  We report on transport measurement study of top-gated field effect
transistors made out of InSb nanowires grown by chemical vapor deposition. The
transistors exhibit ambipolar transport characteristics revealed by three
distinguished gate-voltage regions: In the middle region where the fermi level
resides within the bandgap, the electrical resistance shows an exponential
dependence on temperature and gate voltage. With either more positive or
negative gate voltages, the devices enter the electron and hole transport
regimes, revealed by a resistance decreasing linearly with decreasing
temperature. From the transport measurement data of a 1-$\mu$m-long device made
from a nanowire of 50 nm in diameter, we extract a bandgap energy of 190-220
meV. The off-state current of this device is found to be suppressed within the
measurement noise at a temperature of T = 4 K. A shorter, 260-nm-long device is
found to exhibit a finite off-state current and a hole, on-state,
circumference-normalized current of 11 $\mu$A/$\mu$m at V$_D$ = 50 mV which is
the highest for such a device to our knowledge. The ambipolar transport
characteristics make the InSb nanowires attractive for CMOS electronics, hybrid
electron-hole quantum systems and hole based spin qubits.
