Superjunction has arguably been the most creative and important concept in the power device field since the introduction of the insulated gate bipolar transistor (IGBT) in the 1980s. It is the only concept known today that has challenged and ultimately proved wrong the well-known theoretical study on the limit of silicon in high-voltage devices. This paper deals with the history, device and process development, and the future prospects of Superjunction technologies. It covers fundamental physics, technological challenges as well as aspects of design and modeling of unipolar devices, such as CoolMOS. The superjunction concept is compared to other methods of enhancing the conductivity of power devices (from bipolar to employment of wide-bandgap materials) to derive its set of benefits and limitations. This paper closes with the application of the superjunction concept to other structures or materials, such as terminations, superjunction IGBTs, or silicon carbide Field Effect Transistors (FETs).

Superjunction Power Devices, History, Development, and Future Prospects

A process for an enhancement-mode p-GaN-gated heterostructure field-effect transistor with self-aligned structuring of the p-GaN is proposed. A gate-first process is employed, for which the gate metallization acts as contact and etch mask simultaneously. In the access region, the p-GaN is selectively removed in a dry-etch process by a Cl2/N2/O2 gas mixture. Due to self-aligned processing and precise etch depth control, devices achieve a high saturation current of 554 mA $\cdot$ mm−1, a threshold voltage of 1.08 V, breakdown voltages up to 560 V, and dynamic $R_{ON}$ increase of 45% for 200-V stress bias.

Self-Aligned Process for Selectively Etched p-GaN-Gated AlGaN/GaN-on-Si HFETs

Dynamic $\text {R}_{\mathrm{\scriptscriptstyle ON}}$ and ramped substrate bias measurements are used to demonstrate size- and geometry-dependent dispersion in power transistors. This is due to a novel lateral transport mechanism in the semi-insulating carbon-doped GaN buffer in AlGaN/GaN high-electron-mobility transistors. We propose that the vertical field generates a 2-D hole gas (2DHG) at the bottom of the GaN:C layer, with hole flow extending outside the isolated area. The device-to-device variation is due to a combination of widely spaced preferential leakage paths through the structure and lateral transport from those paths to trapping sites. The spread of the 2DHG outside the active area of the device strongly affects the result of substrate ramp measurements producing major differences between single and multifinger devices. In dynamic $\text {R}_{\mathrm{\scriptscriptstyle ON}}$ recovery measurements, single-finger devices show large device-to-device variation, with multifinger devices showing a small variation with the transient comprising the superposition of the recovery transient of multiple small single-finger devices.

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Lateral Charge Transport in the Carbon-Doped Buffer in AlGaN/GaN-on-Si HEMTs

GaN-based devices are seen as ideal candidates for power-switching applications. For the acceptance of GaN-based devices by module designers, obtaining enhancement-mode (e-mode) behavior in GaN-based heterostructure field-effect transistors (HFETs) has long been in the focus. Although the gate-injection approach appears to be the most promising one to achieve e-mode devices, using a double heterostructure in conjunction with a gate insulator has still its advantages, such as steeper turn-ON characteristics and lower leakage currents. An analytical expression to predict the threshold voltage $V_{\mathrm {th}}$ for a given double heterostructure device has not yet been derived. Moreover, neither an evaluation of the tradeoff between $V_{\mathrm {th}}$ and ON-state resistance $R_{{\text {ds}},\mathrm {\scriptstyle {{ON}}}}$ has been performed to date. This paper addresses these two aspects. We will show an analytical expression for a metal–insulator–semiconductor double HFET (MIS-DHFET), which in certain cases is also valid for the gate-injection transistor. On the basis of this, we will discuss the actual influence of the Al concentration in the backbarrier on $V_{\mathrm {th}}$ . We will further employ technology computer aided design (TCAD) device simulations to evaluate the impact on $R_{{\text {ds}},\mathrm {\scriptstyle {{ON}}}}$ when using MIS-DHFETs. It will be shown that by implementing a double heterostructure in MIS devices, it is possible to suppress the typically observed negative $V_{\mathrm {th}}$ –oxide thickness relationship while maintaining a constant $R_{{\text {ds}},\mathrm {\scriptstyle {{ON}}}}$ .

The III-Nitride Double Heterostructure Revisited: Benefits for Threshold Voltage Engineering of MIS Devices

High-efficiency multijunction laser power converters are demonstrated for low temperature applications with an optical input at 808 nm. The photovoltaic power converting III-V semiconductor devices are designed with GaAs absorbing layers, here with 5 thin subcells (PT5), connected by transparent tunnel junctions. Unprecedented conversion efficiencies of up to 74.7% are measured at temperatures around 150 K. At temperatures around 77 K, a remarkably low bandgap offset value of Woc = 71 mV is obtained at an optical input intensity of ~7 W/cm2. At 77 K, the PT5 retains an efficiency of 65% with up to 0.3 W of converted output power.

/pdf/74-7-efficient-gaas-based-laser-power-converters-at-808-nm-3kkh7y38.pdf

74.7% Efficient GaAs-Based Laser Power Converters at 808 nm at 150 K

GaN-based heterostructure FETs (HFETs) featuring a 2-D electron gas (2DEG) can offer very attractive device performance for power-switching applications. This performance can be assessed by evaluation of the dynamic on-resistance Ron,dyn vs. the breakdown voltage Vbd. In literature, it has been shown that with a high Vbd, Ron,dyn is deteriorated. The impairment of Ron,dyn is mainly driven by electron injection into surface, barrier, and buffer traps. Electron injection itself depends on the electric field which typically peaks at the gate edge towards the drain. A concept suitable to circumvent this issue is the charge-balancing concept which employs a 2-D hole gas (2DHG) on top of the 2DEG allowing for the electric field peak to be suppressed. Furthermore, the 2DEG concentration in the active channel cannot decrease by a change of the surface potential. Hence, beside an improvement in breakdown voltage, also an improvement in dynamic behaviour can be expected. Whereas the first aspect has already been de...

Charge balancing in GaN-based 2-D electron gas devices employing an additional 2-D hole gas and its influence on dynamic behaviour of GaN-based heterostructure field effect transistors

Photovoltaic cells based on In0.53Ga0.47 As absorber material grown lattice-matched on InP substrates are developed, targeting laser power conversion around 1550 nm in the optical C-band. Single- and multi-junction devices are fabricated and characterized. In addition, a thin film route which allows for the implementation of a back surface reflector is developed based on wet-chemical etching of the InP substrate. The influence of the junction type (homo-, front-hetero-, rear-heterojunction) on luminescence coupling in 2-junction devices is explored. Also, a first 10-junction device is investigated. First experimental results will be presented in this conference contribution.

Unlocking 1550 nm Laser Power Conversion by InGaAs Single- and Multi-Junction PV Cells

Photonic power converters (PPCs) attract more and more attention as they enable galvanically isolated optical power transmission. In many power by light applications, conversion efficiency is the key metric for PPCs. A remarkable efficiency of 68.9% was recently achieved by utilizing Fabry-Perot resonances in a GaAs PPC. In this work, we extend this idea of leveraging optical resonance towards strong grating coupled guided mode resonances. A proof of concept is demonstrated by ultrathin GaAs PPCs with an absorber thickness of 160 nm with nanostructured photonic reflectors. Reference ultrathin devices on substrate and with planar mirror show close-to-bandgap EQE of 13% and 20%, respectively, while guided modes resonances yield an EQE of 45% at 840 nm for devices with photonic mirror. Devices were realized on full 4-inch wafers, using processes compatible with industrial manufacturing.

Meike Schauerte

Papers

Charge balancing in GaN-based 2-D electron gas devices employing an additional 2-D hole gas and its influence on dynamic behaviour of GaN-based heterostructure field effect transistors

Unlocking 1550 nm Laser Power Conversion by InGaAs Single- and Multi-Junction PV Cells

Realization of Ultrathin GaAs Photonic Power Converters with Rear-Side Metal Grating on Full 4” Wafers