# Description --------------------------------------------------------------------------------------------------------------
# Simulation of the band structure at the np junction of a diode.  This is a qualitatively reasonable representation
#   using a scaled tanh function.  Ohmic (linear) potential drop across the n and p regions is included.
# Use 0.6 > bias > -1.0
#
# Name:        bands.py
# Author:      W. Hetherington, Physics, Oregon State University
# Created:     2012/02/14
# Modified:    2012
# Copyright:   (c) 2012
# License:     No restrictions
#---------------------------------------------------------------------------------------------------------------------------

from scipy import *
from numpy import *
import matplotlib.pyplot as plt

def BandFunctions(xn, xp, dz, band_gap, evd_zero, e_donor, e_acceptor, bias) :
	# xn and xp are the position arrays for the n and p regions.
	# dz is the zero bias depletion zone width.
	# band_gap in eV, vd_zero is the zero bias potential energy difference in eV across the junction.
	# bias is the foraward bias in eV.
	# e_donor is the donor state energy in eV below the n-type conduction band edge.
	# e_acceptor is the acceptor state energy in eV above the p-type valence band edge.
	x = concatenate((xn, xp))
	#cb = (tanh(x/100.0) + 1.0)/2.0 * (evd_zero - bias/2.0) + e_donor + bias/2.0
	evd = evd_zero - bias
	d = dz * (1.0 - bias/evd_zero)
	cb = tanh(x/d) * evd/2.0 + evd/2.0 + e_donor + bias/2.0 + x * -0.0001 * bias
	#vb = (tanh(x/100.0) - 1.0)/2.0 * (evd_zero - bias/2.0) - e_acceptor - bias/2.0
	vb = tanh(x/d) * evd/2.0 - evd/2.0 - e_acceptor - bias/2.0 + x * -0.0001 * bias
	efn = zeros(len(xn)) + bias/2.0 + xn * -0.0001 * bias
	efp = zeros(len(xp)) - bias/2.0 + xp * -0.0001 * bias
	return cb, vb, x, efn, efp	# efn, efp, x

def Main() :
	pts = 1000
	xn = linspace(-500., 0, pts/2)
	xp = linspace(0, 500., pts/2)
	x = concatenate((xn, xp))
	acceptor_level = 0.05
	donor_level = 0.05
	evd_zero = 0.7
	band_gap = 1.1
	bias = -0.3
	depletion_zone_zero_bias = 100.0
	if bias > evd_zero :
		print('Bias is too large.')
		return
	cb, vb, x, efn, efp = BandFunctions(xn, xp, depletion_zone_zero_bias, band_gap, evd_zero, donor_level, acceptor_level, bias)
	plt.plot(x, cb)
	plt.plot(x, vb)
	spacer = 0
	if bias < 0 :
		spacer = int((-bias) * 1.3 * len(xn))
		print spacer
	if spacer != 0 :
		plt.plot(xn[:-spacer], efn[:-spacer], '-', color='orange')
	else :
		plt.plot(xn, efn, '-', color='red')
	plt.plot(xp[spacer:], efp[spacer:], '-', color='red')
	plt.grid(True)
	legends = ['conduction band', 'valence band', 'donor level', 'acceptor level']
	plt.legend(legends, 'lower right')		# , 'center right')
	plt.xlabel('Position (arbitrary unit)')
	plt.ylabel('Potential Energy in eV')
	plt.title('Diode Band Structure for Bias = ' + str(bias) + ' eV')
	plt.show()



#-------------------------------------------------------------------------------------------------------------------

if __name__ == '__main__' :
	Main()