Question

Quantum well lasers achieve lower threshold current because of

• A. Larger active volume
• B. Carrier confinement
• C. Higher losses
• D. Lower gain

Hint:

Shrinking the active layer down to a quantum well (\(< 20 \text{ nm}\)) changes the density of states from a broad curve to sharp steps. This concentrates carriers into a tiny space (carrier confinement), allowing the laser to turn on at a much lower current.

Answer 1

The Correct Option is B

Concept: A quantum well is a very thin heterostructure layer where a narrow-bandgap semiconductor is sandwiched between two wider-bandgap semiconductor layers. The thickness of this central active layer is reduced to dimensions comparable to the de Broglie wavelength of the charge carriers (typically less than \(20 \text{ nm}\)). This extreme thinness restricts the motion of electrons and holes to a two-dimensional plane, forcing the system into quantum confinement.

Step 1: Quantization and changes to the Density of States.
In a standard bulk semiconductor laser, the electronic density of states (\(g(E)\)) scales continuously with energy as a square root function: \[ g_{\text{bulk}}(E) \propto \sqrt{E - E_c} \] This continuous distribution spreads the injected electrons and holes across a broad energy range. As a result, a very large injection current is required to fill all these states and achieve the population inversion necessary for laser action. In a quantum well structure, the restriction of movement along the growth axis transforms the density of states into a discrete, stair-step distribution pattern: \[ g_{\text{quantum\_well}}(E) = \sum_{n} \frac{m^*}{\pi \hbar^2} \Theta(E - E_n) \] Where \(\Theta\) is the Heaviside step function. This step-like profile concentrates a high density of available electronic states right at the bottom edge of the conduction sub-bands.

Step 2: Mechanism behind lower threshold current.
This modification provides two major operational advantages:
• Carrier Confinement: Injected electrons and holes are physically trapped within the ultra-thin well layer. Because they are confined to a tiny volume, their local concentration increases rapidly even at low absolute injection currents.
• High Optical Gain: Because the density of states is sharp and step-like, nearly all injected carriers contribute to radiative recombination at the exact target laser frequency, rather than being wasted across a broad thermal spectrum. Consequently, population inversion and optical gain are achieved at a fraction of the current required for bulk semiconductor lasers, drastically lowering the threshold current (\(I_{\text{th}}\)). This matches option (B).