def xvariation(C0: float, er: float, area_cm2: float):
    """
    Estimates the quantity
    
    eps*A / C0 = x_e + eps*F_e/rho_e
    
    which corresponds to variations in the depletion width over approximately 
    the same distance scale.
    
    Parameters
    ----------
    C0: float
        The fitted value of C0 in pico Farads (C^2/J)
    er: float
        The relative permittivity of the dielectric
    area_cm2: float
        The area of the device in cm^2
    
    Returns
    -------
    float:
        eps*A/C0 in cm
    """
    # e0 = 8.854187817620389e-14 C^2 / J / cm
    # q = 1.6021766208e-19
    x = er*8.854187817620389*area_cm2/C0/100
    
    return x