Transistors, in particular MOS transistors, like MOSFET or IGBT, can be used as electronic switches. In those applications a load path of the transistor is connected in series to the load, with the series circuit including the transistor and the load being connected between voltage supply terminals. The transistor can be switched on and off by applying a suitable control signal to its control input.
In many applications it is desired to measure the current through the load. For measuring the current through a load a sense transistor may be used. The sense transistor is coupled to the load transistor and may be operated in the same operation point as the load transistor. If the load transistor and the sense transistor are operated in the same operation point, a sense current flowing through the sense transistor is proportional to a current flowing through the load transistor. A proportionality factor between the load current and the sense current is dependent on the ratio between the active transistor areas (channel areas) of the sense transistor and the load transistor.
The proportionality factor is equal to the ratio between the active device areas, if the load transistor and the sense transistor have identical device characteristics. However, in reality the device characteristics of the load transistor and the sense transistor may differ. In one embodiment the threshold voltages of the two transistors may be different. This difference in the device characteristics may result from systematic differences in the processes that are used for manufacturing the load transistor and the sense transistor.
Usually the load transistor and the sense transistor are integrated in a common semiconductor body (die, chip), where each of the load transistor and the sense transistor may include a number of transistor cells, that can be produced using identical processes. A first number of transistor cells is connected in parallel, thereby forming the load transistor, and a second number of transistor cells is connected in parallel, thereby forming the sense transistor. The transistor cells of the load and the sense transistor may have a common control terminal and one common first load terminal, but two different second load terminals. If the transistors are MOSFET the control terminal is a gate terminal, the first load terminal may be a drain terminal, and the second load terminals may be source terminals. Producing two different second load terminals (source terminals) may result in process variations that differently influence the threshold voltage of the load transistor cells and the sense transistor cells.
For these and other reasons there is a need for the present invention.