In Schottky‐barrier (Pt–nGaAs) Read‐type IMPATT diodes (Hi‐Lo or Lo‐Hi‐Lo) it is now well known that the thermionic field‐effect carrier injection (TFE) can decrease the diode microwave performances but also improve the noise properties under large signal levels. We present an experimental study to determine the TFE current which sets off the avalanche for voltage operation corresponding to oscillation or amplification conditions by measuring the back‐bias effect in a pulse operation mode. Then using the tunnel current which has been checked by the present method, the variation of the optimum frequency operation for a high‐low doping profile diode is calculated.