This paper reports on the ambipolar conduction for the λ-Deoxyribonucleic Acid (DNA) field effect transistor (FET) with 450, 400 and 250 base pair experimentally and theoretically. It was found that the drain current of the p-type DNA/Si FET increased as the ratio of the guanine-cytosine (GC) pair increased and that of the n-type DNA/Si FET decreased as the ratio of the adenine-thymine (AT) pair decreased, and the ratio of the GC pair and AT pair was controlled by the total number of the base pair. In addition, it was found that the hole conduction mechanism of the 400 bp DNA/Si FET was polaron hopping and its activation energy was 0.13eV. By considering the electron affinity of the adenine, thymine, guanine, and cytosine, the ambipolar characteristics of the DNA/Si FET was understood. The holes are injected to the guanine base for the negative gate voltage, and the electrons are injected to the adenine, thymine, and cytosine for the positive gate voltage.

The influence of the gate voltage or base pair ratio modulation on the λ-DNA FET performance was examined. The result of the gate voltage modulation indicated that the captured electrons in the guanine base of the λ-DNA molecules greatly influenced the Id-Vd characteristics, and that of the base pair ratio modulation indicated that the tendency of the conductivity was partly clarified by considering the activation energy of holes and electrons and the length and numbers of the serial AT or GC sequences over which the holes or electrons jumped. In addition, the influence of the dimensionality of the DNA molecule on the conductivity was discussed theoretically.