Numerical modeling of abrupt heterojunctions using a thermionic-field emission boundary condition

We present a numerical model in which the thermionic and tunneling mechanisms across an abrupt heterojunction interface are taken into account on the basis of the one dimensional drift-diffusion formulation. We use an expression of thermionic-field emission current formulated based on the WKB approximation as a boundary condition at the abrupt heterointerface which eventually limits the current transport over the barrier while maintaining the current continuity. The I-V characteristics of three types of GaAs/AlGaAs heterojunctions are analyzed by varying device dimension, doping density, and temperature and compared with those obtained by the thermionic emission model to illustrate the significance of both tunneling and thermionic emission mechanisms. We demonstrate that the role of tunneling in the overall current transport is very important in these abrupt heterojunctions especially at high doping densities and low temperatures. In the case of an MBE-grown AlGaAs triangular heterojunction barrier, the temperature-dependent I-V characteristics are measured and compared with the theoretical results. Good agreement is obtained when the tunneling process is taken into account by employing the thermionic-field emission boundary condition presented here.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30948/1/0000620.pd

Electronic properties of pseudomorphic InGaAs/AlGaAs (on GaAs) and InGaAs/InAlAs (on InP) Modfet structures

Pseudomorphic (strained channel) modulation doped field effect transistors (MODFETs) have recently received a considerable amount of attention. These devices provide potential for both improved device performance and new physics studies. In this paper we present theoretical studies of n-type and p-type strained channel MODFETs. Information on carrier masses, subband occupation, and the charge control picture as a function of strain in the channel is presented. The n-MODFET studies are based on using the results of tight binding calculations for bandstructure in the strained channel. The p-MODFET problem involves the use of the Kohn Luttinger hamiltonian. Self consistent solution of the Schrodinger equation and the Poisson equation then allows us to study the MODFET properties. In p-type MODFETs, the control of heavy hole-light hole coupling via strain allows the possibility of tailoring hole masses. Comparisons with some of the experimental works previously published are also presented.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27558/1/0000602.pd

Diffusion effects in short-channel GaAs MESFETs

Diffusion effects in short-channel GaAs MESFETs are studied using a two-dimensional electron temperature simulation. A structure that consists of two n+-n contacts on a thin channel region was used as a test vehicle for the study. This structure was found to have a higher cutoff frequency than conventionally doped devices with the same gate length due to decreased gate capacitance and a selective modulation of the device transconductance. These results suggest that the performance of short-channel microwave MESFETs may be influenced strongly by electron diffusion.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28031/1/0000470.pd

Bias circuit instabilities and their effect on the d.c. current-voltage characteristics of double-barrier resonant tunneling diodes

Bias circuit stability has important implications for the study and application of double-barrier resonant tunneling structures. Stability criteria for resonant tunneling diodes are investigated for the common bias circuit topologies. A systematic study was made of the effect of different bias circuit elements on the measured d.c. I-V curves. A double-barrier diode was studied as an example, with experimental and theoretical results. The main results of the paper are (1) stable resonant tunneling diode operation is difficult to obtain, (2) the low-frequency oscillation introduces a characteristic signature in the measured d.c. I-V characteristic.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29472/1/0000558.pd

Built‐in biaxial strain dependence of Γ‐X transport in GaAs/InxAl1−xAs/GaAs pseudomorphic heterojunction barriers (x=0, 0.03, and 0.06)

The effects of built‐in biaxial strain on Γ‐X transport in n‐GaAs/i‐InxAl1−xAs/n‐GaAs pseudomorphic single‐barrier structures (x=0, 0.03, and 0.06) are studied by measuring temperature‐dependent I‐V characteristics. For the accurate characterization of electron transport across each barrier, a self‐consistent numerical model is used to analyze the experimental results. For each structure, the four barrier parameters defined from the thermionic‐field‐emission theory, the effective Richardson constant A∗, the conduction‐band offsets ΔEc1,2, and a tunneling mass mn∗ are extracted by calculating the theoretical I‐V characteristics and fitting them to the experimental I‐V‐T data. The experimentally obtained X‐point conduction‐band shifts with the addition of indium are compared with the theoretical results calculated based on the model‐solid theory. The results indicate that the addition of indium not only splits the degenerate X minima of the InxAl1−xAs barrier, but also shifts the relative barrier heights of both longitudinal and transverse X valleys due to the alloy‐dependent band‐structure modification. The comparison between the experimental and theoretical results illustrates that the transverse X valleys are the main conduction channel for the Γ‐X transport across InxAl1−xAs pseudomorphic barriers. © 1994 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70663/2/JAPIAU-76-12-7907-1.pd

Erratum: ‘‘Evidence for field enhanced electron capture by EL2 centers in semi‐insulating GaAs and the effect on GaAs radiation detectors’’ [J. Appl. Phys. 75, 7910 (1994)]

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69985/2/JAPIAU-77-3-1331-1.pd

Evidence for field enhanced electron capture by EL2 centers in semi‐insulating GaAs and the effect on GaAs radiation detectors

The performance of Schottky contact semiconductor radiation detectors fabricated from semi‐insulating GaAs is highly sensitive to charged impurities and defects in the material. The observed behavior of semi‐insulating GaAs Schottky barrier alpha particle detectors does not match well with models that treat the semi‐insulating material as either perfectly intrinsic or as material with deep donors (EL2) of constant capture cross section compensated with shallow acceptors. We propose an explanation for the discrepancy based on enhanced capture of electrons by EL2 centers at high electric fields and the resulting formation of a quasineutral region in the GaAs. Presented is a simple model including field enhanced electron capture which shows good agreement with experimental alpha particle pulse height measurements.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71231/2/JAPIAU-75-12-7910-1.pd

InGaAs/InP hot electron transistors grown by chemical beam epitaxy

In this letter, we report on the dc performance of chemical beam epitaxy grown InGaAs/InP hot electron transistors (HETs). The highest observed differential β (dIC/dIB) is over 100. The HETs have Pd/Ge/Ti/Al shallow ohmic base contacts with diffusion lengths less than 300 Å. Furthermore, we also demonstrated ballistic transport of electrons in an InGaAs/InP HET by obtaining an energy distribution of electrons with ∼60 meV full width at half maximum. The measured conduction band discontinuity of InGaAs/InP is 250.3 meV, which is 39.8% of the band gap difference.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70748/2/APPLAB-61-2-189-1.pd

Effect of remote ischaemic conditioning on clinical outcomes in patients with acute myocardial infarction (CONDI-2/ERIC-PPCI): a single-blind randomised controlled trial.

BACKGROUND: Remote ischaemic conditioning with transient ischaemia and reperfusion applied to the arm has been shown to reduce myocardial infarct size in patients with ST-elevation myocardial infarction (STEMI) undergoing primary percutaneous coronary intervention (PPCI). We investigated whether remote ischaemic conditioning could reduce the incidence of cardiac death and hospitalisation for heart failure at 12 months. METHODS: We did an international investigator-initiated, prospective, single-blind, randomised controlled trial (CONDI-2/ERIC-PPCI) at 33 centres across the UK, Denmark, Spain, and Serbia. Patients (age >18 years) with suspected STEMI and who were eligible for PPCI were randomly allocated (1:1, stratified by centre with a permuted block method) to receive standard treatment (including a sham simulated remote ischaemic conditioning intervention at UK sites only) or remote ischaemic conditioning treatment (intermittent ischaemia and reperfusion applied to the arm through four cycles of 5-min inflation and 5-min deflation of an automated cuff device) before PPCI. Investigators responsible for data collection and outcome assessment were masked to treatment allocation. The primary combined endpoint was cardiac death or hospitalisation for heart failure at 12 months in the intention-to-treat population. This trial is registered with ClinicalTrials.gov (NCT02342522) and is completed. FINDINGS: Between Nov 6, 2013, and March 31, 2018, 5401 patients were randomly allocated to either the control group (n=2701) or the remote ischaemic conditioning group (n=2700). After exclusion of patients upon hospital arrival or loss to follow-up, 2569 patients in the control group and 2546 in the intervention group were included in the intention-to-treat analysis. At 12 months post-PPCI, the Kaplan-Meier-estimated frequencies of cardiac death or hospitalisation for heart failure (the primary endpoint) were 220 (8·6%) patients in the control group and 239 (9·4%) in the remote ischaemic conditioning group (hazard ratio 1·10 [95% CI 0·91-1·32], p=0·32 for intervention versus control). No important unexpected adverse events or side effects of remote ischaemic conditioning were observed. INTERPRETATION: Remote ischaemic conditioning does not improve clinical outcomes (cardiac death or hospitalisation for heart failure) at 12 months in patients with STEMI undergoing PPCI. FUNDING: British Heart Foundation, University College London Hospitals/University College London Biomedical Research Centre, Danish Innovation Foundation, Novo Nordisk Foundation, TrygFonden

Heterojunction Transistors.

The objective of this investigation is to study the properties of GaAs-based heterojunction transistors in order to better underst and their high-frequency performance. The high-frequency limitations of high-mobility materials are studied by comparing the switching performance of diodes and transistors based on two transport models: a conventional drift-diffusion transport model and a more accurate energy-momentum balance transport model. The drift-diffusion model predicts high-frequency performance limitations because of velocity saturation and hot electron effects. The energy-momentum model predicts poorer high-frequency performance based on velocity overshoot and cool electron effects. It also predicts an underdamped electron velocity due to the inductive nature of the electron velocity. The properties of heterojunction interfaces are studied by comparing three interface models: the Boltzmann model, the Fermi-Dirac model and a model based on quantum statistics. The choice of model is a tradeoff between accuracy and cost. Over the range of temperatures and interface conditions studied, the Fermi-Dirac model predicts interface conditions close to the more accurate quantum-based model using approximately the same amount of computer time as the simple Boltzmann model. The Fermi-Dirac interface model is used as a basis for a two-dimensional model for a high electron mobility transistor structure. The structure is studied over a range of bias, doping and Al fraction conditions in order to better underst and the device operation. The device operation is found to be closer to bipolar transistor operation than conventional field-effect transistor operation. The two-dimensional model is also used to study the properties of ballistic injection structures and ballistic transistors. A simple Monte Carlo model is used to study ballistic injection. The results show that very high velocities can be obtained by ballistic injection, but that the effects occur over relatively short distances. This limits the performance of ballistic transistors.Ph.D.Electrical engineeringUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/160947/1/8612509.pd