P.A. Houston

Electron drift velocity in n-GaAs at high electric fields

Measurements have been made on the drift velocity of electrons in n-GaAs by the time-of-flight technique. The method of determining the νE characteristics from the transit time across the non-uniform field depletion region of the sample is described. Velocity/field curves are presented in the field range 20 to 113 kV/cm at various temperatures from 130 to 400°K. All curves show the velocity to decrease slowly as the field is increased and velocity saturation is approached at the highest field values used. The characteristics match up well with the generally accepted experimental curves at the lower fields, and a simple theory is described which predicts well the saturation velocity.

Gate leakage effects and breakdown voltage in metalorganic vapor phase epitaxy AlGaN/GaN heterostructure field-effect transistors

The gate leakage behavior in AlGaN/GaN heterostructure field-effect transistors was studied as a function of applied bias, temperature, and surface periphery. A surface hopping conduction mechanism with an activation energy of 0.21 eV is proposed for the gate–drain leakage for voltages that exceed pinchoff. The reverse breakdown voltage of the device exhibited a negative temperature coefficient of −0.11 V K−1, suggesting that a breakdown mechanism other than impact ionization, such as thermal runaway, may be responsible.

Investigation of impact ionization in thin GaAs diodes

The electron and hole multiplication coefficients, M/sub e/ and M/sub h/, respectively, have been measured in thin GaAs homojunction PIN and NIP diodes and from conventional ionization analysis the effective electron and hole ionization coefficients, /spl alpha/ and /spl beta/, respectively, have been determined. The nominal intrinsic region thickness w of these structures ranges from 1.0 /spl mu/m down to 25 nm. In the thicker structures, bulk-like behavior is observed; however, in the thinner structures, significant differences are found. As the i-regions become thinner and the electric fields increase, the M/sub e//M/sub h/ ratio is seen to approach unity. The experimental results are modeled and interpreted using a semianalytical solution of the Boltzmann equation. In thin (w/spl les/0.1 /spl mu/m) devices the dead space effect reduces effective ionization coefficients below their bulk values at low values of carrier multiplication. However, overshoot effects compensate for this at extremely high fields (/spl ges/1/spl times/10/sup 3/ kV/cm).

Comparison of different surface passivation dielectrics in AlGaN/GaN heterostructure field-effect transistors

Different dielectrics were used for post-processing surface passivation of AlGaN/GaN heterostructure field-effect transistors (HFETs) and the resulting electrical characteristics examined. An increase in the maximum drain current of approximately 25% was observed after Si3N4 and SiO2 deposition and ~15% for annealed SiO on AlGaN/GaN HFETs. In all cases, the passivation was found to increase the gate leakage current with an observed reduction in the leakage activation energy. However, the rise in gate leakage current was least for SiO. The plasma enhanced chemical vapour deposition method was found not to contribute to the passivation mechanism, whilst the presence of Si appears to be an important factor.

Gain of a heterojunction bipolar phototransistor

Analytical expressions have been derived for the collector current, optical gain, and quantum efficiency for a heterojunction, bi-polar phototransistor (HPT). These expressions can be utilized to optimize the current gain and quantum efficiency for HPT design. The presence of avalanche multiplication in the base-collector junction has been taken into account and shown to be a significant factor in determining the gain of an InGaAs/InP phototransistor. Experimental results of optical gain versus the collector-emitter voltage can only be explained in terms Of avalanche multiplication.

Conduction‐band discontinuity in InGaP/GaAs measured using both current‐voltage and photoemission methods

Both current‐voltage and photoemission measurements of the conduction‐band discontinuity of the same InGaP/GaAs p‐n heterojunction have been carried out. Interpretation of the current‐voltage results using thermionic emission theory applied to a heterojunction bipolar transistor have resulted in a conduction‐band offset value of 0.21 eV in the case of a compositionally abrupt junction. This figure has been confirmed by performing independent photoemission measurements on the same junction.

GaAs solid state detectors for particle physics

Abstract We report on progress with Schottky diode and p-i-n diode GaAs detectors for minimum ionising particles. The radiation hardness and potential speed of simple diodes is shown to be more than competitive with silicon detector. A discussion is given of the present understanding of the charge transport mechanism in the detectors since it influences their charge collection efficiency. Early results from microstrip detectors are also described.

Second-harmonic generation from a first-order quasi-phase-matched GaAs/AlGaAs waveguide crystal

We demonstrate, for the first time to our knowledge, the generation of second-harmonic pulses by use of a novel methodology for achieving first-order quasi-phase matching in a semiconductor waveguide crystal. This methodology is based on a periodic modulation of the susceptibility coefficient along the direction of light-beam propagation in which advantage is taken of the fact that chi((2))(GaAs)>chi((2))(Al(x)Ga(1-x)As) . Efficient second-harmonic generation at 975 nm of a pump wavelength of 1950 nm has been demonstrated for a crystal with a nonuniform domain dimension (duty cycle, ~39/61).

Avalanche multiplication in GaInP/GaAs single heterojunction bipolar transistors

The electron multiplication factors in GaInP/GaAs single heterojunction bipolar transistors (HBTs) have been measured as a function of base-collector bias for a range of GaAs collector doping densities. In the lowest doped (5/spl times/10/sup 14/ cm/sup -3/) thick collector the multiplication is determined by the local electric field. As the collector doping increases, the measured multiplication is found to be significantly reduced at low values of multiplication from that predicted by the electric field profile. However, good agreement is always found at high multiplication, close to breakdown. This reduction in multiplication at low electric fields is attributed to the dead space, the minimum distance over which carriers must travel before gaining the ionization threshold energy. A simple correction for the dead space is proposed, allowing the multiplication to be accurately predicted even in heavily doped structures.

Diffusion of Cd And Zn In InP between 550 and 650°C

A large number of diffusions have been carried out in sealed quartz ampoules in the temperature range 550–650°C using Zn and Cd in InP. Three-fronted profiles were observed at 650°C for both Cd and Zn and the diffused samples were extensively analysed using Hall measurements, electron beam induced current, electrochemical carrier concentration profiling and thermal probing. These electrical measurements identified the p/n junction unambiguously and indicated a region of high compensation between the p/n junction and the third line. The weight of P added to the ampoule had little effect up to 0.4mg in 0.75cm3 volume at which point a dramatic reduction in diffusion depth for all fronts occurred for increasing P. Diffusion coefficients for Cd and Zn were estimated and plotted as a function of temperature.