We report the direct current (DC) and microwave performance of BeMgZnO/ZnO heterostructure field effect transistors (HFETs) on sapphire substrates. The devices fabricated using Al2O3 as the gate dielectric with a gate length of 1.5 um and a gate width of 75 um exhibited a pinch-off voltage of −4.0 V and a maximum peak transconductance of 63 mS/mm. A current gain cutoff frequency fT of 5.0 GHz was achieved, highest among ZnO-based FETs. The corresponding electron velocity of above 1E7 cm/s estimated based on the gate transit time inches closer to the theoretical peak velocity in ZnO (3.5E7 cm/s). This value is significantly higher than the previously reported values in ZnO-based HFETs, which is attributed to the two-dimensional electron gas (2DEG) concentration at or near the resonance of longitudinal optical (LO)-phonon and plasmon frequencies as well as the improved quality of the heterostructure owing to optimized ZnO buffer growth and BeO and MgO alloying in the barrier. To probe the high-frequency response of the HFETs, extrinsic and intrinsic parameters of the small-signal equivalent circuit for the BeMgZnO/ZnO HFETs were investigated using the hybrid extraction method.
Accumulation of non-equilibrium hot longitudinal optical (LO) phonons limits the electron drift velocity for electronic devices operating under high electric field. Ultrafast decay of hot phonons can take place via plasmon-LO phonon resonance, which leads to fast electron energy relaxation and hence high electron drift velocity and optimum operation of the devices. This need motivates us to create heterostructures with 2DEG density close to the plasmon-LO phonon resonance region. Through incorporating a few percent of Be into the BeMgZnO barrier to switch the strain sign in the barrier from compressive to tensile, we have achieved 2DEG densities over a wide range in Zn-polar BeMgZnO/ZnO heterostructures with moderate Mg content (below 30%) grown by molecular beam epitaxy. We have obtained electron mobility of 250 cm2/Vs at room temperature (293 K) and 1800 cm2/Vs at 13 K in Be0.02Mg0.26ZnO/ZnO heterostructures. Via capacitance-voltage (CV) spectroscopy, we have explored the depth profiles of the apparent carrier density of samples grown under different conditions. The correlations between electrical properties and MBE growth parameters of Zn-polar BeMgZnO/ZnO heterostructures are discussed.
Advances in frequency performance of heterostructure field-effect transistors (HFETs) are discussed in terms
of dissipative processes. The conditions for fastest dissipation coincide reasonably well with those for fastest
operation and slowest device degradation. The correlation has its genesis in dissipation of the hot-phonon heat
accumulated by non-equilibrium optical phonons launched by hot electrons. The hot-phonon heat causes defect
formation and additional electron scattering in a different manner as compared with the effects due to conventional
heat accumulated by acoustic phonons. The desirable ultrafast conversion of hot phonons into acoustic
phonons is assisted by plasmons as demonstrated through measurement of hot-phonon lifetime. Signatures of
plasmons have been also resolved in hot-electron transport, transistor frequency performance, phase noise, and
device reliability. The plasmon-assisted ultrafast dissipation of hot-phonon heat explains the known necessity
for application a stronger negative gate bias to a channel with higher as-grown electron density.
Low-frequency noise and current-transient measurements were applied to analyze the degradation of nearly latticematched
InAlN/AlN/GaN heterostructure field-effect transistors caused by electrical stress. Almost identical devices
on the same wafer were stresses 7 hr. at a fixed DC drain bias of VDS=20 V and different gate biases. We noted up to
32 dB/Hz higher low-frequency noise for stressed devices over the entire frequency range of 1 Hz- 100 kHz. The
measurements showed the minimum degradation at a gate-controlled two-dimensional electron gas density of
9.4x1012 cm-2. This result is in good agreement with the reported stress effect on drain-current degradation and
current-gain-cutoff-frequency measurements, and consistent with the ultrafast decay of hot-phonons due to the
phonon-plasmon coupling. Moreover, the current transient (gate-lag) measurements were also performed on pristine
and highly degraded devices up to 5 ms pulse durations. Drain current is almost totally lost in degraded HFETs as
opposed to a very small drop for pristine devices and no recovery observed for both indicating that generation of
deep traps in GaN buffer.
The effect of active layer design on the efficiency of InGaN light emitting diodes (LEDs) with the light emission in blue
(~420 nm) has been studied. Correlation between the internal quantum efficiency (IQE) and relative external quantum
efficiency (EQE) and salient features of structures on c-plane InGaN LEDs which contain multiple quantum wells
(MQWs) of different barrier height (either In0.01Ga0.99N or In0.06Ga0.94N barriers) and thickness (3 nm and 12 nm) as well
as different double heterostructure (DH) designs (3 nm, dual 3 nm, 6 nm, dual 6 nm, 9 nm and 11 nm) with inserted 3
nm In0.06Ga0.94N barrier. Pulsed electroluminescence (EL) and optical excitation power-dependent photoluminescence
(PL) measurements indicated that the thinner and lower In0.06Ga0.94N barriers bode well for high EQE and IQE.
Furthermore, increase of the effective active region thickness by multiple InGaN DH structures (dual, quad and hex)
separated by 3 nm In0.06Ga0.94N barriers is promising at high injection levels. Although increasing the single DH
thickness from 3 to 6 nm improves the peak relative EQE by nearly 3.6 times due to increased density of states and
increased emitting volume, the IQE suffers a nearly 30% loss. Further increase in the DH thickness to 9 and 11 nm
results in a significantly slower rate of increase of EQE with current injection and lower peak EQE values presumably
due to degradation of the InGaN layer. Increasing the number of 3 nm DH active regions with 3 nm In0.06Ga0.94N
barriers improves EQE, while still maintaining high IQE (above 95% at a carrier concentration of 1018 cm-3) and
showing negligible EQE degradation up to 550 A/cm2 due to increased emitting volume and high radiative
recombination coefficients and high IQE.
InGaN light emitting diodes (LEDs), which have become key components of the lighting technology owing to
their improved power conversion efficiencies and brightness, still suffer from efficiency degradation at high
injection levels. Experiments showing sizeable impact of the barrier height provided by an electron blocking layer
(EBL) or the electron cooling layer prior to electron injection into the active region strongly suggest that the electron
overflow resulting from ballistic and quasi-ballistic transport is the major cause of efficiency loss with increasing
injection. Our previous report using a first order simple overflow model based on hot electrons and constant LO
phonon scattering rates describes well the experimental observations of electron spillover and the associated
efficiency degradation in both nonpolar m-plane and polar c-plane LEDs with different barrier height EBLs and
electron injection layers. LEDs without EBLs show three to five times lower efficiencies than those with
Al0.15Ga0.85N EBLs due to significant electron overflow to the p-type region in the former. For effective means of
thermalization in the active region within their residence time and possibly longitudinal optical phonon lifetime, the
electrons were cooled prior to their injection via a staircase electron injector, i.e. an InGaN staircase structure with
step-wise increased In composition. The investigated m-plane and c-plane LEDs with incorporation of staircase
electron injector show comparable electroluminescence performance regardless of the status of EBL. This paper
discusses hot electron effects on efficiency loss, means to cool the electrons prior to injection.
The electric fields present in a heterostructure field effect transistor (HFET) give rise to large densities of
hot electrons and hot phonons not in equilibrium with the lattice (thus the term "hot"). Hot electrons give
rise to an accumulation of hot phonons, which play a deleterious role in device performance and reliability,
particularly as 2DEG densities increase. Thus, it is important that hot phonon effects be mitigated. The hot
phonon effect arises due to the fact that LO phonon (longitudinal optical phonon) scattering is the primary
scattering mechanism at high fields in GaN, and that LO phonons tend to accumulate in the channel due to
long lifetimes and low group velocity; minimizing the existence in time of hot phonons in the channel is the
overall goal. That said, the lifetime of the LO phonons decaying into propagating LA modes is not constant,
and it is through the exploitation of this fact that one gains the ability to enhance the performance and
reliability of HFETs. This has been evidenced by operating HFETs at particular bias conditions which tend
to lend themselves to relatively short LO phonon lifetimes. Alternatively, minimization of hot phonon
lifetimes may be achieved through novel device designs. In this work, we outline designs for minimizing
hot phonon effects in HFET devices with sheet densities >2 x 1013 cm-2. We propose "camel-back"
structures with wavefunction "spreading layers" to maximize 2DEG charge while mitigating hot phonon
effects arising from high density 2DEGs.
A bottleneck for heat dissipation is discussed in terms of plasmon-assisted decay of nonequilibrium longitudinal
optical phonons launched by hot electrons. According to experiment at low and moderate electric fields, the
fastest decay takes place at electron density of ~6.7×1012 cm-2 and ~2.7×1012 cm-2 for heterostructures with
2DEG channels located in GaN and GaInAs, respectively. Hot-electron temperature and gate voltage can be
used to shift the optimal density. A GaN-based heterostructure field effect transistor degrades slower when
dissipation of the LO-mode heat is faster.
Non-equilibrium (hot) occupancy of longitudinal-optical (LO) phonon modes is considered for InAlN/AlN/GaN
and related structures with two-dimensional electron gas (2DEG) channels located in GaN and subjected to
high electric fields. The channel operation is found to depend critically on the LO-mode occupancy controlled
by hot-phonon lifetime. The lifetime dependences on the occupancy and the 2DEG density are presented and
discussed in terms of plasmon-assisted decay of hot-phonons. The optimal 2DEG density for the fastest removal
of the LO-mode heat is estimated.
Interaction of hot electrons with longitudinal optical (LO) phonons and the resultant accumulation of the phonons
is investigated in the GaN-based channels subjected to high electric fields. Physical background of microwave
noise technique for experimental investigation of hot phonons is described. The technique is applied to the 2DEG
and 3DEG channels located in AlGaN/GaN and AlGaN/AlN/GaN heterostructures and silicon-doped GaN.
The noise technique has yielded the pioneering result on the hot-phonon lifetime in the standard AlGaN/GaN
channel of interest for microwave power transistors; the result is confirmed later by independent time-resolved
intersubband absorption technique.
At the high electron density, typical for 2DEG channels, the hot-phonon lifetime is independent of the lattice
temperature; the lifetime is essentially shorter as compared with the values for bulk GaN at a low electron
density. The dependence of lifetime on electron density available from time-resolved Raman hot-phonon lifetime
measurements is discussed in terms of plasmon-assisted disintegration of LO-phonon-like quasiparticles launched
by high-energy electrons.
Microwave noise technique is applied to study fast and ultrafast correlations in nitride and arsenide heterostructures containing a two-dimensional electron gas subjected to a strong electric field applied in the plane of electron confinement. The main attention is paid to experimental investigation of electron energy dissipation, hot phonons, and high-energy electrons shared by the adjacent layers (real-space transfer). The typical experimental values for the time of electron energy relaxation range from several picoseconds at low electric fields and low ambient temperatures to hundreds of femtoseconds at a high field. The measured dependence of the electron energy relaxation time on the bias is compared with those obtained through Monte Carlo simulation for different models. An essential contribution due to hot phonons and electron gas degeneracy is evidenced. Dependence of hot-phonon temperature on the electron temperature is deduced from the experimental results on the microwave noise and the dissipated power. The adjacent layers share the high-energy electrons unless the heterojunction barrier is high. Random transitions between the confined and the shared states cause microwave noise. The relaxation time of the occupancy fluctuations is estimated from the measured spectral intensity of current fluctuations. Hot phonons are found to reduce the threshold field for this noise source. The experimental data on AlGaN/GaN, AlGaAs/GaAs and AlInAs/GaInAs/AlInAs are compared.
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