Theoretical and experimental analysis of a second-harmonic InP transferred electron device is presented for the 63-170 GHz frequency band. This broadband device produces 30 mW at 63 GHz, 85 mW at 122 GHz, and 8 mW at 170 GHz; in all cases the diode is operating in the second-harmonic mode. A continuously-tunable cavity has been used to produce 30-40 mW of output power over the 119-147.5 GHz range without any detectable frequency jumps or power dips. High frequency structure simulator (HFSS) and drift-diffusion harmonic-balance analysis (DDHB) are used to self-consistently analyze the second-harmonic TEO operation. Numerical simulation results are presented that explain the broadband behavior of the device, and determine the optimal device embedding impedance. Simulations are capable of predicting operating frequencies to within several GHz and output powers to within about 20% accuracy.