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It is known that a depletion MOS device behaves quite differently from an enhancement device. Because of its intrinsic difference, depletion I-V characteristics are much more complicated than are the I-V functions of an enhancement MOSFET. The conventional transistor I-V equation developed for enhancement transistors cannot be applied directly to depletion transistors. In this paper, seven operating modes a depletion device can have are discussed. The concept of extended depletion regions is introduced to calculate transistor I-V characteristics. A graphical representation of the universal channel depth versus the channel potential is proposed to help in understanding the engineering significance of operational modes depletion devices can assume. A flowchart is developed to calculate the depletion transistor I-V characteristics. The simulation and analysis aid can be easily adapted to computer-oriented circuit and device investigations.