LOW-TEMPERATURE CARRIER DISTRIBUTIONS IN WIDE QUANTUM-WELLS OF DIFFERENT SHAPES FROM CAPACITANCE-VOLTAGE MEASUREMENTS

The carrier distributions in modulation-doped wide graded quantum well s that a measurement of the capacitance C between a surface gate and a n ohmic contact to the carriers as a function of the applied bias V wo uld yield are calculated. These capacitance-voltage (C-V) distribution s are found to agree inexactly, but closely, with the calculated true carrier distributions. Density modulation features, induced by superla ttices or by abrupt changes in the curvature of band-gap grading, are strikingly reproduced. Electron distributions extracted from actual me asurements on a wide parabolic well and on a parabolic well with super imposed superlattice are in good agreement with theory. For the case o f the parabolic well, the occupancy of a finite number of subbands is manifested as structure in the C-V distributions. This technique is re levant to the measurement of carrier distributions in any wide carrier system with more than one electric subband occupied.