Extended hot-electron model and current bistability in insulating <math><mrow><mi>a</mi><mtext>-</mtext><msub><mi mathvariant="normal">Y</mi><mi>x</mi></msub><msub><mi>Si</mi><mrow><mn>1</mn><mtext>-</mtext><mi>x</mi></mrow></msub></mrow></math> films