Is designing the hardware circuits more difficult than designing software? Can a hardware circuit designer earn more money than a software designer? What subjects do I need to study if I want to make a career as a hardware circuit designer?
It depends on what type of hardware you want to design, analog or digital. Digital design is basically software design. You'll use a hardware description language like VHDL or Verilog to design the circuits. If you go the analog route, depending on what you specialize you'll use a different software package. Analog is less programming, and more estimated calcalcations and running computer simulations.
In terms of which pays more, it really depends. I would say, don't do it for the money, but rather which type of circuit design do you enjoy more.