What was Atomera's revenue in 2021 
an imagine, this is a very beneficial trend for us. We also believe it's a strong vote of confidence in Atomera's technology when companies with very limited R&D wafer capability are allocating it to development using MST technology.
We continue to see very strong CapEx growth by almost all players in the semiconductor industry. Inevitably, the most advanced nodes are getting a lot of that CapEx, but legacy nodes are now starting to get long-overdue attention with capital directed toward building new factories or expanding production in existing fabs to correct long-term capacity issues. Today's Intel announcement of the Tower acquisition is a good example of this type of investment. We believe this allocation of capital provides a very strong opportunity for Atomera since MST can uniquely enhance older nodes that have run out of other options.
Our MST-SP product is a great example of how our technology can be used to improve both performance and die size, and therefore, capacity in a legacy fab. We are at the beginning of an unprecedented period of CapEx growth in the semiconductor industry, and with that growth come exceptional opportunities for Atomera. I think you can agree that the last three months have been very productive for Atomera. Our company achieved several important milestones, including publicly announcing MST-SP, reaching 300 patents granted and pending, inclusion in Forbes List of Best American Small Companies for 2022, and more recently announcing a new license with a foundry and fulfillment of the technical milestones specified in our JDA.
We are clearly entering 2022 with strong momentum, which is something we hope to carry forward through the remainder of the year. Now Frank will review our financials.
Frank Laurencio -- Chief Financial Officer
Thank you, Scott. At the close of the market today, we issued a press release announcing our fourth quarter and full-year 2022 results. This slide shows our summary financials. Our GAAP net loss for the year ended December 31, 2021, was $15.7 million or $0.70 per share, compared to a net loss of $14.9 million or $0.79 per share in 2020.
Revenue increased by $338,000 from $62,000 in 2020 to $400,000 in 2021. GAAP operating expenses were $15.9 million in 2021, which was an increase of $960,000 from $15.0 million in 2020. Non-GAAP net loss for 2021 was $12.5 million and reflected $12.9 million in non-GAAP operating expenses. In 2020, our non-GAAP net loss was $11.7 million, the same as our non-GAAP operating expenses for that year.
Stock compensation expense, which is the largest component of non-GAAP costs, was $3 million in both years. Our press release and this slide contain a reconciliation between our GAAP and non-GAAP results. In the fourth quarter of 2021, our GAAP net loss was $4.2 million or $0.18 per share compared to a net loss of $3.9 million or $0.19 per share in Q4 of 2020, reflecting higher operating expenses. Sequentially, the Q4 net loss in 2021 was approximately flat compared to Q3.
On a non-GAAP basis, our Q4 2021 net loss was $3.4 million compared to a loss of $3 million in Q4 2020. The $3.4 million in non-GAAP net loss in Q4 was unchanged sequentially from Q3 2021. Focusing now on the components of operating expenses, our full-year research and development expenses in 2021 were $8.8 million, an increase of $355,000 from $8.4 million in 2020. This increase was mainly due to the new epi tool lease, which we started making payments on in August of 2021.
General and administrative expenses increased to $6.2 million in 2021 from $5.6 million in 2020, mainly reflecting higher insurance and payroll costs. Sales and marketing expenses in 2021 were $986,000 compared to $921,000 in the prior year. Our cash balance at December 31, 2021, was $28.7 million, compared to $37.9 million at the end of 2020. The $9.2 million decline over the year reflects $12.4 million of cash used in operating activities, offset by $3.3 million of cash received from financing activities.
As of December 31, 2021, we had 23.2 million shares outstanding. Last year, we recognized
an imagine, this is a very beneficial trend for us. We also believe it's a strong vote of confidence in Atomera's technology when companies with very limited R&D wafer capability are allocating it to development using MST technology.
We continue to see very strong CapEx growth by almost all players in the semiconductor industry. Inevitably, the most advanced nodes are getting a lot of that CapEx, but legacy nodes are now starting to get long-overdue attention with capital directed toward building new factories or expanding production in existing fabs to correct long-term capacity issues. Today's Intel announcement of the Tower acquisition is a good example of this type of investment. We believe this allocation of capital provides a very strong opportunity for Atomera since MST can uniquely enhance older nodes that have run out of other options.
Our MST-SP product is a great example of how our technology can be used to improve both performance and die size, and therefore, capacity in a legacy fab. We are at the beginning of an unprecedented period of CapEx growth in the semiconductor industry, and with that growth come exceptional opportunities for Atomera. I think you can agree that the last three months have been very productive for Atomera. Our company achieved several important milestones, including publicly announcing MST-SP, reaching 300 patents granted and pending, inclusion in Forbes List of Best American Small Companies for 2022, and more recently announcing a new license with a foundry and fulfillment of the technical milestones specified in our JDA.
We are clearly entering 2022 with strong momentum, which is something we hope to carry forward through the remainder of the year. Now Frank will review our financials.
Frank Laurencio -- Chief Financial Officer
Thank you, Scott. At the close of the market today, we issued a press release announcing our fourth quarter and full-year 2022 results. This slide shows our summary financials. Our GAAP net loss for the year ended December 31, 2021, was $15.7 million or $0.70 per share, compared to a net loss of $14.9 million or $0.79 per share in 2020.
 Revenue increased by $338,000 from $62,000 in 2020 to $400,000 in 2021. GAAP operating expenses were $15.9 million in 2021, which was an increase of $960,000 from $15.0 million in 2020. Non-GAAP net loss for 2021 was $12.5 million and reflected $12.9 million in non-GAAP operating expenses. In 2020, our non-GAAP net loss was $11.7 million, the same as our non-GAAP operating expenses for that year.
Stock compensation expense, which is the largest component of non-GAAP costs, was $3 million in both years. Our press release and this slide contain a reconciliation between our GAAP and non-GAAP results. In the fourth quarter of 2021, our GAAP net loss was $4.2 million or $0.18 per share compared to a net loss of $3.9 million or $0.19 per share in Q4 of 2020, reflecting higher operating expenses. Sequentially, the Q4 net loss in 2021 was approximately flat compared to Q3.
On a non-GAAP basis, our Q4 2021 net loss was $3.4 million compared to a loss of $3 million in Q4 2020. The $3.4 million in non-GAAP net loss in Q4 was unchanged sequentially from Q3 2021. Focusing now on the components of operating expenses, our full-year research and development expenses in 2021 were $8.8 million, an increase of $355,000 from $8.4 million in 2020. This increase was mainly due to the new epi tool lease, which we started making payments on in August of 2021.
General and administrative expenses increased to $6.2 million in 2021 from $5.6 million in 2020, mainly reflecting higher insurance and payroll costs. Sales and marketing expenses in 2021 were $986,000 compared to $921,000 in the prior year. Our cash balance at December 31, 2021, was $28.7 million, compared to $37.9 million at the end of 2020. The $9.2 million decline over the year reflects $12.4 million of cash used in operating activities, offset by $3.3 million of cash received from financing activities.
 As of December 31, 2021, we had 23.2 million shares outstanding. Last year, we recognized