A UNIFIED EQUILIBRIUM TREATMENT OF MODULATION-DOPED HETEROJUNCTIONS AND GROSSLY ASYMMETRIC HOMOJUNCTIONS, AND ITS APPLICATION TO MODFET DESIGN

Homo- and hetero- ''grossly asymmetric junctions,'' i.e., ''junctions between a heavily doped and a lightly doped layer,'' are important bui lding blocks of modern p(+)-i-n(+) and n(+)-i-n(+) diodes, BJT's and M ODFET's. We establish a hitherto unhighlighted aspect of the unity und erlying the physics of these junctions, based on a simple yet original deduction from available surface field-potential relations. We show t hat, apart from pursuing the scientific quest for unification, the ded uction also leads to three new results of practical significance. Firs tly, simple expressions are obtained for important parameters of the s pace-charge layer of a general grossly asymmetric junction under equil ibrium. These parameters include the width of the partially depleted r egion on the heavily doped side of the junction, that could not be obt ained from earlier analyses. Secondly, applying this partial depletion width expression to the MODFET heterojunction, a nonlinear MODFET 2-D EG charge versus gate voltage model is derived, which is very useful f or accurately simulating the effects of gradual saturation charge-volt age nonlinearity on dc and ac performance of analogue circuits. This c harge-voltage model is expressed directly in terms of device parameter s and temperature, unlike earlier nonlinear charge-voltage models whos e parameters were empirical and could be extracted only by fitting to experimental data or complex numerical calculations. Thirdly, a new co ncept has emerged, as per which the effects of electron confinement, p artial impurity ionization, Fermi-Dirac statistics and small geometry in a grossly asymmetric junction can be treated simply as apparent ban d discontinuity narrowing phenomena, and thus represented in an additi ve form by dimensionally identical parameters. The concept facilitates a comparison of different modulation doped heterojunction systems. We present calculations illustrating the above results.