The Johnson-Lark-Harowitz effect, also known as the JLH effect, is a phenomenon in semiconductor devices, particularly in bipolar junction transistors (BJTs). It refers to the increase in the base current of a BJT due to the parasitic capacitance between the collector and the base.

In a BJT, the collector-base junction forms a capacitor due to the depletion region between the collector and the base regions. This capacitor has a certain capacitance, which becomes significant at high frequencies. When the transistor is operating at high frequencies, the changing voltage across this capacitor results in a displacement current. This displacement current adds to the base current, effectively increasing it beyond what would be expected based solely on the input signal.

The Johnson-Lark-Harowitz effect can have several implications in the operation of BJTs, particularly in high-frequency applications. It can lead to increased power dissipation, reduced gain, and distortion in the output signal. Therefore, it is essential for designers to consider this effect when designing high-frequency circuits using bipolar transistors.

The effect is named after its discoverers: P.B. Johnson, R.A. Lark, and L. Harowitz, who described it in a paper published in the Proceedings of the IEEE in 1962.

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