James R Biard

A model of the avalanche photodiode

A general model for the avalanche photodiode is presented. It is shown that the diode consists of four regions: 1) guard ring, 2) uniform avalanche region, 3) high-field absorption region and 4) zero-field absorption region. Expressions are given for the ac quantum efficiency, the dc quantum efficiency, and the transit time cutoff frequency. Material requirements are discussed. Based on an entire detector system, an expression is derived for the signal-to-noise ratio. An example is given with the result that a noise-equivalent power (NEP) of 10-12W/Hz1/2is obtained with an optimum avalanche gain of approximately 23.

Optoelectronics as applied to functional electronic blocks

The combination of a GaAs light source and a silicon photodetector in a Functional Electronic Block (FEB) provides signal coupling with electrical isolation. Circuit functions not previously available in microelectronic form can be realized with optical coupling. The characteristics of the GaAs light source, the silicon photodetector and optical coupling techniques of importance in FEB design are discussed. Several useful FEBs which employ optical coupling to achieve electrical isolation are described; these include a multiplex switch, an isolated gate p-n-p-n-type switch, a photon-coupled transistor and an optoelectronic pulse amplifier.

Electrical characteristics of proton-implanted vertical-cavity surface-emitting lasers

The electrical characteristics of proton-implanted GaAs quantum-well vertical-cavity surface-emitting semiconductor lasers (VCSELs) have been studied. We show that the 2 kT current, observed over many decades in these VCSELs, is primarily due to nonradiative recombination mechanisms. These include surface recombination at the edges of the proton-implanted region and bulk recombination at defects and heterojunction interface traps. The contribution of these mechanisms to the total nonradiative current and the threshold current density has been calculated. Lateral spontaneous emission measurements have been used to prove that the radiative current has a kT behavior in the subthreshold region. Electrical derivative measurements have been used to identify leakage current paths through the proton-implanted region in the low-bias regions. In addition, electrical derivative measurements have been used to measure the variation of series resistance with current near the lasing threshold. From a consideration of the various current paths in the VCSEL, a lumped circuit equivalent model for the VCSEL has been developed.

GaAs Infrared source

A diffused junction GaAs diode has been developed for use as a fast, high-efficiency, infrared source. The diode is forward biased with the light being generated by the recombination of minority carriers in the semiconductor bulk. The device is a P+-N structure with a spaced contact covering a minor portion of the N-type face of the wafer through which the radiation is emitted. The infrared radiation is characterized by a sharply defined peak at slightly less than band gap (E gap=1.43ev at 300° K) and a broad peak at about 1.0 ev. The measured radiation pattern closely follows that expected of a flat plate, i.e., the intensity decreases as the cosine of the angle from the normal. The diode V-I curve is characterized by exp (qV/2kT) at low current (up to 1 ma), exp (qV/kT) at moderate current (5-50 ma) and series IR drop at high current (above 200 ma).

An Iterative Model for the Steady-State Current Distribution in Oxide-Confined VCSELs

This paper presents an iterative model for the analysis of the current distribution in vertical-cavity surface-emitting lasers (VCSELs) using a SPICE-like approach. The model includes a degeneracy correction for operation at and above threshold. The effect of the resistance due to the p-distributed Bragg reflector (p-DBR) mirror layers and the oxide layer on performance is investigated. Higher sheet resistance under the oxide layer reduces the threshold current, but reduces the current range over which single transverse mode operation occurs. The voltage drop across the p-DBR region dominates spatial hole burning, which is moderated by lateral drift and diffusion. This simple iterative model is applied to commercially available oxide-confined VCSELs.

A high frequency silicon avalanche photodiode

An NPπP silicon avalanche photodiode with an NπP guardring is described. The diode was designed for high-frequency operation in the visible and near infrared spectrum and was fabricated on epitaxial material using standard silicon planar techniques. The active avalanche region is 10 mils in diameter and the depletion layer width determined by the epitaxial layer is 4.7 microns. The diode is mounted in a microwave pill package, that allows for easy application in coaxial connectors or printed circuit boards. Measurements of the low frequency noise characteristics show them to be in agreement with the theory of McIntyre. For avalanche gain, M, less than 100 the noise power increases with M to the 2.4 power. For larger gains there is a more rapid increase in the noise. Noise measurements at one GHz show the same slope of 2.4 on the noise power versus dc gain curve at M up to 100. At gains above this the diode gain-bandwidth product (100 GHz) limits the noise power to a slope of 1. Gains up to 200 were observed at low frequencies without the presence of excessive noise due to microplasmas. Calculations show the diode in a typical system would have an N.E.P. of 10-12WHZ-1/2, for a one GHz bandwittt and an avalanche gain of 30.