MODFET and HBT

[shiks]

Both MODFET and HBTs are high speed devices..But as far as I know, MODFETs are still faster than HBTs..Then why would anybody like to prefer HBTs over MODFETs? I mean what are the advantages of HBTs?

[shiks]

can one arbitrarily replace a BJT by an HBT or there are certain criteria like frquency etc?

[Sukhbinder]

what about an IGBJT (Insulated Gate Bipolar Junction Transistor) it has the input stage of a Mosfet and the output if a BJT. so u get the high input impedance of a MOS and lower working voltages and high working temperatures of a BJT