J.A. del Alamo

On-state breakdown in power HEMTs: measurements and modeling

A new definition of and measurement technique for on-state breakdown in high electron mobility transistors (HEMTs) is presented. The new gate current extraction technique is unambiguous, simple, and non-destructive. Using this technique in conjunction with sidegate and temperature-dependent measurements, we illuminate the different roles that thermionic field emission and impact ionization play in HEMT breakdown. This physical understanding allows the creation of a phenomenological model for breakdown, and demonstrates that depending on device design, either on-state or off-state breakdown can limit maximum power.

A new gate current extraction technique for measurement of on-state breakdown voltage in HEMTs

We present a new simple three-terminal technique for measuring the on-state breakdown voltage in HEMTs. The gate current extraction technique involves grounding the source, and extracting a constant current from the gate. The drain current is then ramped from the off-state to the on-state, and the locus of drain voltage is measured. This locus of drain current versus drain voltage provides a simple, unambiguous definition of the on-state breakdown voltage which is consistent with the accepted definition of off-state breakdown. The technique is relatively safe and repeatable so that temperature dependent measurements of on-state breakdown can be carried out. This helps illuminate the physics of both off-state and on-state breakdown.

Nonlinear source and drain resistance in recessed-gate heterostructure field-effect transistors

We have profiled the parasitic source and drain resistances versus current in recessed-gate HFETs with heavily-doped caps, using an InAlAs/n/sup +/-InP HFET as a vehicle. We observe a dramatic reduction in the parasitic resistances at moderate-to-high currents as significant current passes through the cap. Consequently, we note very little dependence in g, on the length of the extrinsic gate-source region. This is an experimental verification of predictions of two-layer models in the literature.

Mesa-sidewall gate leakage in InAlAs/InGaAs heterostructure field-effect transistors

InAlAs/InGaAs HFETs fabricated by conventional mesa isolation have a potential parasitic gate-leakage path where the gate metallization overlaps the exposed channel edge at the mesa sidewall. The existence of this path has been proven by fabricating special heterojunction diodes with different mesa-sidewall gate-metal overlap lengths. It is found that sidewall leakage is a function of the crystallographic orientation of the sidewall, and increases with channel thicknesses, sidewall overlap area, and InAs mole fraction in the channel. In HFETs fabricated alongside the diodes, sidewall leakage increased the subthreshold and forward gate leakage currents, and reduced the breakdown voltage. >

A new physical model for the kink effect on InAlAs/InGaAs HEMTs

New measurements providing direct evidence linking the kink effect and impact ionization in InAlAs/InGaAs HEMTs are reported. Current kink models are not consistent with our findings. We propose a new mechanism, barrier-induced hole pile-up at the source, to explain the kink. The new model is shown to be consistent with both room temperature and low temperature measurements. These results allow formulation of a simple equivalent circuit model of the kink.

Off-state breakdown in power pHEMTs: the impact of the source

Summary form only given. Conventional wisdom suggests that in pseudomorphic high electron mobility transistors (pHEMTs), the field between the drain and the gate determines off-state breakdown, and that the drain to gate voltage therefore sets the breakdown voltage of the device. Thus, the two terminal breakdown voltage is a widely used figure of merit, and most models for breakdown focus on the depletion region in the gate-drain gap, while altogether ignoring the source. We present new measurements and simulations that demonstrate that for power pHEMTs, the electrostatic interaction of the source seriously degrades the devices gate-drain breakdown, and must be taken into consideration in device design. As a vehicle for this study we have used a state-of-the-art L/sub G/=0.25 /spl mu/m double heterostructure pHEMT with excellent power performance (P/sub 0/= 1W, Gain= 11 dB, and PAE=60% at 10 GHz for W/sub G/=1200 /spl mu/m) and high breakdown voltage (BV/sub DG/=21 V at I/sub D/=1 mA/mm).

FAR-INFRARED RADIATION-INDUCED THERMOPOWER IN A QUANTUM POINT CONTACT

We have observed a far‐infrared (0.3–2.5 THz) radiation‐induced photovoltaic signal in an antenna‐coupled quantum point contract, which oscillates with the gate voltage and peaks at the onset of each subband. The polarity of this photovoltaic signal can be reversed by shifting the far‐infrared beam from the drain to the source, or vice versa. This signal has been unambiguously attributed to thermopower generated by asymmetric heating of the drain and the source by the far‐infrared radiation. Quantitative agreement has been obtained between measurements and calculations based on ballistic transport in one‐dimensional electron systems and the electrical and thermal circuit elements in our experimental system.

Temperature dependence of breakdown voltage in InAlAs/InGaAs HEMTs: theory and experiments

We present results of an experimental and theoretical study of the temperature dependence of the off-state breakdown voltage of InAlAs/InGaAs high electron mobility transistors (HEMTs). We find that the breakdown voltage (BV) has a negative temperature coefficient that is more prominent for lower values of the extrinsic sheet carrier concentration (n/sub s/). Structural parameters such as the insulator thickness and top-to-bottom delta doping ratio have little effect on BV if n/sub s/ is held constant. These results are consistent with an extension of a new tunneling model for breakdown in HEMTs to include thermionic-field emission.

Impact ionization and light emission in InAlAs/InGaAs heterostructure field-effect transistors

We present measurements on impact ionization effects, real space transfer of holes and electrons, and light emission occurring in n-channel InAlAs/InGaGs heterostructure Field-Effect Transistors based on InP operated at high electric fields and at different temperatures. The channel electrons heated by the lateral electric field give rise to impact ionization and light emission. By comparing the electrical characteristics and the integrated light intensity in different energy ranges and at different temperatures, we were able to identify two main different light emission mechanisms: conduction to conduction-band transitions for low energy photons and conduction to valence-band transitions for high energy photons. The correlation between the gate current and the light intensity allowed us to separately evaluate the electron and hole components of the gate current. >

High quality InGaAs/InP and InAlAs/InP heterostructures beyond the Matthews-Blakeslee critical layer thickness

Mismatched epitaxial layers of InGaAs and InAlAs were grown on InP by molecular beam epitaxy. Double-crystal X-ray diffraction measurements show that the crystalline quality of the layers consistently remains unperturbed to thicknesses up to 3-8 times the Matthews-Blakeslee critical layer thickness. The findings are applied to the growth of high-performance mismatched InAlAs/InGaAs/InP heterostructure field-effect transistors.<<ETX>>