Current–Voltage (I–V) characteristics of GaAs/AlAs double-barrier resonant tunneling diodes (RTDs) with a Si-planar-doped barrier are studied. At 4.2 K the peak-to-valley current ratio decreased when Si was introduced to the barrier next to the electrode from which electrons are emitted (emitter barrier). On the other hand, the ratio did not decrease considerably when Si was introduced to the other barrier, i.e., next to the electrode into which electrons flow (collector barrier). To investigate the origin of the decrease in the peak-to-valley ratio, we evaluated the transmission coefficient experimentally, and found that the resonance transmission widths for the double-barrier RTDs are almost the same. This means that the reduction in the peak-to-valley current ratio is not caused by the broadening of resonance transmission width. To explain why the peak-to-valley current ratio is degraded, we then introduced a new model in which an excess current flows through donor levels in the emitter barrier. To confirm this model, we investigated the I–V characteristics of GaAs/AlAs single-barrier diodes. We found an excess current, which is probably due to a tunneling current through the donor level created in the barrier, when the barrier is planar-doped. It is, therefore, concluded that the decrease in the peak-to-valley current ratio observed for the RTD with the doped emitter barrier is due to this excess current flowing through the doped emitter barrier.