Of course, we can use Darlington pair transistors to increase the current gain, but this kind of arrangement still requires far more controlling current than an equivalent power IGFET: It would be nice from the standpoint of control circuitry to have power transistors with high current gain, so that far less current is needed for control of load current. To give an example, in order for a power BJT with a β of 20 to conduct a collector current of 100 amps, there must be at least 5 amps of base current, a substantial amount of current in itself for miniature discrete or integrated control circuitry to handle: If a bipolar junction transistor is used to control a large collector current, there must be a substantial base current sourced or sunk by some control circuitry, in accordance with the β ratio. This high current gain would at first seem to place IGFET technology at a decided advantage over bipolar transistors for the control of very large currents.
The only current we see through the gate terminal of an IGFET, then, is whatever transient (brief surge) may be required to charge the gate-channel capacitance and displace the depletion region as the transistor switches from an “on” state to an “off” state, or vice versa. Because of their insulated gates, IGFETs of all types have extremely high current gain: there can be no sustained gate current if there is no continuous gate circuit in which electrons may continually flow.
0 kommentar(er)
