gb at this time the control signal ig 2 is low the gate electrode 19b serves as potential barriers since the constant voltage g 3 of 3 volts is applied a part q 2 of the charge q 1 introduced from the impurity layer 32 overflows to the impurity layer 33 when the control signal ig 1 becomes low the gate electrode 30 serves as a potential barrier and thus interrupts the passage of any charge between the impurity layer 33 and the potential well below the gate electrode gb as a result a signal charge q b corresponding to electrode 31 and the potential well below the gate electrode gb is stored at the potential well below the gate electrode gb that is with reference to fig5 a and 5b the charge q b equals the difference q 1 q 2 which is proportional to the difference between the voltages applied to the gate electrodes 31 and gb i e 4 3 1 v the measurement of the prescribed amount of charge is thus completed when the control signals ig 1 ig 2 and u03c6 2a are respectively asserted low 5 v high 5 v and high 5 v at time t 2 the potential level below the.