  We theoretically study the spin current and spin-transfer torque generation
in a conventional spin- valve hybrid structure of type ferromagnetic/normal
metal/ferromagnetic (FM/NM/FM) made of the topological insulator (TI), in which
a gate voltage is attached to the normal layer. We demonstrate the penetration
of the spin-transfer torque into the right ferromagnetic layer and show that,
unlike graphene spin-valve junction, the spin-transfer torque in TI is very
sensitive to the chemical potential of the NM region. As an important result,
by changing the chemical potential of the NM spacer and magnetization
directions, one can control all components of the STT. Interestingly, both the
resonance spin current and the resonance spin-transfer torque appear for
energies determined from a resonance equation. By increasing the chemical
potential of the NM spacer, the amplitude of the STTs decreases while at large
chemical potentials of $\mu_N$ there are intervals of chemical potential in
which both the spin current and the spin-transfer torque become zero. These
findings could open new perspectives for applications in spin-transfer torque
magnetic random access memory (STT-MRAM) devices based on TI.
