We demonstrate the characteristics of p+-Ge/n-Si and n+-Ge/p-Si heterojunction diodes with a very high on/off ratio and very low leakage current using heteroepitaxial Ge grown directly on Si. The current ratio of p+-Ge/n-Si and n+-Ge/p-Si heterojunction is ∼5×107 and ∼3×106, respectively. The remarkably low off current density is 2.1 µA/cm2 for p+-Ge/n-Si and 19 µA/cm2 for n+-Ge/p-Si at a reverse bias of |VR|=±1 V, respectively. High on current density (125 A/cm2 for p+-Ge/n-Si and 54 A/cm2 for n+-Ge/p-Si) with a forward bias |VF|=±1 V is obtained with a GeO2 passivation and an Al2O3 isolation.
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