How can I add a new textbox on every click of a button, in javascript?

Here is the code I have at the moment…

<button id="a" onclick="c()"></button>

<div id="b"></div>

function c()
{
    var Html = '<input type="text" id="a">';
    document.getElementById('b').innerHTML = Html;
}

The problem is that, when the button is clicked several times, it only adds one textbox. I need it to add a textbox on every click.