Processes occurring at low amplitudes

In this condition, the level of carriers injection is very low, so the process of accumulation and cleaning out of minority carriers into the base don’t influence the duration of the diode switching time. Generally the shape of pulses depends on a barrier capacitance.

At the moment when a pulse is passing through a diode (operating with a current oscillator), a hole current consists of a capacitance component only. That is why, a diode voltage changes at once abruptly because of voltage drop across a base resistance (Fig. 3.14a). The diode voltage increases as a barrier capacitance takes a charge. After diode disconnecting, its voltage comes down abruptly, but the barrier capacitance voltage still remains. Capacitance and diode voltage come down as this capacitance discharges through the diode p-n junction resistance. When operating with a voltage oscillator, there is no effect of a base resistance modulation because of a low injection level. So, the diode impedance has only a capacitance component, and the diode current increases abruptly at the moment after the diode switching off (Fig. 3.14b). The value of a diode current is limited by the base impedance. While the barrier capacitance takes a charge, the p-n junction voltage and diode current are seeking steady level, which is defined by an active component of a p-n junction impedance.

Fig.3.14. Voltage and current oscillograms of a diode at operating with current (a) and voltage (b) oscillators under the low-amplitudes condition

After voltages in turned on the barrier capacitance discharges, and the current reaches a steady level for a while.

Pulse diodes are specified not only by t_cl.out, t_rest, C_bar, but by direct forward voltage and current, reverse voltage and current, limited reverse voltage, and a pulse height.