This thesis describes self-sensing cantilevers made from GaAs/AlGaAs heterostructures containing two-dimensional electron gases (2DEGs). The cantilevers have micron size lateral dimensions, nanometer size thicknesses, and deflection sensors composed of strain-sensing field-effect transistors (FETs) integrated into their base.; We fabricated strain sensing FETs from a GaAs/Al{dollar}rmsb{lcub}0.3{rcub}Gasb{lcub}0.7{rcub}As{dollar} heterostructure containing a 2DEG 520 A beneath the surface. We measure FET noise corresponding to a gate charge noise {dollar}<{dollar}0.2 electrons/{dollar}surd{dollar}Hz. GaAs Field-effect transistors sense strain via the piezoelectric effect. We measured volume strain sensitivity {dollar}2times10sp{lcub}-9{rcub}/surd{dollar}Hz at T = 4.2 K and {dollar}4times10sp{lcub}-9{rcub}/surd{dollar}Hz at T = 77 K limited by FET noise. Such FETs with small size, low noise, high operating speed, and low power dissipation are ideal for integration into small GaAs/AlGaAs mechanical systems as strain sensors.; Three self-sensing cantilevers were demonstrated. The first, with dimensions {dollar}rm65times11.4times0.5 mu msp3,{dollar} showed the measurement of the cantilever resonance frequency 88.2 kHz using the strain sensing FET. The second, with dimensions {dollar}rm65times11.4times0.25 mu msp3,{dollar} was used as a scanning probe microscope cantilever at T = 4.2 K. Measured deflection noise 10 A/{dollar}surd{dollar}Hz at 100 Hz corresponded to a force noise 19 pN/{dollar}surd{dollar}Hz at 100 Hz limited by FET 1/f noise. The third, a scanning probe microscope cantilever {dollar}rm3times2times0.129 mu msp3,{dollar} had a calculated spring constant 4.7 N/m and resonance frequency 11 MHz. The measured FET charge noise {dollar}<{dollar}0.001 e/{dollar}surd{dollar}Hz combined with the increased deflection sensitivity found in smaller cantilevers gives a projected deflection sensitivity of 0.002A/{dollar}surd{dollar}Hz.
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