How to write conditional statement/loop?

I need to plot with an expression for V<0 and another expression for V>0.

clc
clear all
n = 1.02;    %Ideality factor
A = 33.65;  %Richardson constant
q = 1.602*10^-19;    % electron charge
K = 1.38*10^-23; %Boltzmann constant
T = 300;    % Absolute temperature
phi_b = 0.28; %Barrier Height
J_0 = A* (T* T)* exp((-q* phi_b)./(K* T));
V = linspace(0,2);
if V<0
    J = -J_0;
else
    J = J_0.* exp((q* V)./(n* K* T)).* (1 - (exp(-q* V)./(K* T)));
end
plot(V, log(J ./(1 - exp(-q* V)/(K *T))));

Answer 1

[Jan]

For V = linspace(0, 2) there is no V < 0. But in general:

V = linspace(-2, 2);
J        = J_0.* exp((q * V) ./ (n * K * T)) .* (1 - (exp(-q * V) ./ (K * T)));
J(V < 0) = -J_0;

[Ranjan Kandasamy]

So, a If Else statement doesn't work here?

[Jan]

Of course it would work, but it is slower, less elegant and offers more chances for typos:

V = linspace(-2, 2, 100);
J = NaN(size(V));  % Pre-allocation
for k = 1:numel(V)
   if V(k) < 0
      J(k) = -J_0;
   else
      J(k) = J_0 * exp((q * V(k)) / (n * K * T)) * (1 - (exp(-q * V(k)) / (K * T)));
   end
end

% Or slightly simpler:
V = linspace(-2, 2, 100);
J = repmat(-J_0, size(V));  % Pre-allocation with default value
for k = 1:numel(V)
   if V(k) >= 0
      J(k) = J_0 * exp((q * V(k)) / (n * K * T)) * (1 - (exp(-q * V(k)) / (K * T)));
   end
end