If you have access to signal processing toolbox you can do this with pulstrain
I'm sure there's a cleaner/clearer way to use this but here's what I worked out for an arbitrary width and frequency. Note that each pulse has a time and amplitude, you can make the amplitude +/-1 to achieve a biphasic pulse.
t=linspace(0,1,1000);
w = .02;
dPos = .1:.1:1;
dNeg = dPos + w;
dT = [dPos dNeg]';
dAmp = [ones(numel(dPos),1); -1 * ones(numel(dNeg),1)];
y = pulstran(t,[dT dAmp],'rectpuls',w);
plot(t,y)
ylim padded
