a transient blocking unit includes at least two depletion mode transistors connected to each other such that they can rapidly switch from a normal low - impedance state to a high - impedance current blocking state in response to an over - voltage or over - current condition . this behavior makes tbus useful for protecting electrical devices and circuit from harmful electrical transients . some kinds of transistors can exhibit a phenomenon known as current collapse , where channel conductance is temporarily reduced after exposure to high voltage . although current collapse is undesirable , transistors exhibiting current collapse can have otherwise favorable properties for tbu applications . according to the present invention , a tbu is provided where a diode is placed in parallel with a tbu transistor that can exhibit current collapse . the diode prevents high power dissipation in a current collapsed transistor , thereby reducing the vulnerability of the tbu to permanent damage or destruction in service .