Almost all natural diamonds are impure and contain substitutional nitrogen. Clusters of N atoms are present in Type 1a diamonds (the most common defect structure), while Type 1b diamonds contain well-separated N atoms. 

(a) What is meant by ‘substitutional nitrogen’? 

(b) In what type of environment will an N atom reside in natural diamond? 

(c) Sketch a diagram to illustrate the band structure of diamond, assuming no defects. Explain why diamond is an insulator. 

(d) The donor-level associated with the nitrogen lies about 1.7 eV below the conduction band. How does this donor level arise? Why do natural diamonds not behave as semiconductors?