mm-wave source with significant bandwidth. This device is readily scalable to other frequencybands, particularly those in themm-and sub-mmwave frequency bands from 100 GHz to 1 THz.

Due to its remarkably high Johnson figure of merit, gallium nitride (GaN) has become the material of choice for applications requiring high output power at high frequencies. In this paper, we have reviewed high-speed switching work to date and we demonstrate a 28 V, 865-MHz switching speed step-down converter that can be applied to envelope tracking (ET). We report a GaN monolithic microwave integrated circuit technology utilized to fabricate a gate driver IC at a switching speed > 1 GHz with <;95-ps transitions, monolithically integrated with a 100 V breakdown GaN power switch. The on-chip integration of the power switch and gate driver allows in-circuit characterization of the power switch switching speed. Using the integrated gate driver and switch, we report a slew rate of the power switch of 152 V/ns at 50 V, which enables a down converter IC capable of ET with over 50-MHz tracking bandwidth. This paper shows the first demonstration of a GaN low-side gate driver capable of > 1-GHz switching, monolithic integration with a 0.15-μm T-gate suitable for up to Ka-band operation, and a > 100 V breakdown voltage power switch on the same chip.

Envelope tracking (ET) is an appealing alternative to the widely used Doherty power amplifier (PA) due to its potential to increase efficiency, particularly for high data rate transmissions. In this work, we demonstrate a 28 V, 865 MHz switching speed step-down converter that can be applied to envelope tracking. The converter's ET capability is demonstrated at up to 865 MHz PWM carrier frequency. An integrated RF GaN HEMT and 100 V GaN power switch process is reported, which enables fabrication of a power switch and gate driver on the same IC, necessary for achieving the 865 MHz switching speed. We report a slew rate of the main power switch of 152 V/ns at 50 V, which enables ET with up to 20% less power than that required without ET. To the authors' best knowledge, this is the first demonstration of a GaN MMIC process that integrates a GaN HEMT suitable for X through Ka-band power amplifiers and a high voltage power switch on the same IC.

Short-Abstract—Recent advances in InP semiconductor device design and high-yield processing have led to the demonstration of transistors with power gain cut-off frequencies (fmax) greater than 1 THz. More importantly, the devices are supported by low-loss interconnects, accurate models and robust design kits. Employing a systematic design methodology that includes significant EM and thermal modeling, we have demonstrated circuit components for transceivers up to 670 GHz and power amplifiers with >100mW of output power operating at 230 GHz. Insertion of these high frequency chips requires additional breakthroughs in packaging, and thermal management. Other applications of the technology would allow the high gain-bandwidth to be traded for high dynamic range and efficiency at mmWave frequencies.

We report on progress in developing a travelling wave tube amplifier with significant gain and power at 220 GHz. This paper provides an overview of the program, describing fabrication and test of slow-wave structures with bandwidths exceeding 50 GHz centered at 220 GHz, the production of a sheet electron beam, development of a solid state preamplifier delivering 50 mW to the tube with > 17 dB of gain and beam-wave simulation of the entire circuit leading to expected output powers of over 50 W. Two further papers from the group are also submitted to IVEC: from UC Davis describing the interaction structure fabrication and hot test, and from CPI describing the sheet electron beam, TWT design and beam - wave simulations. The tube is currently under test and results will be reported in this paper.

Gallium Nitride is an attractive material for a high voltage handling, high current drive power switch technology. More precisely the high electron mobility and large bandgap of GaN offers a technology with a low Ron*Qg product of 50 mOhm-nC with ~600V blocking. However, the most mature device is a lateral GaN high electron mobility transistor (HEMT) that presents scaling and packaging challenges. In addition, enhancement mode operation remains a challenge in a manufacturable process that retains the two key benefits of breakdown and drive current.

Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs) are thermally limited much below the electrical capability of the devices. The unique challenge of a GaN HEMT is its ultra-high heat flux at the micro-scale gate fingers. The traditional packaging and base plate level liquid cooling have limited capability and is far from the heat source, resulting in high thermal resistance from the device junction to the coolant, and ultimately limiting the RF power. We present a device-level high heat flux cooling solution with liquid micro-jet impingement within ~100 μm distance of the heat source. A preliminary demonstration of the technique on a GaN-on-Silicon device shows 50% higher heat dissipation capability, compared to the state-of-the-art pin fin base plate liquid cooling, while maintaining the device junction temperature at 150°C. If the dissipation power level is held constant at 35 W of dissipation power, the technique reduces the device junction temperature by 45°C.

We have demonstrated 40-nm In0.7Ga0.3As Metamorphic HEMTs (MHEMTs) with a record value in f(T). The devices feature a Pt gate sinking process to effectively thin down the In0.52Al0.48As barrier layer, together with dual Si d-doping in the barrier to lower the potential barrier in the S/D access region. The fabricated device with L-g = 40-nm exhibits V-T = 0.05 V, g(m,max) = 2.7 mS/mu m, f(T) = 688 GHz and f(max) = 800 GHz. In addition, we have developed an analytical model of f(T) in a III-V HEMT based on a small-signal equivalent circuit, which provides an excellent agreement with measured f(T). This in turns guides a realistic way to further improve f(T) beyond THz.

We report on the design and fabrication of a sheet beam traveling-wave tube (TWT) amplifier being developed under the DARPA HiFIVE program. The device is designed to achieve over 50 W output power at 220 GHz with a power-bandwidth product exceeding 1 kW-GHz.

Abstract We have demonstrated 50-nm enhancement-mode (E-mode) In0.7Ga0.3As PHEMTs with fmax in excess of 1 THz. The devices feature a Pt gate sinking process to effectively thin down the In0.52Al0.48As barrier layer, together with a two-step recess process. The fabricated device with Lg = 50-nm exhibits VT = 0.1 V, gm,max = 1.75 mS/μm, fT = 465 GHz and fmax = 1.06 THz at a moderate value of VDS = 0.75 V. In addition, we have physically modeled the abnormal peaky behavior in Mason’s unilateral gain (Ug) at high values of VDS. A revised small signal model that includes a shunting Rgd-NDR with negative value successfully describes the behavior of the device from 1 to 67 GHz. .

We have demonstrated 50-nm enhancement-mode (E-mode) In0.7Ga0.3As PHEMTs with f(max) in excess of 1 THz. The devices feature a Pt gate sinking process to effectively thin down the In0.52Al0.48As barrier layer, together with a two-step recess process. The fabricated device with L-g = 50-nm exhibits V-T = 0.1 V, g(m,max) = 1.75 mS/mu m, f(T) = 465 GHz and f(max) = 1.06 THz at a moderate value of V-DS = 0.75 V. In addition, we have physically modeled the abnormal peaky behavior in Mason's unilateral gain (U-g) at high values of V-DS. A revised small signal model that includes a shunting Rgd-NDR with negative value successfully describes the behavior of the device from 1 to 67 GHz.

Indium phosphide heterojunction bipolar transistors (HBTs) find applications in very wide-band digital and mixed-signal integrated circuits (ICs). Devices fabricated in high-yield process flows at 500 nm feature size obtain 450 GHz cutoff frequencies and 5 V breakdown and enable high yield fabrication of integrated circuits having more than 3000 transistors. Laboratory devices at 250 nm feature size obtain 755 GHz . We describe device and circuit bandwidth limits associated with HBTs, develop scaling roadmaps for HBTs having lithographic minimum feature sizes between 512 and 64 nm, and identify key technological challenges in realizing 480-GHz digital ICs and 1000-GHz amplifiers. Key features of manufacturable self-aligned dielectric sidewall processes are described in detail.

In the above titled paper (ibid., vol. 96, no. 2, pp. 271-286, Feb 08), there are errors. Corrections are presented here.

Our direct growth approach of integrating compound semiconductors (CS) and silicon CMOS is based on a unique silicon template wafer with an embedded CS template layer of Germanium (Ge). It enables selective placement of CS devices in arbitrary locations on a Silicon CMOS wafer for simple, high yield, monolithic integration and optimal circuit performance. HBTs demonstrate a peak current gain cutoff frequency ft of 170GHz at a nominal collector current density of 2mA/μm2. To the best of our knowledge this represents the first demonstration of an InP-based HBT fabricated on a silicon wafer.